Lexical Analysis With Flex, for Flex 2.6.4

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This manual describes flex, a tool for generating programs that perform pattern-matching on text. The manual includes both tutorial and reference sections.

This edition of The flex Manual documents flex version 2.6.4. It was last updated on 6 May 2017.

This manual was written by Vern Paxson, Will Estes and John Millaway.

Table of Contents

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2 Reporting Bugs

If you find a bug in flex, please report it using GitHub’s issue tracking facility at https://github.com/westes/flex/issues/

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3 Introduction

flex is a tool for generating scanners. A scanner is a program which recognizes lexical patterns in text. The flex program reads the given input files, or its standard input if no file names are given, for a description of a scanner to generate. The description is in the form of pairs of regular expressions and C code, called rules. flex generates as output a C source file, lex.yy.c by default, which defines a routine yylex(). This file can be compiled and linked with the flex runtime library to produce an executable. When the executable is run, it analyzes its input for occurrences of the regular expressions. Whenever it finds one, it executes the corresponding C code.

4 Some Simple Examples

First some simple examples to get the flavor of how one uses flex.

The following flex input specifies a scanner which, when it encounters the string ‘username’ will replace it with the user’s login name:

    username    printf( "%s", getlogin() );

By default, any text not matched by a flex scanner is copied to the output, so the net effect of this scanner is to copy its input file to its output with each occurrence of ‘username’ expanded. In this input, there is just one rule. ‘username’ is the pattern and the ‘printf’ is the action. The ‘%%’ symbol marks the beginning of the rules.

Here’s another simple example:

            int num_lines = 0, num_chars = 0;

    \n      ++num_lines; ++num_chars;
    .       ++num_chars;


    int main()
            printf( "# of lines = %d, # of chars = %d\n",
                    num_lines, num_chars );

This scanner counts the number of characters and the number of lines in its input. It produces no output other than the final report on the character and line counts. The first line declares two globals, num_lines and num_chars, which are accessible both inside yylex() and in the main() routine declared after the second ‘%%’. There are two rules, one which matches a newline (‘\n’) and increments both the line count and the character count, and one which matches any character other than a newline (indicated by the ‘.’ regular expression).

A somewhat more complicated example:

    /* scanner for a toy Pascal-like language */

    /* need this for the call to atof() below */
    #include <math.h>

    DIGIT    [0-9]
    ID       [a-z][a-z0-9]*


    {DIGIT}+    {
                printf( "An integer: %s (%d)\n", yytext,
                        atoi( yytext ) );

    {DIGIT}+"."{DIGIT}*        {
                printf( "A float: %s (%g)\n", yytext,
                        atof( yytext ) );

    if|then|begin|end|procedure|function        {
                printf( "A keyword: %s\n", yytext );

    {ID}        printf( "An identifier: %s\n", yytext );

    "+"|"-"|"*"|"/"   printf( "An operator: %s\n", yytext );

    "{"[^{}\n]*"}"     /* eat up one-line comments */

    [ \t\n]+          /* eat up whitespace */

    .           printf( "Unrecognized character: %s\n", yytext );


    int main( int argc, char **argv )
        ++argv, --argc;  /* skip over program name */
        if ( argc > 0 )
                yyin = fopen( argv[0], "r" );
                yyin = stdin;


This is the beginnings of a simple scanner for a language like Pascal. It identifies different types of tokens and reports on what it has seen.

The details of this example will be explained in the following sections.

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5 Format of the Input File

The flex input file consists of three sections, separated by a line containing only ‘%%’.

    user code

5.1 Format of the Definitions Section

The definitions section contains declarations of simple name definitions to simplify the scanner specification, and declarations of start conditions, which are explained in a later section.

Name definitions have the form:

    name definition

The ‘name’ is a word beginning with a letter or an underscore (‘_’) followed by zero or more letters, digits, ‘_’, or ‘-’ (dash). The definition is taken to begin at the first non-whitespace character following the name and continuing to the end of the line. The definition can subsequently be referred to using ‘{name}’, which will expand to ‘(definition)’. For example,

    DIGIT    [0-9]
    ID       [a-z][a-z0-9]*

Defines ‘DIGIT’ to be a regular expression which matches a single digit, and ‘ID’ to be a regular expression which matches a letter followed by zero-or-more letters-or-digits. A subsequent reference to


is identical to


and matches one-or-more digits followed by a ‘.’ followed by zero-or-more digits.

An unindented comment (i.e., a line beginning with ‘/*’) is copied verbatim to the output up to the next ‘*/’.

Any indented text or text enclosed in ‘%{’ and ‘%}’ is also copied verbatim to the output (with the %{ and %} symbols removed). The %{ and %} symbols must appear unindented on lines by themselves.

A %top block is similar to a ‘%{’ ... ‘%}’ block, except that the code in a %top block is relocated to the top of the generated file, before any flex definitions 1. The %top block is useful when you want certain preprocessor macros to be defined or certain files to be included before the generated code. The single characters, ‘{’ and ‘}’ are used to delimit the %top block, as show in the example below:

        /* This code goes at the "top" of the generated file. */
        #include <stdint.h>
        #include <inttypes.h>

Multiple %top blocks are allowed, and their order is preserved.

5.2 Format of the Rules Section

The rules section of the flex input contains a series of rules of the form:

    pattern   action

where the pattern must be unindented and the action must begin on the same line. See Patterns, for a further description of patterns and actions.

In the rules section, any indented or %{ %} enclosed text appearing before the first rule may be used to declare variables which are local to the scanning routine and (after the declarations) code which is to be executed whenever the scanning routine is entered. Other indented or %{ %} text in the rule section is still copied to the output, but its meaning is not well-defined and it may well cause compile-time errors (this feature is present for POSIX compliance. See Incompatibilities with Lex and Posix, for other such features).

Any indented text or text enclosed in ‘%{’ and ‘%}’ is copied verbatim to the output (with the %{ and %} symbols removed). The %{ and %} symbols must appear unindented on lines by themselves.

5.3 Format of the User Code Section

The user code section is simply copied to lex.yy.c verbatim. It is used for companion routines which call or are called by the scanner. The presence of this section is optional; if it is missing, the second ‘%%’ in the input file may be skipped, too.

5.4 Comments in the Input

Flex supports C-style comments, that is, anything between ‘/*’ and ‘*/’ is considered a comment. Whenever flex encounters a comment, it copies the entire comment verbatim to the generated source code. Comments may appear just about anywhere, but with the following exceptions:

  • Comments may not appear in the Rules Section wherever flex is expecting a regular expression. This means comments may not appear at the beginning of a line, or immediately following a list of scanner states.
  • Comments may not appear on an ‘%option’ line in the Definitions Section.

If you want to follow a simple rule, then always begin a comment on a new line, with one or more whitespace characters before the initial ‘/*’). This rule will work anywhere in the input file.

All the comments in the following example are valid:

/* code block */

/* Definitions Section */

    /* Rules Section */
ruleA   /* after regex */ { /* code block */ } /* after code block */
        /* Rules Section (indented) */
ruleC   ECHO;
ruleD   ECHO;
/* code block */
/* User Code Section */

6 Patterns

The patterns in the input (see Format of the Rules Section) are written using an extended set of regular expressions. These are:


match the character ’x’


any character (byte) except newline


a character class; in this case, the pattern matches either an ’x’, a ’y’, or a ’z’


a "character class" with a range in it; matches an ’a’, a ’b’, any letter from ’j’ through ’o’, or a ’Z’


a "negated character class", i.e., any character but those in the class. In this case, any character EXCEPT an uppercase letter.


any character EXCEPT an uppercase letter or a newline


the lowercase consonants


zero or more r’s, where r is any regular expression


one or more r’s


zero or one r’s (that is, “an optional r”)


anywhere from two to five r’s


two or more r’s


exactly 4 r’s


the expansion of the ‘name’ definition (see Format of the Input File).


the literal string: ‘[xyz]"foo


if X is ‘a’, ‘b’, ‘f’, ‘n’, ‘r’, ‘t’, or ‘v’, then the ANSI-C interpretation of ‘\x’. Otherwise, a literal ‘X’ (used to escape operators such as ‘*’)


a NUL character (ASCII code 0)


the character with octal value 123


the character with hexadecimal value 2a


match an ‘r’; parentheses are used to override precedence (see below)


apply option ‘r’ and omit option ‘s’ while interpreting pattern. Options may be zero or more of the characters ‘i’, ‘s’, or ‘x’.

i’ means case-insensitive. ‘-i’ means case-sensitive.

s’ alters the meaning of the ‘.’ syntax to match any single byte whatsoever. ‘-s’ alters the meaning of ‘.’ to match any byte except ‘\n’.

x’ ignores comments and whitespace in patterns. Whitespace is ignored unless it is backslash-escaped, contained within ‘""’s, or appears inside a character class.

The following are all valid:

(?:foo)         same as  (foo)
(?i:ab7)        same as  ([aA][bB]7)
(?-i:ab)        same as  (ab)
(?s:.)          same as  [\x00-\xFF]
(?-s:.)         same as  [^\n]
(?ix-s: a . b)  same as  ([Aa][^\n][bB])
(?x:a  b)       same as  ("ab")
(?x:a\ b)       same as  ("a b")
(?x:a" "b)      same as  ("a b")
(?x:a[ ]b)      same as  ("a b")
    /* comment */
    c)          same as  (abc)
(?# comment )

omit everything within ‘()’. The first ‘)’ character encountered ends the pattern. It is not possible to for the comment to contain a ‘)’ character. The comment may span lines.


the regular expression ‘r’ followed by the regular expression ‘s’; called concatenation


either an ‘r’ or an ‘s


an ‘r’ but only if it is followed by an ‘s’. The text matched by ‘s’ is included when determining whether this rule is the longest match, but is then returned to the input before the action is executed. So the action only sees the text matched by ‘r’. This type of pattern is called trailing context. (There are some combinations of ‘r/s’ that flex cannot match correctly. See Limitations, regarding dangerous trailing context.)


an ‘r’, but only at the beginning of a line (i.e., when just starting to scan, or right after a newline has been scanned).


an ‘r’, but only at the end of a line (i.e., just before a newline). Equivalent to ‘r/\n’.

Note that flex’s notion of “newline” is exactly whatever the C compiler used to compile flex interprets ‘\n’ as; in particular, on some DOS systems you must either filter out ‘\r’s in the input yourself, or explicitly use ‘r/\r\n’ for ‘r$’.


an ‘r’, but only in start condition s (see Start Conditions for discussion of start conditions).


same, but in any of start conditions s1, s2, or s3.


an ‘r’ in any start condition, even an exclusive one.


an end-of-file.


an end-of-file when in start condition s1 or s2

Note that inside of a character class, all regular expression operators lose their special meaning except escape (‘\’) and the character class operators, ‘-’, ‘]]’, and, at the beginning of the class, ‘^’.

The regular expressions listed above are grouped according to precedence, from highest precedence at the top to lowest at the bottom. Those grouped together have equal precedence (see special note on the precedence of the repeat operator, ‘{}’, under the documentation for the ‘--posix’ POSIX compliance option). For example,


is the same as


since the ‘*’ operator has higher precedence than concatenation, and concatenation higher than alternation (‘|’). This pattern therefore matches either the string ‘fooor the string ‘ba’ followed by zero-or-more ‘r’’s. To match ‘foo’ or zero-or-more repetitions of the string ‘bar’, use:


And to match a sequence of zero or more repetitions of ‘foo’ and ‘bar’:


In addition to characters and ranges of characters, character classes can also contain character class expressions. These are expressions enclosed inside ‘[:’ and ‘:]’ delimiters (which themselves must appear between the ‘[’ and ‘]’ of the character class. Other elements may occur inside the character class, too). The valid expressions are:

    [:alnum:] [:alpha:] [:blank:]
    [:cntrl:] [:digit:] [:graph:]
    [:lower:] [:print:] [:punct:]
    [:space:] [:upper:] [:xdigit:]

These expressions all designate a set of characters equivalent to the corresponding standard C isXXX function. For example, ‘[:alnum:]’ designates those characters for which isalnum() returns true - i.e., any alphabetic or numeric character. Some systems don’t provide isblank(), so flex defines ‘[:blank:]’ as a blank or a tab.

For example, the following character classes are all equivalent:


A word of caution. Character classes are expanded immediately when seen in the flex input. This means the character classes are sensitive to the locale in which flex is executed, and the resulting scanner will not be sensitive to the runtime locale. This may or may not be desirable.

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7 How the Input Is Matched

When the generated scanner is run, it analyzes its input looking for strings which match any of its patterns. If it finds more than one match, it takes the one matching the most text (for trailing context rules, this includes the length of the trailing part, even though it will then be returned to the input). If it finds two or more matches of the same length, the rule listed first in the flex input file is chosen.

Once the match is determined, the text corresponding to the match (called the token) is made available in the global character pointer yytext, and its length in the global integer yyleng. The action corresponding to the matched pattern is then executed (see Actions), and then the remaining input is scanned for another match.

If no match is found, then the default rule is executed: the next character in the input is considered matched and copied to the standard output. Thus, the simplest valid flex input is:


which generates a scanner that simply copies its input (one character at a time) to its output.

Note that yytext can be defined in two different ways: either as a character pointer or as a character array. You can control which definition flex uses by including one of the special directives %pointer or %array in the first (definitions) section of your flex input. The default is %pointer, unless you use the ‘-l’ lex compatibility option, in which case yytext will be an array. The advantage of using %pointer is substantially faster scanning and no buffer overflow when matching very large tokens (unless you run out of dynamic memory). The disadvantage is that you are restricted in how your actions can modify yytext (see Actions), and calls to the unput() function destroys the present contents of yytext, which can be a considerable porting headache when moving between different lex versions.

The advantage of %array is that you can then modify yytext to your heart’s content, and calls to unput() do not destroy yytext (see Actions). Furthermore, existing lex programs sometimes access yytext externally using declarations of the form:

    extern char yytext[];

This definition is erroneous when used with %pointer, but correct for %array.

The %array declaration defines yytext to be an array of YYLMAX characters, which defaults to a fairly large value. You can change the size by simply #define’ing YYLMAX to a different value in the first section of your flex input. As mentioned above, with %pointer yytext grows dynamically to accommodate large tokens. While this means your %pointer scanner can accommodate very large tokens (such as matching entire blocks of comments), bear in mind that each time the scanner must resize yytext it also must rescan the entire token from the beginning, so matching such tokens can prove slow. yytext presently does not dynamically grow if a call to unput() results in too much text being pushed back; instead, a run-time error results.

Also note that you cannot use %array with C++ scanner classes (see Generating C++ Scanners).

8 Actions

Each pattern in a rule has a corresponding action, which can be any arbitrary C statement. The pattern ends at the first non-escaped whitespace character; the remainder of the line is its action. If the action is empty, then when the pattern is matched the input token is simply discarded. For example, here is the specification for a program which deletes all occurrences of ‘zap me’ from its input:

    "zap me"

This example will copy all other characters in the input to the output since they will be matched by the default rule.

Here is a program which compresses multiple blanks and tabs down to a single blank, and throws away whitespace found at the end of a line:

    [ \t]+        putchar( ' ' );
    [ \t]+$       /* ignore this token */

If the action contains a ‘{’, then the action spans till the balancing ‘}’ is found, and the action may cross multiple lines. flex knows about C strings and comments and won’t be fooled by braces found within them, but also allows actions to begin with ‘%{’ and will consider the action to be all the text up to the next ‘%}’ (regardless of ordinary braces inside the action).

An action consisting solely of a vertical bar (‘|’) means “same as the action for the next rule”. See below for an illustration.

Actions can include arbitrary C code, including return statements to return a value to whatever routine called yylex(). Each time yylex() is called it continues processing tokens from where it last left off until it either reaches the end of the file or executes a return.

Actions are free to modify yytext except for lengthening it (adding characters to its end–these will overwrite later characters in the input stream). This however does not apply when using %array (see How the Input Is Matched). In that case, yytext may be freely modified in any way.

Actions are free to modify yyleng except they should not do so if the action also includes use of yymore() (see below).

There are a number of special directives which can be included within an action:


copies yytext to the scanner’s output.


followed by the name of a start condition places the scanner in the corresponding start condition (see below).


directs the scanner to proceed on to the “second best” rule which matched the input (or a prefix of the input). The rule is chosen as described above in How the Input Is Matched, and yytext and yyleng set up appropriately. It may either be one which matched as much text as the originally chosen rule but came later in the flex input file, or one which matched less text. For example, the following will both count the words in the input and call the routine special() whenever ‘frob’ is seen:

            int word_count = 0;

    frob        special(); REJECT;
    [^ \t\n]+   ++word_count;

Without the REJECT, any occurrences of ‘frob’ in the input would not be counted as words, since the scanner normally executes only one action per token. Multiple uses of REJECT are allowed, each one finding the next best choice to the currently active rule. For example, when the following scanner scans the token ‘abcd’, it will write ‘abcdabcaba’ to the output:

    a        |
    ab       |
    abc      |
    abcd     ECHO; REJECT;
    .|\n     /* eat up any unmatched character */

The first three rules share the fourth’s action since they use the special ‘|’ action.

REJECT is a particularly expensive feature in terms of scanner performance; if it is used in any of the scanner’s actions it will slow down all of the scanner’s matching. Furthermore, REJECT cannot be used with the ‘-Cf’ or ‘-CF’ options (see Scanner Options).

Note also that unlike the other special actions, REJECT is a branch. Code immediately following it in the action will not be executed.


tells the scanner that the next time it matches a rule, the corresponding token should be appended onto the current value of yytext rather than replacing it. For example, given the input ‘mega-kludge’ the following will write ‘mega-mega-kludge’ to the output:

    mega-    ECHO; yymore();
    kludge   ECHO;

First ‘mega-’ is matched and echoed to the output. Then ‘kludge’ is matched, but the previous ‘mega-’ is still hanging around at the beginning of yytext so the ECHO for the ‘kludge’ rule will actually write ‘mega-kludge’.

Two notes regarding use of yymore(). First, yymore() depends on the value of yyleng correctly reflecting the size of the current token, so you must not modify yyleng if you are using yymore(). Second, the presence of yymore() in the scanner’s action entails a minor performance penalty in the scanner’s matching speed.

yyless(n) returns all but the first n characters of the current token back to the input stream, where they will be rescanned when the scanner looks for the next match. yytext and yyleng are adjusted appropriately (e.g., yyleng will now be equal to n). For example, on the input ‘foobar’ the following will write out ‘foobarbar’:

    foobar    ECHO; yyless(3);
    [a-z]+    ECHO;

An argument of 0 to yyless() will cause the entire current input string to be scanned again. Unless you’ve changed how the scanner will subsequently process its input (using BEGIN, for example), this will result in an endless loop.

Note that yyless() is a macro and can only be used in the flex input file, not from other source files.

unput(c) puts the character c back onto the input stream. It will be the next character scanned. The following action will take the current token and cause it to be rescanned enclosed in parentheses.

    int i;
    /* Copy yytext because unput() trashes yytext */
    char *yycopy = strdup( yytext );
    unput( ')' );
    for ( i = yyleng - 1; i >= 0; --i )
        unput( yycopy[i] );
    unput( '(' );
    free( yycopy );

Note that since each unput() puts the given character back at the beginning of the input stream, pushing back strings must be done back-to-front.

An important potential problem when using unput() is that if you are using %pointer (the default), a call to unput() destroys the contents of yytext, starting with its rightmost character and devouring one character to the left with each call. If you need the value of yytext preserved after a call to unput() (as in the above example), you must either first copy it elsewhere, or build your scanner using %array instead (see How the Input Is Matched).

Finally, note that you cannot put back ‘EOF’ to attempt to mark the input stream with an end-of-file.

input() reads the next character from the input stream. For example, the following is one way to eat up C comments:

    "/*"        {
                int c;

                for ( ; ; )
                    while ( (c = input()) != '*' &&
                            c != EOF )
                        ;    /* eat up text of comment */

                    if ( c == '*' )
                        while ( (c = input()) == '*' )
                        if ( c == '/' )
                            break;    /* found the end */

                    if ( c == EOF )
                        error( "EOF in comment" );

(Note that if the scanner is compiled using C++, then input() is instead referred to as yyinput(), in order to avoid a name clash with the C++ stream by the name of input.)

YY_FLUSH_BUFFER; flushes the scanner’s internal buffer so that the next time the scanner attempts to match a token, it will first refill the buffer using YY_INPUT() (see The Generated Scanner). This action is a special case of the more general yy_flush_buffer; function, described below (see Multiple Input Buffers)

yyterminate() can be used in lieu of a return statement in an action. It terminates the scanner and returns a 0 to the scanner’s caller, indicating “all done”. By default, yyterminate() is also called when an end-of-file is encountered. It is a macro and may be redefined.

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9 The Generated Scanner

The output of flex is the file lex.yy.c, which contains the scanning routine yylex(), a number of tables used by it for matching tokens, and a number of auxiliary routines and macros. By default, yylex() is declared as follows:

    int yylex()
        ... various definitions and the actions in here ...

(If your environment supports function prototypes, then it will be int yylex( void ).) This definition may be changed by defining the YY_DECL macro. For example, you could use:

    #define YY_DECL float lexscan( a, b ) float a, b;

to give the scanning routine the name lexscan, returning a float, and taking two floats as arguments. Note that if you give arguments to the scanning routine using a K&R-style/non-prototyped function declaration, you must terminate the definition with a semi-colon (;).

flex generates ‘C99’ function definitions by default. Flex used to have the ability to generate obsolete, er, ‘traditional’, function definitions. This was to support bootstrapping gcc on old systems. Unfortunately, traditional definitions prevent us from using any standard data types smaller than int (such as short, char, or bool) as function arguments. Furthermore, traditional definitions support added extra complexity in the skeleton file. For this reason, current versions of flex generate standard C99 code only, leaving K&R-style functions to the historians.

Whenever yylex() is called, it scans tokens from the global input file yyin (which defaults to stdin). It continues until it either reaches an end-of-file (at which point it returns the value 0) or one of its actions executes a return statement.

If the scanner reaches an end-of-file, subsequent calls are undefined unless either yyin is pointed at a new input file (in which case scanning continues from that file), or yyrestart() is called. yyrestart() takes one argument, a FILE * pointer (which can be NULL, if you’ve set up YY_INPUT to scan from a source other than yyin), and initializes yyin for scanning from that file. Essentially there is no difference between just assigning yyin to a new input file or using yyrestart() to do so; the latter is available for compatibility with previous versions of flex, and because it can be used to switch input files in the middle of scanning. It can also be used to throw away the current input buffer, by calling it with an argument of yyin; but it would be better to use YY_FLUSH_BUFFER (see Actions). Note that yyrestart() does not reset the start condition to INITIAL (see Start Conditions).

If yylex() stops scanning due to executing a return statement in one of the actions, the scanner may then be called again and it will resume scanning where it left off.

By default (and for purposes of efficiency), the scanner uses block-reads rather than simple getc() calls to read characters from yyin. The nature of how it gets its input can be controlled by defining the YY_INPUT macro. The calling sequence for YY_INPUT() is YY_INPUT(buf,result,max_size). Its action is to place up to max_size characters in the character array buf and return in the integer variable result either the number of characters read or the constant YY_NULL (0 on Unix systems) to indicate ‘EOF’. The default YY_INPUT reads from the global file-pointer yyin.

Here is a sample definition of YY_INPUT (in the definitions section of the input file):

    #define YY_INPUT(buf,result,max_size) \
        { \
        int c = getchar(); \
        result = (c == EOF) ? YY_NULL : (buf[0] = c, 1); \

This definition will change the input processing to occur one character at a time.

When the scanner receives an end-of-file indication from YY_INPUT, it then checks the yywrap() function. If yywrap() returns false (zero), then it is assumed that the function has gone ahead and set up yyin to point to another input file, and scanning continues. If it returns true (non-zero), then the scanner terminates, returning 0 to its caller. Note that in either case, the start condition remains unchanged; it does not revert to INITIAL.

If you do not supply your own version of yywrap(), then you must either use %option noyywrap (in which case the scanner behaves as though yywrap() returned 1), or you must link with ‘-lfl’ to obtain the default version of the routine, which always returns 1.

For scanning from in-memory buffers (e.g., scanning strings), see Scanning Strings. See Multiple Input Buffers.

The scanner writes its ECHO output to the yyout global (default, stdout), which may be redefined by the user simply by assigning it to some other FILE pointer.

10 Start Conditions

flex provides a mechanism for conditionally activating rules. Any rule whose pattern is prefixed with ‘<sc>’ will only be active when the scanner is in the start condition named sc. For example,

    <STRING>[^"]*        { /* eat up the string body ... */

will be active only when the scanner is in the STRING start condition, and

    <INITIAL,STRING,QUOTE>\.        { /* handle an escape ... */

will be active only when the current start condition is either INITIAL, STRING, or QUOTE.

Start conditions are declared in the definitions (first) section of the input using unindented lines beginning with either ‘%s’ or ‘%x’ followed by a list of names. The former declares inclusive start conditions, the latter exclusive start conditions. A start condition is activated using the BEGIN action. Until the next BEGIN action is executed, rules with the given start condition will be active and rules with other start conditions will be inactive. If the start condition is inclusive, then rules with no start conditions at all will also be active. If it is exclusive, then only rules qualified with the start condition will be active. A set of rules contingent on the same exclusive start condition describe a scanner which is independent of any of the other rules in the flex input. Because of this, exclusive start conditions make it easy to specify “mini-scanners” which scan portions of the input that are syntactically different from the rest (e.g., comments).

If the distinction between inclusive and exclusive start conditions is still a little vague, here’s a simple example illustrating the connection between the two. The set of rules:

    %s example

    <example>foo   do_something();

    bar            something_else();

is equivalent to

    %x example

    <example>foo   do_something();

    <INITIAL,example>bar    something_else();

Without the <INITIAL,example> qualifier, the bar pattern in the second example wouldn’t be active (i.e., couldn’t match) when in start condition example. If we just used <example> to qualify bar, though, then it would only be active in example and not in INITIAL, while in the first example it’s active in both, because in the first example the example start condition is an inclusive (%s) start condition.

Also note that the special start-condition specifier <*> matches every start condition. Thus, the above example could also have been written:

    %x example

    <example>foo   do_something();

    <*>bar    something_else();

The default rule (to ECHO any unmatched character) remains active in start conditions. It is equivalent to:

    <*>.|\n     ECHO;

BEGIN(0) returns to the original state where only the rules with no start conditions are active. This state can also be referred to as the start-condition INITIAL, so BEGIN(INITIAL) is equivalent to BEGIN(0). (The parentheses around the start condition name are not required but are considered good style.)

BEGIN actions can also be given as indented code at the beginning of the rules section. For example, the following will cause the scanner to enter the SPECIAL start condition whenever yylex() is called and the global variable enter_special is true:

            int enter_special;

    %x SPECIAL
            if ( enter_special )

    ...more rules follow...

To illustrate the uses of start conditions, here is a scanner which provides two different interpretations of a string like ‘123.456’. By default it will treat it as three tokens, the integer ‘123’, a dot (‘.’), and the integer ‘456’. But if the string is preceded earlier in the line by the string ‘expect-floats’ it will treat it as a single token, the floating-point number ‘123.456’:

    #include <math.h>
    %s expect

    expect-floats        BEGIN(expect);

    <expect>[0-9]+.[0-9]+      {
                printf( "found a float, = %f\n",
                        atof( yytext ) );
    <expect>\n           {
                /* that's the end of the line, so
                 * we need another "expect-number"
                 * before we'll recognize any more
                 * numbers

    [0-9]+      {
                printf( "found an integer, = %d\n",
                        atoi( yytext ) );

    "."         printf( "found a dot\n" );

Here is a scanner which recognizes (and discards) C comments while maintaining a count of the current input line.

    %x comment
            int line_num = 1;

    "/*"         BEGIN(comment);

    <comment>[^*\n]*        /* eat anything that's not a '*' */
    <comment>"*"+[^*/\n]*   /* eat up '*'s not followed by '/'s */
    <comment>\n             ++line_num;
    <comment>"*"+"/"        BEGIN(INITIAL);

This scanner goes to a bit of trouble to match as much text as possible with each rule. In general, when attempting to write a high-speed scanner try to match as much possible in each rule, as it’s a big win.

Note that start-conditions names are really integer values and can be stored as such. Thus, the above could be extended in the following fashion:

    %x comment foo
            int line_num = 1;
            int comment_caller;

    "/*"         {
                 comment_caller = INITIAL;


    <foo>"/*"    {
                 comment_caller = foo;

    <comment>[^*\n]*        /* eat anything that's not a '*' */
    <comment>"*"+[^*/\n]*   /* eat up '*'s not followed by '/'s */
    <comment>\n             ++line_num;
    <comment>"*"+"/"        BEGIN(comment_caller);

Furthermore, you can access the current start condition using the integer-valued YY_START macro. For example, the above assignments to comment_caller could instead be written

    comment_caller = YY_START;

Flex provides YYSTATE as an alias for YY_START (since that is what’s used by AT&T lex).

For historical reasons, start conditions do not have their own name-space within the generated scanner. The start condition names are unmodified in the generated scanner and generated header. See option-header. See option-prefix.

Finally, here’s an example of how to match C-style quoted strings using exclusive start conditions, including expanded escape sequences (but not including checking for a string that’s too long):

    %x str

            char string_buf[MAX_STR_CONST];
            char *string_buf_ptr;

    \"      string_buf_ptr = string_buf; BEGIN(str);

    <str>\"        { /* saw closing quote - all done */
            *string_buf_ptr = '\0';
            /* return string constant token type and
             * value to parser

    <str>\n        {
            /* error - unterminated string constant */
            /* generate error message */

    <str>\\[0-7]{1,3} {
            /* octal escape sequence */
            int result;

            (void) sscanf( yytext + 1, "%o", &result );

            if ( result > 0xff )
                    /* error, constant is out-of-bounds */

            *string_buf_ptr++ = result;

    <str>\\[0-9]+ {
            /* generate error - bad escape sequence; something
             * like '\48' or '\0777777'

    <str>\\n  *string_buf_ptr++ = '\n';
    <str>\\t  *string_buf_ptr++ = '\t';
    <str>\\r  *string_buf_ptr++ = '\r';
    <str>\\b  *string_buf_ptr++ = '\b';
    <str>\\f  *string_buf_ptr++ = '\f';

    <str>\\(.|\n)  *string_buf_ptr++ = yytext[1];

    <str>[^\\\n\"]+        {
            char *yptr = yytext;

            while ( *yptr )
                    *string_buf_ptr++ = *yptr++;

Often, such as in some of the examples above, you wind up writing a whole bunch of rules all preceded by the same start condition(s). Flex makes this a little easier and cleaner by introducing a notion of start condition scope. A start condition scope is begun with:


where <SCs> is a list of one or more start conditions. Inside the start condition scope, every rule automatically has the prefix <SCs> applied to it, until a ‘}’ which matches the initial ‘{’. So, for example,

        "\\n"   return '\n';
        "\\r"   return '\r';
        "\\f"   return '\f';
        "\\0"   return '\0';

is equivalent to:

    <ESC>"\\n"  return '\n';
    <ESC>"\\r"  return '\r';
    <ESC>"\\f"  return '\f';
    <ESC>"\\0"  return '\0';

Start condition scopes may be nested.

The following routines are available for manipulating stacks of start conditions:

Function: void yy_push_state ( int new_state )

pushes the current start condition onto the top of the start condition stack and switches to new_state as though you had used BEGIN new_state (recall that start condition names are also integers).

Function: void yy_pop_state ()

pops the top of the stack and switches to it via BEGIN.

Function: int yy_top_state ()

returns the top of the stack without altering the stack’s contents.

The start condition stack grows dynamically and so has no built-in size limitation. If memory is exhausted, program execution aborts.

To use start condition stacks, your scanner must include a %option stack directive (see Scanner Options).

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11 Multiple Input Buffers

Some scanners (such as those which support “include” files) require reading from several input streams. As flex scanners do a large amount of buffering, one cannot control where the next input will be read from by simply writing a YY_INPUT() which is sensitive to the scanning context. YY_INPUT() is only called when the scanner reaches the end of its buffer, which may be a long time after scanning a statement such as an include statement which requires switching the input source.

To negotiate these sorts of problems, flex provides a mechanism for creating and switching between multiple input buffers. An input buffer is created by using:

Function: YY_BUFFER_STATE yy_create_buffer ( FILE *file, int size )

which takes a FILE pointer and a size and creates a buffer associated with the given file and large enough to hold size characters (when in doubt, use YY_BUF_SIZE for the size). It returns a YY_BUFFER_STATE handle, which may then be passed to other routines (see below). The YY_BUFFER_STATE type is a pointer to an opaque struct yy_buffer_state structure, so you may safely initialize YY_BUFFER_STATE variables to ((YY_BUFFER_STATE) 0) if you wish, and also refer to the opaque structure in order to correctly declare input buffers in source files other than that of your scanner. Note that the FILE pointer in the call to yy_create_buffer is only used as the value of yyin seen by YY_INPUT. If you redefine YY_INPUT() so it no longer uses yyin, then you can safely pass a NULL FILE pointer to yy_create_buffer. You select a particular buffer to scan from using:

Function: void yy_switch_to_buffer ( YY_BUFFER_STATE new_buffer )

The above function switches the scanner’s input buffer so subsequent tokens will come from new_buffer. Note that yy_switch_to_buffer() may be used by yywrap() to set things up for continued scanning, instead of opening a new file and pointing yyin at it. If you are looking for a stack of input buffers, then you want to use yypush_buffer_state() instead of this function. Note also that switching input sources via either yy_switch_to_buffer() or yywrap() does not change the start condition.

Function: void yy_delete_buffer ( YY_BUFFER_STATE buffer )

is used to reclaim the storage associated with a buffer. (buffer can be NULL, in which case the routine does nothing.) You can also clear the current contents of a buffer using:

Function: void yypush_buffer_state ( YY_BUFFER_STATE buffer )

This function pushes the new buffer state onto an internal stack. The pushed state becomes the new current state. The stack is maintained by flex and will grow as required. This function is intended to be used instead of yy_switch_to_buffer, when you want to change states, but preserve the current state for later use.

Function: void yypop_buffer_state ( )

This function removes the current state from the top of the stack, and deletes it by calling yy_delete_buffer. The next state on the stack, if any, becomes the new current state.

Function: void yy_flush_buffer ( YY_BUFFER_STATE buffer )

This function discards the buffer’s contents, so the next time the scanner attempts to match a token from the buffer, it will first fill the buffer anew using YY_INPUT().

Function: YY_BUFFER_STATE yy_new_buffer ( FILE *file, int size )

is an alias for yy_create_buffer(), provided for compatibility with the C++ use of new and delete for creating and destroying dynamic objects.

YY_CURRENT_BUFFER macro returns a YY_BUFFER_STATE handle to the current buffer. It should not be used as an lvalue.

Here are two examples of using these features for writing a scanner which expands include files (the <<EOF>> feature is discussed below).

This first example uses yypush_buffer_state and yypop_buffer_state. Flex maintains the stack internally.

    /* the "incl" state is used for picking up the name
     * of an include file
    %x incl
    include             BEGIN(incl);

    [a-z]+              ECHO;
    [^a-z\n]*\n?        ECHO;

    <incl>[ \t]*      /* eat the whitespace */
    <incl>[^ \t\n]+   { /* got the include file name */
            yyin = fopen( yytext, "r" );

            if ( ! yyin )
                error( ... );

			yypush_buffer_state(yy_create_buffer( yyin, YY_BUF_SIZE ));


    <<EOF>> {

            if ( !YY_CURRENT_BUFFER )

The second example, below, does the same thing as the previous example did, but manages its own input buffer stack manually (instead of letting flex do it).

    /* the "incl" state is used for picking up the name
     * of an include file
    %x incl

    #define MAX_INCLUDE_DEPTH 10
    int include_stack_ptr = 0;

    include             BEGIN(incl);

    [a-z]+              ECHO;
    [^a-z\n]*\n?        ECHO;

    <incl>[ \t]*      /* eat the whitespace */
    <incl>[^ \t\n]+   { /* got the include file name */
            if ( include_stack_ptr >= MAX_INCLUDE_DEPTH )
                fprintf( stderr, "Includes nested too deeply" );
                exit( 1 );

            include_stack[include_stack_ptr++] =

            yyin = fopen( yytext, "r" );

            if ( ! yyin )
                error( ... );

                yy_create_buffer( yyin, YY_BUF_SIZE ) );


    <<EOF>> {
            if ( --include_stack_ptr == 0 )

                yy_delete_buffer( YY_CURRENT_BUFFER );
                     include_stack[include_stack_ptr] );

The following routines are available for setting up input buffers for scanning in-memory strings instead of files. All of them create a new input buffer for scanning the string, and return a corresponding YY_BUFFER_STATE handle (which you should delete with yy_delete_buffer() when done with it). They also switch to the new buffer using yy_switch_to_buffer(), so the next call to yylex() will start scanning the string.

Function: YY_BUFFER_STATE yy_scan_string ( const char *str )

scans a NUL-terminated string.

Function: YY_BUFFER_STATE yy_scan_bytes ( const char *bytes, int len )

scans len bytes (including possibly NULs) starting at location bytes.

Note that both of these functions create and scan a copy of the string or bytes. (This may be desirable, since yylex() modifies the contents of the buffer it is scanning.) You can avoid the copy by using:

Function: YY_BUFFER_STATE yy_scan_buffer (char *base, yy_size_t size)

which scans in place the buffer starting at base, consisting of size bytes, the last two bytes of which must be YY_END_OF_BUFFER_CHAR (ASCII NUL). These last two bytes are not scanned; thus, scanning consists of base[0] through base[size-2], inclusive.

If you fail to set up base in this manner (i.e., forget the final two YY_END_OF_BUFFER_CHAR bytes), then yy_scan_buffer() returns a NULL pointer instead of creating a new input buffer.

Data type: yy_size_t

is an integral type to which you can cast an integer expression reflecting the size of the buffer.

12 End-of-File Rules

The special rule <<EOF>> indicates actions which are to be taken when an end-of-file is encountered and yywrap() returns non-zero (i.e., indicates no further files to process). The action must finish by doing one of the following things:

<<EOF>> rules may not be used with other patterns; they may only be qualified with a list of start conditions. If an unqualified <<EOF>> rule is given, it applies to all start conditions which do not already have <<EOF>> actions. To specify an <<EOF>> rule for only the initial start condition, use:


These rules are useful for catching things like unclosed comments. An example:

    %x quote

    ...other rules for dealing with quotes...

    <quote><<EOF>>   {
             error( "unterminated quote" );
   <<EOF>>  {
             if ( *++filelist )
                 yyin = fopen( *filelist, "r" );

13 Miscellaneous Macros

The macro YY_USER_ACTION can be defined to provide an action which is always executed prior to the matched rule’s action. For example, it could be #define’d to call a routine to convert yytext to lower-case. When YY_USER_ACTION is invoked, the variable yy_act gives the number of the matched rule (rules are numbered starting with 1). Suppose you want to profile how often each of your rules is matched. The following would do the trick:

    #define YY_USER_ACTION ++ctr[yy_act]

where ctr is an array to hold the counts for the different rules. Note that the macro YY_NUM_RULES gives the total number of rules (including the default rule), even if you use ‘-s)’, so a correct declaration for ctr is:

    int ctr[YY_NUM_RULES];

The macro YY_USER_INIT may be defined to provide an action which is always executed before the first scan (and before the scanner’s internal initializations are done). For example, it could be used to call a routine to read in a data table or open a logging file.

The macro yy_set_interactive(is_interactive) can be used to control whether the current buffer is considered interactive. An interactive buffer is processed more slowly, but must be used when the scanner’s input source is indeed interactive to avoid problems due to waiting to fill buffers (see the discussion of the ‘-I’ flag in Scanner Options). A non-zero value in the macro invocation marks the buffer as interactive, a zero value as non-interactive. Note that use of this macro overrides %option always-interactive or %option never-interactive (see Scanner Options). yy_set_interactive() must be invoked prior to beginning to scan the buffer that is (or is not) to be considered interactive.

The macro yy_set_bol(at_bol) can be used to control whether the current buffer’s scanning context for the next token match is done as though at the beginning of a line. A non-zero macro argument makes rules anchored with ‘^’ active, while a zero argument makes ‘^’ rules inactive.

The macro YY_AT_BOL() returns true if the next token scanned from the current buffer will have ‘^’ rules active, false otherwise.

In the generated scanner, the actions are all gathered in one large switch statement and separated using YY_BREAK, which may be redefined. By default, it is simply a break, to separate each rule’s action from the following rule’s. Redefining YY_BREAK allows, for example, C++ users to #define YY_BREAK to do nothing (while being very careful that every rule ends with a break or a return!) to avoid suffering from unreachable statement warnings where because a rule’s action ends with return, the YY_BREAK is inaccessible.

14 Values Available To the User

This chapter summarizes the various values available to the user in the rule actions.

char *yytext

holds the text of the current token. It may be modified but not lengthened (you cannot append characters to the end).

If the special directive %array appears in the first section of the scanner description, then yytext is instead declared char yytext[YYLMAX], where YYLMAX is a macro definition that you can redefine in the first section if you don’t like the default value (generally 8KB). Using %array results in somewhat slower scanners, but the value of yytext becomes immune to calls to unput(), which potentially destroy its value when yytext is a character pointer. The opposite of %array is %pointer, which is the default.

You cannot use %array when generating C++ scanner classes (the ‘-+’ flag).

int yyleng

holds the length of the current token.

FILE *yyin

is the file which by default flex reads from. It may be redefined but doing so only makes sense before scanning begins or after an EOF has been encountered. Changing it in the midst of scanning will have unexpected results since flex buffers its input; use yyrestart() instead. Once scanning terminates because an end-of-file has been seen, you can assign yyin at the new input file and then call the scanner again to continue scanning.

void yyrestart( FILE *new_file )

may be called to point yyin at the new input file. The switch-over to the new file is immediate (any previously buffered-up input is lost). Note that calling yyrestart() with yyin as an argument thus throws away the current input buffer and continues scanning the same input file.

FILE *yyout

is the file to which ECHO actions are done. It can be reassigned by the user.


returns a YY_BUFFER_STATE handle to the current buffer.


returns an integer value corresponding to the current start condition. You can subsequently use this value with BEGIN to return to that start condition.

15 Interfacing with Yacc

One of the main uses of flex is as a companion to the yacc parser-generator. yacc parsers expect to call a routine named yylex() to find the next input token. The routine is supposed to return the type of the next token as well as putting any associated value in the global yylval. To use flex with yacc, one specifies the ‘-d’ option to yacc to instruct it to generate the file y.tab.h containing definitions of all the %tokens appearing in the yacc input. This file is then included in the flex scanner. For example, if one of the tokens is TOK_NUMBER, part of the scanner might look like:

    #include "y.tab.h"


    [0-9]+        yylval = atoi( yytext ); return TOK_NUMBER;

16 Scanner Options

The various flex options are categorized by function in the following menu. If you want to lookup a particular option by name, See Index of Scanner Options.

Even though there are many scanner options, a typical scanner might only specify the following options:

%option   8bit reentrant bison-bridge
%option   warn nodefault
%option   yylineno
%option   outfile="scanner.c" header-file="scanner.h"

The first line specifies the general type of scanner we want. The second line specifies that we are being careful. The third line asks flex to track line numbers. The last line tells flex what to name the files. (The options can be specified in any order. We just divided them.)

flex also provides a mechanism for controlling options within the scanner specification itself, rather than from the flex command-line. This is done by including %option directives in the first section of the scanner specification. You can specify multiple options with a single %option directive, and multiple directives in the first section of your flex input file.

Most options are given simply as names, optionally preceded by the word ‘no’ (with no intervening whitespace) to negate their meaning. The names are the same as their long-option equivalents (but without the leading ‘--’ ).

flex scans your rule actions to determine whether you use the REJECT or yymore() features. The REJECT and yymore options are available to override its decision as to whether you use the options, either by setting them (e.g., %option reject) to indicate the feature is indeed used, or unsetting them to indicate it actually is not used (e.g., %option noyymore).

A number of options are available for lint purists who want to suppress the appearance of unneeded routines in the generated scanner. Each of the following, if unset (e.g., %option nounput), results in the corresponding routine not appearing in the generated scanner:

    input, unput
    yy_push_state, yy_pop_state, yy_top_state
    yy_scan_buffer, yy_scan_bytes, yy_scan_string

    yyget_extra, yyset_extra, yyget_leng, yyget_text,
    yyget_lineno, yyset_lineno, yyget_in, yyset_in,
    yyget_out, yyset_out, yyget_lval, yyset_lval,
    yyget_lloc, yyset_lloc, yyget_debug, yyset_debug

(though yy_push_state() and friends won’t appear anyway unless you use %option stack).

16.1 Options for Specifying Filenames

--header-file=FILE, %option header-file="FILE"

instructs flex to write a C header to FILE. This file contains function prototypes, extern variables, and types used by the scanner. Only the external API is exported by the header file. Many macros that are usable from within scanner actions are not exported to the header file. This is due to namespace problems and the goal of a clean external API.

While in the header, the macro yyIN_HEADER is defined, where ‘yy’ is substituted with the appropriate prefix.

The ‘--header-file’ option is not compatible with the ‘--c++’ option, since the C++ scanner provides its own header in yyFlexLexer.h.

-oFILE, --outfile=FILE, %option outfile="FILE"

directs flex to write the scanner to the file FILE instead of lex.yy.c. If you combine ‘--outfile’ with the ‘--stdout’ option, then the scanner is written to stdout but its #line directives (see the ‘-l’ option above) refer to the file FILE.

-t, --stdout, %option stdout

instructs flex to write the scanner it generates to standard output instead of lex.yy.c.

-SFILE, --skel=FILE

overrides the default skeleton file from which flex constructs its scanners. You’ll never need this option unless you are doing flex maintenance or development.


Write serialized scanner dfa tables to FILE. The generated scanner will not contain the tables, and requires them to be loaded at runtime. See serialization.


This option is for flex development. We document it here in case you stumble upon it by accident or in case you suspect some inconsistency in the serialized tables. Flex will serialize the scanner dfa tables but will also generate the in-code tables as it normally does. At runtime, the scanner will verify that the serialized tables match the in-code tables, instead of loading them.

16.2 Options Affecting Scanner Behavior

-i, --case-insensitive, %option case-insensitive

instructs flex to generate a case-insensitive scanner. The case of letters given in the flex input patterns will be ignored, and tokens in the input will be matched regardless of case. The matched text given in yytext will have the preserved case (i.e., it will not be folded). For tricky behavior, see case and character ranges.

-l, --lex-compat, %option lex-compat

turns on maximum compatibility with the original AT&T lex implementation. Note that this does not mean full compatibility. Use of this option costs a considerable amount of performance, and it cannot be used with the ‘--c++’, ‘--full’, ‘--fast’, ‘-Cf’, or ‘-CF’ options. For details on the compatibilities it provides, see Incompatibilities with Lex and Posix. This option also results in the name YY_FLEX_LEX_COMPAT being #define’d in the generated scanner.

-B, --batch, %option batch

instructs flex to generate a batch scanner, the opposite of interactive scanners generated by ‘--interactive’ (see below). In general, you use ‘-B’ when you are certain that your scanner will never be used interactively, and you want to squeeze a little more performance out of it. If your goal is instead to squeeze out a lot more performance, you should be using the ‘-Cf’ or ‘-CF’ options, which turn on ‘--batch’ automatically anyway.

-I, --interactive, %option interactive

instructs flex to generate an interactive scanner. An interactive scanner is one that only looks ahead to decide what token has been matched if it absolutely must. It turns out that always looking one extra character ahead, even if the scanner has already seen enough text to disambiguate the current token, is a bit faster than only looking ahead when necessary. But scanners that always look ahead give dreadful interactive performance; for example, when a user types a newline, it is not recognized as a newline token until they enter another token, which often means typing in another whole line.

flex scanners default to interactive unless you use the ‘-Cf’ or ‘-CF’ table-compression options (see Performance Considerations). That’s because if you’re looking for high-performance you should be using one of these options, so if you didn’t, flex assumes you’d rather trade off a bit of run-time performance for intuitive interactive behavior. Note also that you cannot use ‘--interactive’ in conjunction with ‘-Cf’ or ‘-CF’. Thus, this option is not really needed; it is on by default for all those cases in which it is allowed.

You can force a scanner to not be interactive by using ‘--batch

-7, --7bit, %option 7bit

instructs flex to generate a 7-bit scanner, i.e., one which can only recognize 7-bit characters in its input. The advantage of using ‘--7bit’ is that the scanner’s tables can be up to half the size of those generated using the ‘--8bit’. The disadvantage is that such scanners often hang or crash if their input contains an 8-bit character.

Note, however, that unless you generate your scanner using the ‘-Cf’ or ‘-CF’ table compression options, use of ‘--7bit’ will save only a small amount of table space, and make your scanner considerably less portable. Flex’s default behavior is to generate an 8-bit scanner unless you use the ‘-Cf’ or ‘-CF’, in which case flex defaults to generating 7-bit scanners unless your site was always configured to generate 8-bit scanners (as will often be the case with non-USA sites). You can tell whether flex generated a 7-bit or an 8-bit scanner by inspecting the flag summary in the ‘--verbose’ output as described above.

Note that if you use ‘-Cfe’ or ‘-CFeflex still defaults to generating an 8-bit scanner, since usually with these compression options full 8-bit tables are not much more expensive than 7-bit tables.

-8, --8bit, %option 8bit

instructs flex to generate an 8-bit scanner, i.e., one which can recognize 8-bit characters. This flag is only needed for scanners generated using ‘-Cf’ or ‘-CF’, as otherwise flex defaults to generating an 8-bit scanner anyway.

See the discussion of ‘--7bit’ above for flex’s default behavior and the tradeoffs between 7-bit and 8-bit scanners.

--default, %option default

generate the default rule.

--always-interactive, %option always-interactive

instructs flex to generate a scanner which always considers its input interactive. Normally, on each new input file the scanner calls isatty() in an attempt to determine whether the scanner’s input source is interactive and thus should be read a character at a time. When this option is used, however, then no such call is made.

--never-interactive, --never-interactive

instructs flex to generate a scanner which never considers its input interactive. This is the opposite of always-interactive.

-X, --posix, %option posix

turns on maximum compatibility with the POSIX 1003.2-1992 definition of lex. Since flex was originally designed to implement the POSIX definition of lex this generally involves very few changes in behavior. At the current writing the known differences between flex and the POSIX standard are:

  • In POSIX and AT&T lex, the repeat operator, ‘{}’, has lower precedence than concatenation (thus ‘ab{3}’ yields ‘ababab’). Most POSIX utilities use an Extended Regular Expression (ERE) precedence that has the precedence of the repeat operator higher than concatenation (which causes ‘ab{3}’ to yield ‘abbb’). By default, flex places the precedence of the repeat operator higher than concatenation which matches the ERE processing of other POSIX utilities. When either ‘--posix’ or ‘-l’ are specified, flex will use the traditional AT&T and POSIX-compliant precedence for the repeat operator where concatenation has higher precedence than the repeat operator.
--stack, %option stack

enables the use of start condition stacks (see Start Conditions).

--stdinit, %option stdinit

if set (i.e., %option stdinit) initializes yyin and yyout to stdin and stdout, instead of the default of NULL. Some existing lex programs depend on this behavior, even though it is not compliant with ANSI C, which does not require stdin and stdout to be compile-time constant. In a reentrant scanner, however, this is not a problem since initialization is performed in yylex_init at runtime.

--yylineno, %option yylineno

directs flex to generate a scanner that maintains the number of the current line read from its input in the global variable yylineno. This option is implied by %option lex-compat. In a reentrant C scanner, the macro yylineno is accessible regardless of the value of %option yylineno, however, its value is not modified by flex unless %option yylineno is enabled.

--yywrap, %option yywrap

if unset (i.e., --noyywrap), makes the scanner not call yywrap() upon an end-of-file, but simply assume that there are no more files to scan (until the user points yyin at a new file and calls yylex() again).

16.3 Code-Level And API Options

--ansi-definitions, %option ansi-definitions

Deprecated, ignored

--ansi-prototypes, %option ansi-prototypes

Deprecated, ignored

--bison-bridge, %option bison-bridge

instructs flex to generate a C scanner that is meant to be called by a GNU bison parser. The scanner has minor API changes for bison compatibility. In particular, the declaration of yylex is modified to take an additional parameter, yylval. See C Scanners with Bison Parsers.

--bison-locations, %option bison-locations

instruct flex that GNU bison %locations are being used. This means yylex will be passed an additional parameter, yylloc. This option implies %option bison-bridge. See C Scanners with Bison Parsers.

-L, --noline, %option noline

instructs flex not to generate #line directives. Without this option, flex peppers the generated scanner with #line directives so error messages in the actions will be correctly located with respect to either the original flex input file (if the errors are due to code in the input file), or lex.yy.c (if the errors are flex’s fault – you should report these sorts of errors to the email address given in Reporting Bugs).

-R, --reentrant, %option reentrant

instructs flex to generate a reentrant C scanner. The generated scanner may safely be used in a multi-threaded environment. The API for a reentrant scanner is different than for a non-reentrant scanner see Reentrant C Scanners). Because of the API difference between reentrant and non-reentrant flex scanners, non-reentrant flex code must be modified before it is suitable for use with this option. This option is not compatible with the ‘--c++’ option.

The option ‘--reentrant’ does not affect the performance of the scanner.

-+, --c++, %option c++

specifies that you want flex to generate a C++ scanner class. See Generating C++ Scanners, for details.

--array, %option array

specifies that you want yytext to be an array instead of a char*

--pointer, %option pointer

specify that yytext should be a char *, not an array. This default is char *.

-PPREFIX, --prefix=PREFIX, %option prefix="PREFIX"

changes the default ‘yy’ prefix used by flex for all globally-visible variable and function names to instead be ‘PREFIX’. For example, ‘--prefix=foo’ changes the name of yytext to footext. It also changes the name of the default output file from lex.yy.c to lex.foo.c. Here is a partial list of the names affected:


(If you are using a C++ scanner, then only yywrap and yyFlexLexer are affected.) Within your scanner itself, you can still refer to the global variables and functions using either version of their name; but externally, they have the modified name.

This option lets you easily link together multiple flex programs into the same executable. Note, though, that using this option also renames yywrap(), so you now must either provide your own (appropriately-named) version of the routine for your scanner, or use %option noyywrap, as linking with ‘-lfl’ no longer provides one for you by default.

--main, %option main

directs flex to provide a default main() program for the scanner, which simply calls yylex(). This option implies noyywrap (see below).

--nounistd, %option nounistd

suppresses inclusion of the non-ANSI header file unistd.h. This option is meant to target environments in which unistd.h does not exist. Be aware that certain options may cause flex to generate code that relies on functions normally found in unistd.h, (e.g. isatty(), read().) If you wish to use these functions, you will have to inform your compiler where to find them. See option-always-interactive. See option-read.

--yyclass=NAME, %option yyclass="NAME"

only applies when generating a C++ scanner (the ‘--c++’ option). It informs flex that you have derived NAME as a subclass of yyFlexLexer, so flex will place your actions in the member function foo::yylex() instead of yyFlexLexer::yylex(). It also generates a yyFlexLexer::yylex() member function that emits a run-time error (by invoking yyFlexLexer::LexerError()) if called. See Generating C++ Scanners.

16.4 Options for Scanner Speed and Size


controls the degree of table compression and, more generally, trade-offs between small scanners and fast scanners.


A lone ‘-C’ specifies that the scanner tables should be compressed but neither equivalence classes nor meta-equivalence classes should be used.

-Ca, --align, %option align

(“align”) instructs flex to trade off larger tables in the generated scanner for faster performance because the elements of the tables are better aligned for memory access and computation. On some RISC architectures, fetching and manipulating longwords is more efficient than with smaller-sized units such as shortwords. This option can quadruple the size of the tables used by your scanner.

-Ce, --ecs, %option ecs

directs flex to construct equivalence classes, i.e., sets of characters which have identical lexical properties (for example, if the only appearance of digits in the flex input is in the character class “[0-9]” then the digits ’0’, ’1’, ..., ’9’ will all be put in the same equivalence class). Equivalence classes usually give dramatic reductions in the final table/object file sizes (typically a factor of 2-5) and are pretty cheap performance-wise (one array look-up per character scanned).


specifies that the full scanner tables should be generated - flex should not compress the tables by taking advantages of similar transition functions for different states.


specifies that the alternate fast scanner representation (described above under the ‘--fast’ flag) should be used. This option cannot be used with ‘--c++’.

-Cm, --meta-ecs, %option meta-ecs

directs flex to construct meta-equivalence classes, which are sets of equivalence classes (or characters, if equivalence classes are not being used) that are commonly used together. Meta-equivalence classes are often a big win when using compressed tables, but they have a moderate performance impact (one or two if tests and one array look-up per character scanned).

-Cr, --read, %option read

causes the generated scanner to bypass use of the standard I/O library (stdio) for input. Instead of calling fread() or getc(), the scanner will use the read() system call, resulting in a performance gain which varies from system to system, but in general is probably negligible unless you are also using ‘-Cf’ or ‘-CF’. Using ‘-Cr’ can cause strange behavior if, for example, you read from yyin using stdio prior to calling the scanner (because the scanner will miss whatever text your previous reads left in the stdio input buffer). ‘-Cr’ has no effect if you define YY_INPUT() (see The Generated Scanner).

The options ‘-Cf’ or ‘-CF’ and ‘-Cm’ do not make sense together - there is no opportunity for meta-equivalence classes if the table is not being compressed. Otherwise the options may be freely mixed, and are cumulative.

The default setting is ‘-Cem’, which specifies that flex should generate equivalence classes and meta-equivalence classes. This setting provides the highest degree of table compression. You can trade off faster-executing scanners at the cost of larger tables with the following generally being true:

    slowest & smallest
    fastest & largest

Note that scanners with the smallest tables are usually generated and compiled the quickest, so during development you will usually want to use the default, maximal compression.

-Cfe’ is often a good compromise between speed and size for production scanners.

-f, --full, %option full

specifies fast scanner. No table compression is done and stdio is bypassed. The result is large but fast. This option is equivalent to ‘--Cfr

-F, --fast, %option fast

specifies that the fast scanner table representation should be used (and stdio bypassed). This representation is about as fast as the full table representation ‘--full’, and for some sets of patterns will be considerably smaller (and for others, larger). In general, if the pattern set contains both keywords and a catch-all, identifier rule, such as in the set:

    "case"    return TOK_CASE;
    "switch"  return TOK_SWITCH;
    "default" return TOK_DEFAULT;
    [a-z]+    return TOK_ID;

then you’re better off using the full table representation. If only the identifier rule is present and you then use a hash table or some such to detect the keywords, you’re better off using ‘--fast’.

This option is equivalent to ‘-CFr’. It cannot be used with ‘--c++’.

16.5 Debugging Options

-b, --backup, %option backup

Generate backing-up information to lex.backup. This is a list of scanner states which require backing up and the input characters on which they do so. By adding rules one can remove backing-up states. If all backing-up states are eliminated and ‘-Cf’ or -CF is used, the generated scanner will run faster (see the ‘--perf-report’ flag). Only users who wish to squeeze every last cycle out of their scanners need worry about this option. (see Performance Considerations).

-d, --debug, %option debug

makes the generated scanner run in debug mode. Whenever a pattern is recognized and the global variable yy_flex_debug is non-zero (which is the default), the scanner will write to stderr a line of the form:

    -accepting rule at line 53 ("the matched text")

The line number refers to the location of the rule in the file defining the scanner (i.e., the file that was fed to flex). Messages are also generated when the scanner backs up, accepts the default rule, reaches the end of its input buffer (or encounters a NUL; at this point, the two look the same as far as the scanner’s concerned), or reaches an end-of-file.

-p, --perf-report, %option perf-report

generates a performance report to stderr. The report consists of comments regarding features of the flex input file which will cause a serious loss of performance in the resulting scanner. If you give the flag twice, you will also get comments regarding features that lead to minor performance losses.

Note that the use of REJECT, and variable trailing context (see Limitations) entails a substantial performance penalty; use of yymore(), the ‘^’ operator, and the ‘--interactive’ flag entail minor performance penalties.

-s, --nodefault, %option nodefault

causes the default rule (that unmatched scanner input is echoed to stdout) to be suppressed. If the scanner encounters input that does not match any of its rules, it aborts with an error. This option is useful for finding holes in a scanner’s rule set.

-T, --trace, %option trace

makes flex run in trace mode. It will generate a lot of messages to stderr concerning the form of the input and the resultant non-deterministic and deterministic finite automata. This option is mostly for use in maintaining flex.

-w, --nowarn, %option nowarn

suppresses warning messages.

-v, --verbose, %option verbose

specifies that flex should write to stderr a summary of statistics regarding the scanner it generates. Most of the statistics are meaningless to the casual flex user, but the first line identifies the version of flex (same as reported by ‘--version’), and the next line the flags used when generating the scanner, including those that are on by default.

--warn, %option warn

warn about certain things. In particular, if the default rule can be matched but no default rule has been given, the flex will warn you. We recommend using this option always.

16.6 Miscellaneous Options


A do-nothing option included for POSIX compliance.

-h, -?, --help

generates a “help” summary of flex’s options to stdout and then exits.


Another do-nothing option included for POSIX compliance.

-V, --version

prints the version number to stdout and exits.

17 Performance Considerations

The main design goal of flex is that it generate high-performance scanners. It has been optimized for dealing well with large sets of rules. Aside from the effects on scanner speed of the table compression ‘-C’ options outlined above, there are a number of options/actions which degrade performance. These are, from most expensive to least:

    arbitrary trailing context

    pattern sets that require backing up
    %option yylineno

    %option interactive
    %option always-interactive

    ^ beginning-of-line operator

with the first two all being quite expensive and the last two being quite cheap. Note also that unput() is implemented as a routine call that potentially does quite a bit of work, while yyless() is a quite-cheap macro. So if you are just putting back some excess text you scanned, use yyless().

REJECT should be avoided at all costs when performance is important. It is a particularly expensive option.

There is one case when %option yylineno can be expensive. That is when your patterns match long tokens that could possibly contain a newline character. There is no performance penalty for rules that can not possibly match newlines, since flex does not need to check them for newlines. In general, you should avoid rules such as [^f]+, which match very long tokens, including newlines, and may possibly match your entire file! A better approach is to separate [^f]+ into two rules:

%option yylineno

The above scanner does not incur a performance penalty.

Getting rid of backing up is messy and often may be an enormous amount of work for a complicated scanner. In principal, one begins by using the ‘-b’ flag to generate a lex.backup file. For example, on the input:

    foo        return TOK_KEYWORD;
    foobar     return TOK_KEYWORD;

the file looks like:

    State #6 is non-accepting -
     associated rule line numbers:
           2       3
     out-transitions: [ o ]
     jam-transitions: EOF [ \001-n  p-\177 ]

    State #8 is non-accepting -
     associated rule line numbers:
     out-transitions: [ a ]
     jam-transitions: EOF [ \001-`  b-\177 ]

    State #9 is non-accepting -
     associated rule line numbers:
     out-transitions: [ r ]
     jam-transitions: EOF [ \001-q  s-\177 ]

    Compressed tables always back up.

The first few lines tell us that there’s a scanner state in which it can make a transition on an ’o’ but not on any other character, and that in that state the currently scanned text does not match any rule. The state occurs when trying to match the rules found at lines 2 and 3 in the input file. If the scanner is in that state and then reads something other than an ’o’, it will have to back up to find a rule which is matched. With a bit of headscratching one can see that this must be the state it’s in when it has seen ‘fo’. When this has happened, if anything other than another ‘o’ is seen, the scanner will have to back up to simply match the ‘f’ (by the default rule).

The comment regarding State #8 indicates there’s a problem when ‘foob’ has been scanned. Indeed, on any character other than an ‘a’, the scanner will have to back up to accept "foo". Similarly, the comment for State #9 concerns when ‘fooba’ has been scanned and an ‘r’ does not follow.

The final comment reminds us that there’s no point going to all the trouble of removing backing up from the rules unless we’re using ‘-Cf’ or ‘-CF’, since there’s no performance gain doing so with compressed scanners.

The way to remove the backing up is to add “error” rules:

    foo         return TOK_KEYWORD;
    foobar      return TOK_KEYWORD;

    fooba       |
    foob        |
    fo          {
                /* false alarm, not really a keyword */
                return TOK_ID;

Eliminating backing up among a list of keywords can also be done using a “catch-all” rule:

    foo         return TOK_KEYWORD;
    foobar      return TOK_KEYWORD;

    [a-z]+      return TOK_ID;

This is usually the best solution when appropriate.

Backing up messages tend to cascade. With a complicated set of rules it’s not uncommon to get hundreds of messages. If one can decipher them, though, it often only takes a dozen or so rules to eliminate the backing up (though it’s easy to make a mistake and have an error rule accidentally match a valid token. A possible future flex feature will be to automatically add rules to eliminate backing up).

It’s important to keep in mind that you gain the benefits of eliminating backing up only if you eliminate every instance of backing up. Leaving just one means you gain nothing.

Variable trailing context (where both the leading and trailing parts do not have a fixed length) entails almost the same performance loss as REJECT (i.e., substantial). So when possible a rule like:

    mouse|rat/(cat|dog)   run();

is better written:

    mouse/cat|dog         run();
    rat/cat|dog           run();

or as

    mouse|rat/cat         run();
    mouse|rat/dog         run();

Note that here the special ’|’ action does not provide any savings, and can even make things worse (see Limitations).

Another area where the user can increase a scanner’s performance (and one that’s easier to implement) arises from the fact that the longer the tokens matched, the faster the scanner will run. This is because with long tokens the processing of most input characters takes place in the (short) inner scanning loop, and does not often have to go through the additional work of setting up the scanning environment (e.g., yytext) for the action. Recall the scanner for C comments:

    %x comment
            int line_num = 1;

    "/*"         BEGIN(comment);

    <comment>\n             ++line_num;
    <comment>"*"+"/"        BEGIN(INITIAL);

This could be sped up by writing it as:

    %x comment
            int line_num = 1;

    "/*"         BEGIN(comment);

    <comment>[^*\n]*\n      ++line_num;
    <comment>"*"+[^*/\n]*\n ++line_num;
    <comment>"*"+"/"        BEGIN(INITIAL);

Now instead of each newline requiring the processing of another action, recognizing the newlines is distributed over the other rules to keep the matched text as long as possible. Note that adding rules does not slow down the scanner! The speed of the scanner is independent of the number of rules or (modulo the considerations given at the beginning of this section) how complicated the rules are with regard to operators such as ‘*’ and ‘|’.

A final example in speeding up a scanner: suppose you want to scan through a file containing identifiers and keywords, one per line and with no other extraneous characters, and recognize all the keywords. A natural first approach is:

    asm      |
    auto     |
    break    |
    ... etc ...
    volatile |
    while    /* it's a keyword */

    .|\n     /* it's not a keyword */

To eliminate the back-tracking, introduce a catch-all rule:

    asm      |
    auto     |
    break    |
    ... etc ...
    volatile |
    while    /* it's a keyword */

    [a-z]+   |
    .|\n     /* it's not a keyword */

Now, if it’s guaranteed that there’s exactly one word per line, then we can reduce the total number of matches by a half by merging in the recognition of newlines with that of the other tokens:

    asm\n    |
    auto\n   |
    break\n  |
    ... etc ...
    volatile\n |
    while\n  /* it's a keyword */

    [a-z]+\n |
    .|\n     /* it's not a keyword */

One has to be careful here, as we have now reintroduced backing up into the scanner. In particular, while we know that there will never be any characters in the input stream other than letters or newlines, flex can’t figure this out, and it will plan for possibly needing to back up when it has scanned a token like ‘auto’ and then the next character is something other than a newline or a letter. Previously it would then just match the ‘auto’ rule and be done, but now it has no ‘auto’ rule, only a ‘auto\n’ rule. To eliminate the possibility of backing up, we could either duplicate all rules but without final newlines, or, since we never expect to encounter such an input and therefore don’t how it’s classified, we can introduce one more catch-all rule, this one which doesn’t include a newline:

    asm\n    |
    auto\n   |
    break\n  |
    ... etc ...
    volatile\n |
    while\n  /* it's a keyword */

    [a-z]+\n |
    [a-z]+   |
    .|\n     /* it's not a keyword */

Compiled with ‘-Cf’, this is about as fast as one can get a flex scanner to go for this particular problem.

A final note: flex is slow when matching NULs, particularly when a token contains multiple NULs. It’s best to write rules which match short amounts of text if it’s anticipated that the text will often include NULs.

Another final note regarding performance: as mentioned in How the Input Is Matched, dynamically resizing yytext to accommodate huge tokens is a slow process because it presently requires that the (huge) token be rescanned from the beginning. Thus if performance is vital, you should attempt to match “large” quantities of text but not “huge” quantities, where the cutoff between the two is at about 8K characters per token.

18 Generating C++ Scanners

IMPORTANT: the present form of the scanning class is experimental and may change considerably between major releases.

flex provides two different ways to generate scanners for use with C++. The first way is to simply compile a scanner generated by flex using a C++ compiler instead of a C compiler. You should not encounter any compilation errors (see Reporting Bugs). You can then use C++ code in your rule actions instead of C code. Note that the default input source for your scanner remains yyin, and default echoing is still done to yyout. Both of these remain FILE * variables and not C++ streams.

You can also use flex to generate a C++ scanner class, using the ‘-+’ option (or, equivalently, %option c++), which is automatically specified if the name of the flex executable ends in a ’+’, such as flex++. When using this option, flex defaults to generating the scanner to the file lex.yy.cc instead of lex.yy.c. The generated scanner includes the header file FlexLexer.h, which defines the interface to two C++ classes.

The first class in FlexLexer.h, FlexLexer, provides an abstract base class defining the general scanner class interface. It provides the following member functions:

const char* YYText()

returns the text of the most recently matched token, the equivalent of yytext.

int YYLeng()

returns the length of the most recently matched token, the equivalent of yyleng.

int lineno() const

returns the current input line number (see %option yylineno), or 1 if %option yylineno was not used.

void set_debug( int flag )

sets the debugging flag for the scanner, equivalent to assigning to yy_flex_debug (see Scanner Options). Note that you must build the scanner using %option debug to include debugging information in it.

int debug() const

returns the current setting of the debugging flag.

Also provided are member functions equivalent to yy_switch_to_buffer(), yy_create_buffer() (though the first argument is an istream& object reference and not a FILE*), yy_flush_buffer(), yy_delete_buffer(), and yyrestart() (again, the first argument is a istream& object reference).

The second class defined in FlexLexer.h is yyFlexLexer, which is derived from FlexLexer. It defines the following additional member functions:

yyFlexLexer( istream* arg_yyin = 0, ostream* arg_yyout = 0 )
yyFlexLexer( istream& arg_yyin, ostream& arg_yyout )

constructs a yyFlexLexer object using the given streams for input and output. If not specified, the streams default to cin and cout, respectively. yyFlexLexer does not take ownership of its stream arguments. It’s up to the user to ensure the streams pointed to remain alive at least as long as the yyFlexLexer instance.

virtual int yylex()

performs the same role is yylex() does for ordinary flex scanners: it scans the input stream, consuming tokens, until a rule’s action returns a value. If you derive a subclass S from yyFlexLexer and want to access the member functions and variables of S inside yylex(), then you need to use %option yyclass="S" to inform flex that you will be using that subclass instead of yyFlexLexer. In this case, rather than generating yyFlexLexer::yylex(), flex generates S::yylex() (and also generates a dummy yyFlexLexer::yylex() that calls yyFlexLexer::LexerError() if called).

virtual void switch_streams(istream* new_in = 0, ostream* new_out = 0)
virtual void switch_streams(istream& new_in, ostream& new_out)

reassigns yyin to new_in (if non-null) and yyout to new_out (if non-null), deleting the previous input buffer if yyin is reassigned.

int yylex( istream* new_in, ostream* new_out = 0 )
int yylex( istream& new_in, ostream& new_out )

first switches the input streams via switch_streams( new_in, new_out ) and then returns the value of yylex().

In addition, yyFlexLexer defines the following protected virtual functions which you can redefine in derived classes to tailor the scanner:

virtual int LexerInput( char* buf, int max_size )

reads up to max_size characters into buf and returns the number of characters read. To indicate end-of-input, return 0 characters. Note that interactive scanners (see the ‘-B’ and ‘-I’ flags in Scanner Options) define the macro YY_INTERACTIVE. If you redefine LexerInput() and need to take different actions depending on whether or not the scanner might be scanning an interactive input source, you can test for the presence of this name via #ifdef statements.

virtual void LexerOutput( const char* buf, int size )

writes out size characters from the buffer buf, which, while NUL-terminated, may also contain internal NULs if the scanner’s rules can match text with NULs in them.

virtual void LexerError( const char* msg )

reports a fatal error message. The default version of this function writes the message to the stream cerr and exits.

Note that a yyFlexLexer object contains its entire scanning state. Thus you can use such objects to create reentrant scanners, but see also Reentrant C Scanners. You can instantiate multiple instances of the same yyFlexLexer class, and you can also combine multiple C++ scanner classes together in the same program using the ‘-P’ option discussed above.

Finally, note that the %array feature is not available to C++ scanner classes; you must use %pointer (the default).

Here is an example of a simple C++ scanner:

     // An example of using the flex C++ scanner class.

    #include <iostream>
    using namespace std;
    int mylineno = 0;

    %option noyywrap c++

    string  \"[^\n"]+\"

    ws      [ \t]+

    alpha   [A-Za-z]
    dig     [0-9]
    name    ({alpha}|{dig}|\$)({alpha}|{dig}|[_.\-/$])*
    num1    [-+]?{dig}+\.?([eE][-+]?{dig}+)?
    num2    [-+]?{dig}*\.{dig}+([eE][-+]?{dig}+)?
    number  {num1}|{num2}


    {ws}    /* skip blanks and tabs */

    "/*"    {
            int c;

            while((c = yyinput()) != 0)
                if(c == '\n')

                else if(c == '*')
                    if((c = yyinput()) == '/')

    {number}  cout << "number " << YYText() << '\n';

    \n        mylineno++;

    {name}    cout << "name " << YYText() << '\n';

    {string}  cout << "string " << YYText() << '\n';


	// This include is required if main() is an another source file.
	//#include <FlexLexer.h>

    int main( int /* argc */, char** /* argv */ )
        FlexLexer* lexer = new yyFlexLexer;
        while(lexer->yylex() != 0)
        return 0;

If you want to create multiple (different) lexer classes, you use the ‘-P’ flag (or the prefix= option) to rename each yyFlexLexer to some other ‘xxFlexLexer’. You then can include <FlexLexer.h> in your other sources once per lexer class, first renaming yyFlexLexer as follows:

    #undef yyFlexLexer
    #define yyFlexLexer xxFlexLexer
    #include <FlexLexer.h>

    #undef yyFlexLexer
    #define yyFlexLexer zzFlexLexer
    #include <FlexLexer.h>

if, for example, you used %option prefix="xx" for one of your scanners and %option prefix="zz" for the other.

19 Reentrant C Scanners

flex has the ability to generate a reentrant C scanner. This is accomplished by specifying %option reentrant (‘-R’) The generated scanner is both portable, and safe to use in one or more separate threads of control. The most common use for reentrant scanners is from within multi-threaded applications. Any thread may create and execute a reentrant flex scanner without the need for synchronization with other threads.

19.1 Uses for Reentrant Scanners

However, there are other uses for a reentrant scanner. For example, you could scan two or more files simultaneously to implement a diff at the token level (i.e., instead of at the character level):

    /* Example of maintaining more than one active scanner. */

    do {
        int tok1, tok2;

        tok1 = yylex( scanner_1 );
        tok2 = yylex( scanner_2 );

        if( tok1 != tok2 )
            printf("Files are different.");

   } while ( tok1 && tok2 );

Another use for a reentrant scanner is recursion. (Note that a recursive scanner can also be created using a non-reentrant scanner and buffer states. See Multiple Input Buffers.)

The following crude scanner supports the ‘eval’ command by invoking another instance of itself.

    /* Example of recursive invocation. */

    %option reentrant

    "eval(".+")"  {
                      yyscan_t scanner;
                      YY_BUFFER_STATE buf;

                      yylex_init( &scanner );
                      yytext[yyleng-1] = ' ';

                      buf = yy_scan_string( yytext + 5, scanner );
                      yylex( scanner );

                      yylex_destroy( scanner );

19.2 An Overview of the Reentrant API

The API for reentrant scanners is different than for non-reentrant scanners. Here is a quick overview of the API:

  • %option reentrant must be specified.
  • All functions take one additional argument: yyscanner
  • All global variables are replaced by their macro equivalents. (We tell you this because it may be important to you during debugging.)
  • yylex_init and yylex_destroy must be called before and after yylex, respectively.
  • Accessor methods (get/set functions) provide access to common flex variables.
  • User-specific data can be stored in yyextra.

19.3 Reentrant Example

First, an example of a reentrant scanner:

    /* This scanner prints "//" comments. */

    %option reentrant stack noyywrap
    %x COMMENT


    "//"                 yy_push_state( COMMENT, yyscanner);

    <COMMENT>\n          yy_pop_state( yyscanner );
    <COMMENT>[^\n]+      fprintf( yyout, "%s\n", yytext);


    int main ( int argc, char * argv[] )
        yyscan_t scanner;

        yylex_init ( &scanner );
        yylex ( scanner );
        yylex_destroy ( scanner );
    return 0;

19.4 The Reentrant API in Detail

Here are the things you need to do or know to use the reentrant C API of flex.

19.4.1 Declaring a Scanner As Reentrant

%option reentrant (–reentrant) must be specified.

Notice that %option reentrant is specified in the above example (see Reentrant Example. Had this option not been specified, flex would have happily generated a non-reentrant scanner without complaining. You may explicitly specify %option noreentrant, if you do not want a reentrant scanner, although it is not necessary. The default is to generate a non-reentrant scanner.

19.4.2 The Extra Argument

All functions take one additional argument: yyscanner.

Notice that the calls to yy_push_state and yy_pop_state both have an argument, yyscanner , that is not present in a non-reentrant scanner. Here are the declarations of yy_push_state and yy_pop_state in the reentrant scanner:

    static void yy_push_state  ( int new_state , yyscan_t yyscanner ) ;
    static void yy_pop_state  ( yyscan_t yyscanner  ) ;

Notice that the argument yyscanner appears in the declaration of both functions. In fact, all flex functions in a reentrant scanner have this additional argument. It is always the last argument in the argument list, it is always of type yyscan_t (which is typedef’d to void *) and it is always named yyscanner. As you may have guessed, yyscanner is a pointer to an opaque data structure encapsulating the current state of the scanner. For a list of function declarations, see Functions and Macros Available in Reentrant C Scanners. Note that preprocessor macros, such as BEGIN, ECHO, and REJECT, do not take this additional argument.

19.4.3 Global Variables Replaced By Macros

All global variables in traditional flex have been replaced by macro equivalents.

Note that in the above example, yyout and yytext are not plain variables. These are macros that will expand to their equivalent lvalue. All of the familiar flex globals have been replaced by their macro equivalents. In particular, yytext, yyleng, yylineno, yyin, yyout, yyextra, yylval, and yylloc are macros. You may safely use these macros in actions as if they were plain variables. We only tell you this so you don’t expect to link to these variables externally. Currently, each macro expands to a member of an internal struct, e.g.,

#define yytext (((struct yyguts_t*)yyscanner)->yytext_r)

One important thing to remember about yytext and friends is that yytext is not a global variable in a reentrant scanner, you can not access it directly from outside an action or from other functions. You must use an accessor method, e.g., yyget_text, to accomplish this. (See below).

19.4.4 Init and Destroy Functions

yylex_init and yylex_destroy must be called before and after yylex, respectively.

    int yylex_init ( yyscan_t * ptr_yy_globals ) ;
    int yylex_init_extra ( YY_EXTRA_TYPE user_defined, yyscan_t * ptr_yy_globals ) ;
    int yylex ( yyscan_t yyscanner ) ;
    int yylex_destroy ( yyscan_t yyscanner ) ;

The function yylex_init must be called before calling any other function. The argument to yylex_init is the address of an uninitialized pointer to be filled in by yylex_init, overwriting any previous contents. The function yylex_init_extra may be used instead, taking as its first argument a variable of type YY_EXTRA_TYPE. See the section on yyextra, below, for more details.

The value stored in ptr_yy_globals should thereafter be passed to yylex and yylex_destroy. Flex does not save the argument passed to yylex_init, so it is safe to pass the address of a local pointer to yylex_init so long as it remains in scope for the duration of all calls to the scanner, up to and including the call to yylex_destroy.

The function yylex should be familiar to you by now. The reentrant version takes one argument, which is the value returned (via an argument) by yylex_init. Otherwise, it behaves the same as the non-reentrant version of yylex.

Both yylex_init and yylex_init_extra returns 0 (zero) on success, or non-zero on failure, in which case errno is set to one of the following values:

The function yylex_destroy should be called to free resources used by the scanner. After yylex_destroy is called, the contents of yyscanner should not be used. Of course, there is no need to destroy a scanner if you plan to reuse it. A flex scanner (both reentrant and non-reentrant) may be restarted by calling yyrestart.

Below is an example of a program that creates a scanner, uses it, then destroys it when done:

    int main ()
        yyscan_t scanner;
        int tok;


        while ((tok=yylex(scanner)) > 0)
            printf("tok=%d  yytext=%s\n", tok, yyget_text(scanner));

        return 0;

19.4.5 Accessing Variables with Reentrant Scanners

Accessor methods (get/set functions) provide access to common flex variables.

Many scanners that you build will be part of a larger project. Portions of your project will need access to flex values, such as yytext. In a non-reentrant scanner, these values are global, so there is no problem accessing them. However, in a reentrant scanner, there are no global flex values. You can not access them directly. Instead, you must access flex values using accessor methods (get/set functions). Each accessor method is named yyget_NAME or yyset_NAME, where NAME is the name of the flex variable you want. For example:

    /* Set the last character of yytext to NULL. */
    void chop ( yyscan_t scanner )
        int len = yyget_leng( scanner );
        yyget_text( scanner )[len - 1] = '\0';

The above code may be called from within an action like this:

    .+\n    { chop( yyscanner );}

You may find that %option header-file is particularly useful for generating prototypes of all the accessor functions. See option-header.

19.4.6 Extra Data

User-specific data can be stored in yyextra.

In a reentrant scanner, it is unwise to use global variables to communicate with or maintain state between different pieces of your program. However, you may need access to external data or invoke external functions from within the scanner actions. Likewise, you may need to pass information to your scanner (e.g., open file descriptors, or database connections). In a non-reentrant scanner, the only way to do this would be through the use of global variables. Flex allows you to store arbitrary, “extra” data in a scanner. This data is accessible through the accessor methods yyget_extra and yyset_extra from outside the scanner, and through the shortcut macro yyextra from within the scanner itself. They are defined as follows:

    #define YY_EXTRA_TYPE  void*
    YY_EXTRA_TYPE  yyget_extra ( yyscan_t scanner );
    void           yyset_extra ( YY_EXTRA_TYPE arbitrary_data , yyscan_t scanner);

In addition, an extra form of yylex_init is provided, yylex_init_extra. This function is provided so that the yyextra value can be accessed from within the very first yyalloc, used to allocate the scanner itself.

By default, YY_EXTRA_TYPE is defined as type void *. You may redefine this type using %option extra-type="your_type" in the scanner:

    /* An example of overriding YY_EXTRA_TYPE. */
    #include <sys/stat.h>
    #include <unistd.h>
    %option reentrant
    %option extra-type="struct stat *"

    __filesize__     printf( "%ld", yyextra->st_size  );
    __lastmod__      printf( "%ld", yyextra->st_mtime );
    void scan_file( char* filename )
        yyscan_t scanner;
        struct stat buf;
        FILE *in;

        in = fopen( filename, "r" );
        stat( filename, &buf );

        yylex_init_extra( buf, &scanner );
        yyset_in( in, scanner );
        yylex( scanner );
        yylex_destroy( scanner );

        fclose( in );

19.4.7 About yyscan_t

yyscan_t is defined as:

     typedef void* yyscan_t;

It is initialized by yylex_init() to point to an internal structure. You should never access this value directly. In particular, you should never attempt to free it (use yylex_destroy() instead.)

19.5 Functions and Macros Available in Reentrant C Scanners

The following Functions are available in a reentrant scanner:

    char *yyget_text ( yyscan_t scanner );
    int yyget_leng ( yyscan_t scanner );
    FILE *yyget_in ( yyscan_t scanner );
    FILE *yyget_out ( yyscan_t scanner );
    int yyget_lineno ( yyscan_t scanner );
    YY_EXTRA_TYPE yyget_extra ( yyscan_t scanner );
    int  yyget_debug ( yyscan_t scanner );

    void yyset_debug ( int flag, yyscan_t scanner );
    void yyset_in  ( FILE * in_str , yyscan_t scanner );
    void yyset_out  ( FILE * out_str , yyscan_t scanner );
    void yyset_lineno ( int line_number , yyscan_t scanner );
    void yyset_extra ( YY_EXTRA_TYPE user_defined , yyscan_t scanner );

There are no “set” functions for yytext and yyleng. This is intentional.

The following Macro shortcuts are available in actions in a reentrant scanner:


In a reentrant C scanner, support for yylineno is always present (i.e., you may access yylineno), but the value is never modified by flex unless %option yylineno is enabled. This is to allow the user to maintain the line count independently of flex.

The following functions and macros are made available when %option bison-bridge (‘--bison-bridge’) is specified:

    YYSTYPE * yyget_lval ( yyscan_t scanner );
    void yyset_lval ( YYSTYPE * yylvalp , yyscan_t scanner );

The following functions and macros are made available when %option bison-locations (‘--bison-locations’) is specified:

    YYLTYPE *yyget_lloc ( yyscan_t scanner );
    void yyset_lloc ( YYLTYPE * yyllocp , yyscan_t scanner );

Support for yylval assumes that YYSTYPE is a valid type. Support for yylloc assumes that YYSLYPE is a valid type. Typically, these types are generated by bison, and are included in section 1 of the flex input.

20 Incompatibilities with Lex and Posix

flex is a rewrite of the AT&T Unix lex tool (the two implementations do not share any code, though), with some extensions and incompatibilities, both of which are of concern to those who wish to write scanners acceptable to both implementations. flex is fully compliant with the POSIX lex specification, except that when using %pointer (the default), a call to unput() destroys the contents of yytext, which is counter to the POSIX specification. In this section we discuss all of the known areas of incompatibility between flex, AT&T lex, and the POSIX specification. flex’s ‘-l’ option turns on maximum compatibility with the original AT&T lex implementation, at the cost of a major loss in the generated scanner’s performance. We note below which incompatibilities can be overcome using the ‘-l’ option. flex is fully compatible with lex with the following exceptions:

The following flex features are not included in lex or the POSIX specification:

The feature “multiple actions on a line” refers to the fact that with flex you can put multiple actions on the same line, separated with semi-colons, while with lex, the following:

    foo    handle_foo(); ++num_foos_seen;

is (rather surprisingly) truncated to

    foo    handle_foo();

flex does not truncate the action. Actions that are not enclosed in braces are simply terminated at the end of the line.

21 Memory Management

This chapter describes how flex handles dynamic memory, and how you can override the default behavior.

21.1 The Default Memory Management

Flex allocates dynamic memory during initialization, and once in a while from within a call to yylex(). Initialization takes place during the first call to yylex(). Thereafter, flex may reallocate more memory if it needs to enlarge a buffer. As of version 2.5.9 Flex will clean up all memory when you call yylex_destroy See faq-memory-leak.

Flex allocates dynamic memory for four purposes, listed below 2

16kB for the input buffer.

Flex allocates memory for the character buffer used to perform pattern matching. Flex must read ahead from the input stream and store it in a large character buffer. This buffer is typically the largest chunk of dynamic memory flex consumes. This buffer will grow if necessary, doubling the size each time. Flex frees this memory when you call yylex_destroy(). The default size of this buffer (16384 bytes) is almost always too large. The ideal size for this buffer is the length of the longest token expected, in bytes, plus a little more. Flex will allocate a few extra bytes for housekeeping. Currently, to override the size of the input buffer you must #define YY_BUF_SIZE to whatever number of bytes you want. We don’t plan to change this in the near future, but we reserve the right to do so if we ever add a more robust memory management API.

64kb for the REJECT state. This will only be allocated if you use REJECT.

The size is large enough to hold the same number of states as characters in the input buffer. If you override the size of the input buffer (via YY_BUF_SIZE), then you automatically override the size of this buffer as well.

100 bytes for the start condition stack.

Flex allocates memory for the start condition stack. This is the stack used for pushing start states, i.e., with yy_push_state(). It will grow if necessary. Since the states are simply integers, this stack doesn’t consume much memory. This stack is not present if %option stack is not specified. You will rarely need to tune this buffer. The ideal size for this stack is the maximum depth expected. The memory for this stack is automatically destroyed when you call yylex_destroy(). See option-stack.

40 bytes for each YY_BUFFER_STATE.

Flex allocates memory for each YY_BUFFER_STATE. The buffer state itself is about 40 bytes, plus an additional large character buffer (described above.) The initial buffer state is created during initialization, and with each call to yy_create_buffer(). You can’t tune the size of this, but you can tune the character buffer as described above. Any buffer state that you explicitly create by calling yy_create_buffer() is NOT destroyed automatically. You must call yy_delete_buffer() to free the memory. The exception to this rule is that flex will delete the current buffer automatically when you call yylex_destroy(). If you delete the current buffer, be sure to set it to NULL. That way, flex will not try to delete the buffer a second time (possibly crashing your program!) At the time of this writing, flex does not provide a growable stack for the buffer states. You have to manage that yourself. See Multiple Input Buffers.

84 bytes for the reentrant scanner guts

Flex allocates about 84 bytes for the reentrant scanner structure when you call yylex_init(). It is destroyed when the user calls yylex_destroy().

21.2 Overriding The Default Memory Management

Flex calls the functions yyalloc, yyrealloc, and yyfree when it needs to allocate or free memory. By default, these functions are wrappers around the standard C functions, malloc, realloc, and free, respectively. You can override the default implementations by telling flex that you will provide your own implementations.

To override the default implementations, you must do two things:

  1. Suppress the default implementations by specifying one or more of the following options:
    • %option noyyalloc
    • %option noyyrealloc
    • %option noyyfree.
  2. Provide your own implementation of the following functions: 3
    // For a non-reentrant scanner
    void * yyalloc (size_t bytes);
    void * yyrealloc (void * ptr, size_t bytes);
    void   yyfree (void * ptr);
    // For a reentrant scanner
    void * yyalloc (size_t bytes, void * yyscanner);
    void * yyrealloc (void * ptr, size_t bytes, void * yyscanner);
    void   yyfree (void * ptr, void * yyscanner);

In the following example, we will override all three memory routines. We assume that there is a custom allocator with garbage collection. In order to make this example interesting, we will use a reentrant scanner, passing a pointer to the custom allocator through yyextra.

#include "some_allocator.h"

/* Suppress the default implementations. */
%option noyyalloc noyyrealloc noyyfree
%option reentrant

/* Initialize the allocator. */
#define YY_EXTRA_TYPE  struct allocator*
#define YY_USER_INIT  yyextra = allocator_create();

.|\n   ;

/* Provide our own implementations. */
void * yyalloc (size_t bytes, void* yyscanner) {
    return allocator_alloc (yyextra, bytes);

void * yyrealloc (void * ptr, size_t bytes, void* yyscanner) {
    return allocator_realloc (yyextra, bytes);

void yyfree (void * ptr, void * yyscanner) {      
    /* Do nothing -- we leave it to the garbage collector. */

21.3 A Note About yytext And Memory

When flex finds a match, yytext points to the first character of the match in the input buffer. The string itself is part of the input buffer, and is NOT allocated separately. The value of yytext will be overwritten the next time yylex() is called. In short, the value of yytext is only valid from within the matched rule’s action.

Often, you want the value of yytext to persist for later processing, i.e., by a parser with non-zero lookahead. In order to preserve yytext, you will have to copy it with strdup() or a similar function. But this introduces some headache because your parser is now responsible for freeing the copy of yytext. If you use a yacc or bison parser, (commonly used with flex), you will discover that the error recovery mechanisms can cause memory to be leaked.

To prevent memory leaks from strdup’d yytext, you will have to track the memory somehow. Our experience has shown that a garbage collection mechanism or a pooled memory mechanism will save you a lot of grief when writing parsers.

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22 Serialized Tables

A flex scanner has the ability to save the DFA tables to a file, and load them at runtime when needed. The motivation for this feature is to reduce the runtime memory footprint. Traditionally, these tables have been compiled into the scanner as C arrays, and are sometimes quite large. Since the tables are compiled into the scanner, the memory used by the tables can never be freed. This is a waste of memory, especially if an application uses several scanners, but none of them at the same time.

The serialization feature allows the tables to be loaded at runtime, before scanning begins. The tables may be discarded when scanning is finished.

22.1 Creating Serialized Tables

You may create a scanner with serialized tables by specifying:

    %option tables-file=FILE

These options instruct flex to save the DFA tables to the file FILE. The tables will not be embedded in the generated scanner. The scanner will not function on its own. The scanner will be dependent upon the serialized tables. You must load the tables from this file at runtime before you can scan anything.

If you do not specify a filename to --tables-file, the tables will be saved to lex.yy.tables, where ‘yy’ is the appropriate prefix.

If your project uses several different scanners, you can concatenate the serialized tables into one file, and flex will find the correct set of tables, using the scanner prefix as part of the lookup key. An example follows:

$ flex --tables-file --prefix=cpp cpp.l
$ flex --tables-file --prefix=c   c.l
$ cat lex.cpp.tables lex.c.tables  >  all.tables

The above example created two scanners, ‘cpp’, and ‘c’. Since we did not specify a filename, the tables were serialized to lex.c.tables and lex.cpp.tables, respectively. Then, we concatenated the two files together into all.tables, which we will distribute with our project. At runtime, we will open the file and tell flex to load the tables from it. Flex will find the correct tables automatically. (See next section).

22.2 Loading and Unloading Serialized Tables

If you’ve built your scanner with %option tables-file, then you must load the scanner tables at runtime. This can be accomplished with the following function:

Function: int yytables_fload (FILE* fp [, yyscan_t scanner])

Locates scanner tables in the stream pointed to by fp and loads them. Memory for the tables is allocated via yyalloc. You must call this function before the first call to yylex. The argument scanner only appears in the reentrant scanner. This function returns ‘0’ (zero) on success, or non-zero on error.

The loaded tables are not automatically destroyed (unloaded) when you call yylex_destroy. The reason is that you may create several scanners of the same type (in a reentrant scanner), each of which needs access to these tables. To avoid a nasty memory leak, you must call the following function:

Function: int yytables_destroy ([yyscan_t scanner])

Unloads the scanner tables. The tables must be loaded again before you can scan any more data. The argument scanner only appears in the reentrant scanner. This function returns ‘0’ (zero) on success, or non-zero on error.

The functions yytables_fload and yytables_destroy are not thread-safe. You must ensure that these functions are called exactly once (for each scanner type) in a threaded program, before any thread calls yylex. After the tables are loaded, they are never written to, and no thread protection is required thereafter – until you destroy them.

22.3 Tables File Format

This section defines the file format of serialized flex tables.

The tables format allows for one or more sets of tables to be specified, where each set corresponds to a given scanner. Scanners are indexed by name, as described below. The file format is as follows:

                 TABLE SET 1
        Header  | uint32          th_magic;     |
                | uint32          th_hsize;     |
                | uint32          th_ssize;     |
                | uint16          th_flags;     |
                | char            th_version[]; |
                | char            th_name[];    |
                | uint8           th_pad64[];   |
        Table 1 | uint16          td_id;        |
                | uint16          td_flags;     |
                | uint32          td_hilen;     |
                | uint32          td_lolen;     |
                | void            td_data[];    |
                | uint8           td_pad64[];   |
        Table 2 |                               |
           .    .                               .
           .    .                               .
           .    .                               .
           .    .                               .
        Table n |                               |
                 TABLE SET 2
                 TABLE SET N

The above diagram shows that a complete set of tables consists of a header followed by multiple individual tables. Furthermore, multiple complete sets may be present in the same file, each set with its own header and tables. The sets are contiguous in the file. The only way to know if another set follows is to check the next four bytes for the magic number (or check for EOF). The header and tables sections are padded to 64-bit boundaries. Below we describe each field in detail. This format does not specify how the scanner will expand the given data, i.e., data may be serialized as int8, but expanded to an int32 array at runtime. This is to reduce the size of the serialized data where possible. Remember, all integer values are in network byte order.

Fields of a table header:


Magic number, always 0xF13C57B1.


Size of this entire header, in bytes, including all fields plus any padding.


Size of this entire set, in bytes, including the header, all tables, plus any padding.


Bit flags for this table set. Currently unused.


Flex version in NULL-terminated string format. e.g., ‘2.5.13a’. This is the version of flex that was used to create the serialized tables.


Contains the name of this table set. The default is ‘yytables’, and is prefixed accordingly, e.g., ‘footables’. Must be NULL-terminated.


Zero or more NULL bytes, padding the entire header to the next 64-bit boundary as calculated from the beginning of the header.

Fields of a table:


Specifies the table identifier. Possible values are:





YYTD_ID_CHK (0x03)


YYTD_ID_DEF (0x04)


YYTD_ID_EC (0x05)






YYTD_ID_NXT (0x08)

yy_nxt. This array may be two dimensional. See the td_hilen field below.




yy_start_state_list. This array is handled specially because it is an array of pointers to structs. See the td_flags field below.


yy_transition. This array is handled specially because it is an array of structs. See the td_lolen field below.




Bit flags describing how to interpret the data in td_data. The data arrays are one-dimensional by default, but may be two dimensional as specified in the td_hilen field.

YYTD_DATA8 (0x01)

The data is serialized as an array of type int8.

YYTD_DATA16 (0x02)

The data is serialized as an array of type int16.

YYTD_DATA32 (0x04)

The data is serialized as an array of type int32.


The data is a list of indexes of entries in the expanded yy_transition array. Each index should be expanded to a pointer to the corresponding entry in the yy_transition array. We count on the fact that the yy_transition array has already been seen.


The data is a list of yy_trans_info structs, each of which consists of two integers. There is no padding between struct elements or between structs. The type of each member is determined by the YYTD_DATA* bits.


If td_hilen is non-zero, then the data is a two-dimensional array. Otherwise, the data is a one-dimensional array. td_hilen contains the number of elements in the higher dimensional array, and td_lolen contains the number of elements in the lowest dimension.

Conceptually, td_data is either sometype td_data[td_lolen], or sometype td_data[td_hilen][td_lolen], where sometype is specified by the td_flags field. It is possible for both td_lolen and td_hilen to be zero, in which case td_data is a zero length array, and no data is loaded, i.e., this table is simply skipped. Flex does not currently generate tables of zero length.


Specifies the number of elements in the lowest dimension array. If this is a one-dimensional array, then it is simply the number of elements in this array. The element size is determined by the td_flags field.


The table data. This array may be a one- or two-dimensional array, of type int8, int16, int32, struct yy_trans_info, or struct yy_trans_info*, depending upon the values in the td_flags, td_hilen, and td_lolen fields.


Zero or more NULL bytes, padding the entire table to the next 64-bit boundary as calculated from the beginning of this table.

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23 Diagnostics

The following is a list of flex diagnostic messages:

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24 Limitations

Some trailing context patterns cannot be properly matched and generate warning messages (‘dangerous trailing context’). These are patterns where the ending of the first part of the rule matches the beginning of the second part, such as ‘zx*/xy*’, where the ’x*’ matches the ’x’ at the beginning of the trailing context. (Note that the POSIX draft states that the text matched by such patterns is undefined.) For some trailing context rules, parts which are actually fixed-length are not recognized as such, leading to the abovementioned performance loss. In particular, parts using ‘|’ or ‘{n}’ (such as ‘foo{3}’) are always considered variable-length. Combining trailing context with the special ‘|’ action can result in fixed trailing context being turned into the more expensive variable trailing context. For example, in the following:

    abc      |

Use of unput() invalidates yytext and yyleng, unless the %array directive or the ‘-l’ option has been used. Pattern-matching of NULs is substantially slower than matching other characters. Dynamic resizing of the input buffer is slow, as it entails rescanning all the text matched so far by the current (generally huge) token. Due to both buffering of input and read-ahead, you cannot intermix calls to <stdio.h> routines, such as, getchar(), with flex rules and expect it to work. Call input() instead. The total table entries listed by the ‘-v’ flag excludes the number of table entries needed to determine what rule has been matched. The number of entries is equal to the number of DFA states if the scanner does not use REJECT, and somewhat greater than the number of states if it does. REJECT cannot be used with the ‘-f’ or ‘-F’ options.

The flex internal algorithms need documentation.

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25 Additional Reading

You may wish to read more about the following programs:

The following books may contain material of interest:

John Levine, Tony Mason, and Doug Brown, Lex & Yacc, O’Reilly and Associates. Be sure to get the 2nd edition.

M. E. Lesk and E. Schmidt, LEX – Lexical Analyzer Generator

Alfred Aho, Ravi Sethi and Jeffrey Ullman, Compilers: Principles, Techniques and Tools, Addison-Wesley (1986). Describes the pattern-matching techniques used by flex (deterministic finite automata).

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From time to time, the flex maintainer receives certain questions. Rather than repeat answers to well-understood problems, we publish them here.

When was flex born?

Vern Paxson took over the Software Tools lex project from Jef Poskanzer in 1982. At that point it was written in Ratfor. Around 1987 or so, Paxson translated it into C, and a legend was born :-).

How do I expand backslash-escape sequences in C-style quoted strings?

A key point when scanning quoted strings is that you cannot (easily) write a single rule that will precisely match the string if you allow things like embedded escape sequences and newlines. If you try to match strings with a single rule then you’ll wind up having to rescan the string anyway to find any escape sequences.

Instead you can use exclusive start conditions and a set of rules, one for matching non-escaped text, one for matching a single escape, one for matching an embedded newline, and one for recognizing the end of the string. Each of these rules is then faced with the question of where to put its intermediary results. The best solution is for the rules to append their local value of yytext to the end of a “string literal” buffer. A rule like the escape-matcher will append to the buffer the meaning of the escape sequence rather than the literal text in yytext. In this way, yytext does not need to be modified at all.

Why do flex scanners call fileno if it is not ANSI compatible?

Flex scanners call fileno() in order to get the file descriptor corresponding to yyin. The file descriptor may be passed to isatty() or read(), depending upon which %options you specified. If your system does not have fileno() support, to get rid of the read() call, do not specify %option read. To get rid of the isatty() call, you must specify one of %option always-interactive or %option never-interactive.

Does flex support recursive pattern definitions?


block   "{"({block}|{statement})*"}"

No. You cannot have recursive definitions. The pattern-matching power of regular expressions in general (and therefore flex scanners, too) is limited. In particular, regular expressions cannot “balance” parentheses to an arbitrary degree. For example, it’s impossible to write a regular expression that matches all strings containing the same number of ’{’s as ’}’s. For more powerful pattern matching, you need a parser, such as GNU bison.

How do I skip huge chunks of input (tens of megabytes) while using flex?

Use fseek() (or lseek()) to position yyin, then call yyrestart().

Flex is not matching my patterns in the same order that I defined them.

flex picks the rule that matches the most text (i.e., the longest possible input string). This is because flex uses an entirely different matching technique (“deterministic finite automata”) that actually does all of the matching simultaneously, in parallel. (Seems impossible, but it’s actually a fairly simple technique once you understand the principles.)

A side-effect of this parallel matching is that when the input matches more than one rule, flex scanners pick the rule that matched the most text. This is explained further in the manual, in the section See How the Input Is Matched.

If you want flex to choose a shorter match, then you can work around this behavior by expanding your short rule to match more text, then put back the extra:

data_.*        yyless( 5 ); BEGIN BLOCKIDSTATE;

Another fix would be to make the second rule active only during the <BLOCKIDSTATE> start condition, and make that start condition exclusive by declaring it with %x instead of %s.

A final fix is to change the input language so that the ambiguity for ‘data_’ is removed, by adding characters to it that don’t match the identifier rule, or by removing characters (such as ‘_’) from the identifier rule so it no longer matches ‘data_’. (Of course, you might also not have the option of changing the input language.)

My actions are executing out of order or sometimes not at all.

Most likely, you have (in error) placed the opening ‘{’ of the action block on a different line than the rule, e.g.,

{  <<<--- WRONG!


flex requires that the opening ‘{’ of an action associated with a rule begin on the same line as does the rule. You need instead to write your rules as follows:

^(foo|bar)   {  // CORRECT!


How can I have multiple input sources feed into the same scanner at the same time?

If …

  • your scanner is free of backtracking (verified using flex’s ‘-b’ flag),
  • AND you run your scanner interactively (‘-I’ option; default unless using special table compression options),
  • AND you feed it one character at a time by redefining YY_INPUT to do so,

then every time it matches a token, it will have exhausted its input buffer (because the scanner is free of backtracking). This means you can safely use select() at the point and only call yylex() for another token if select() indicates there’s data available.

That is, move the select() out from the input function to a point where it determines whether yylex() gets called for the next token.

With this approach, you will still have problems if your input can arrive piecemeal; select() could inform you that the beginning of a token is available, you call yylex() to get it, but it winds up blocking waiting for the later characters in the token.

Here’s another way: Move your input multiplexing inside of YY_INPUT. That is, whenever YY_INPUT is called, it select()’s to see where input is available. If input is available for the scanner, it reads and returns the next byte. If input is available from another source, it calls whatever function is responsible for reading from that source. (If no input is available, it blocks until some input is available.) I’ve used this technique in an interpreter I wrote that both reads keyboard input using a flex scanner and IPC traffic from sockets, and it works fine.

Can I build nested parsers that work with the same input file?

This is not going to work without some additional effort. The reason is that flex block-buffers the input it reads from yyin. This means that the “outermost” yylex(), when called, will automatically slurp up the first 8K of input available on yyin, and subsequent calls to other yylex()’s won’t see that input. You might be tempted to work around this problem by redefining YY_INPUT to only return a small amount of text, but it turns out that that approach is quite difficult. Instead, the best solution is to combine all of your scanners into one large scanner, using a different exclusive start condition for each.

How can I match text only at the end of a file?

There is no way to write a rule which is “match this text, but only if it comes at the end of the file”. You can fake it, though, if you happen to have a character lying around that you don’t allow in your input. Then you redefine YY_INPUT to call your own routine which, if it sees an ‘EOF’, returns the magic character first (and remembers to return a real EOF next time it’s called). Then you could write:

<COMMENT>(.|\n)*{EOF_CHAR}    /* saw comment at EOF */

How can I make REJECT cascade across start condition boundaries?

You can do this as follows. Suppose you have a start condition ‘A’, and after exhausting all of the possible matches in ‘<A>’, you want to try matches in ‘<INITIAL>’. Then you could use the following:

%x A
<A>rule_that_is_long    ...; REJECT;
<A>rule                 ...; REJECT; /* shorter rule */
<A>.|\n  {
/* Shortest and last rule in <A>, so
* cascaded REJECTs will eventually
* wind up matching this rule.  We want
* to now switch to the initial state
* and try matching from there instead.
yyless(0);    /* put back matched text */

Why can’t I use fast or full tables with interactive mode?

One of the assumptions flex makes is that interactive applications are inherently slow (they’re waiting on a human after all). It has to do with how the scanner detects that it must be finished scanning a token. For interactive scanners, after scanning each character the current state is looked up in a table (essentially) to see whether there’s a chance of another input character possibly extending the length of the match. If not, the scanner halts. For non-interactive scanners, the end-of-token test is much simpler, basically a compare with 0, so no memory bus cycles. Since the test occurs in the innermost scanning loop, one would like to make it go as fast as possible.

Still, it seems reasonable to allow the user to choose to trade off a bit of performance in this area to gain the corresponding flexibility. There might be another reason, though, why fast scanners don’t support the interactive option.

How much faster is -F or -f than -C?

Much faster (factor of 2-3).

If I have a simple grammar can’t I just parse it with flex?

Is your grammar recursive? That’s almost always a sign that you’re better off using a parser/scanner rather than just trying to use a scanner alone.

Why doesn’t yyrestart() set the start state back to INITIAL?

There are two reasons. The first is that there might be programs that rely on the start state not changing across file changes. The second is that beginning with flex version 2.4, use of yyrestart() is no longer required, so fixing the problem there doesn’t solve the more general problem.

How can I match C-style comments?

You might be tempted to try something like this:

"/*".*"*/"       // WRONG!

or, worse, this:

"/*"(.|\n)"*/"   // WRONG!

The above rules will eat too much input, and blow up on things like:

/* a comment */ do_my_thing( "oops */" );

Here is one way which allows you to track line information:

"/*"              BEGIN(IN_COMMENT);
"*/"      BEGIN(INITIAL);
[^*\n]+   // eat comment in chunks
"*"       // eat the lone star
\n        yylineno++;

The ’.’ isn’t working the way I expected.

Here are some tips for using ‘.’:

  • A common mistake is to place the grouping parenthesis AFTER an operator, when you really meant to place the parenthesis BEFORE the operator, e.g., you probably want this (foo|bar)+ and NOT this (foo|bar+).

    The first pattern matches the words ‘foo’ or ‘bar’ any number of times, e.g., it matches the text ‘barfoofoobarfoo’. The second pattern matches a single instance of foo or a single instance of bar followed by one or more ‘r’s, e.g., it matches the text barrrr .

  • A ‘.’ inside ‘[]’’s just means a literal‘.’ (period), and NOT “any character except newline”.
  • Remember that ‘.’ matches any character EXCEPT ‘\n’ (and ‘EOF’). If you really want to match ANY character, including newlines, then use (.|\n) Beware that the regex (.|\n)+ will match your entire input!
  • Finally, if you want to match a literal ‘.’ (a period), then use ‘[.]’ or ‘"."

Can I get the flex manual in another format?

The flex source distribution includes a texinfo manual. You are free to convert that texinfo into whatever format you desire. The texinfo package includes tools for conversion to a number of formats.

Does there exist a "faster" NDFA->DFA algorithm?

There’s no way around the potential exponential running time - it can take you exponential time just to enumerate all of the DFA states. In practice, though, the running time is closer to linear, or sometimes quadratic.

How does flex compile the DFA so quickly?

There are two big speed wins that flex uses:

  1. It analyzes the input rules to construct equivalence classes for those characters that always make the same transitions. It then rewrites the NFA using equivalence classes for transitions instead of characters. This cuts down the NFA->DFA computation time dramatically, to the point where, for uncompressed DFA tables, the DFA generation is often I/O bound in writing out the tables.
  2. It maintains hash values for previously computed DFA states, so testing whether a newly constructed DFA state is equivalent to a previously constructed state can be done very quickly, by first comparing hash values.

How can I use more than 8192 rules?

Flex is compiled with an upper limit of 8192 rules per scanner. If you need more than 8192 rules in your scanner, you’ll have to recompile flex with the following changes in flexdef.h:

<    #define YY_TRAILING_MASK 0x2000
<    #define YY_TRAILING_HEAD_MASK 0x4000
>    #define YY_TRAILING_MASK 0x20000000
>    #define YY_TRAILING_HEAD_MASK 0x40000000

This should work okay as long as your C compiler uses 32 bit integers. But you might want to think about whether using such a huge number of rules is the best way to solve your problem.

The following may also be relevant:

With luck, you should be able to increase the definitions in flexdef.h for:

#define JAMSTATE -32766 /* marks a reference to the state that always jams */
#define MAXIMUM_MNS 31999
#define BAD_SUBSCRIPT -32767

recompile everything, and it’ll all work. Flex only has these 16-bit-like values built into it because a long time ago it was developed on a machine with 16-bit ints. I’ve given this advice to others in the past but haven’t heard back from them whether it worked okay or not...

How do I abandon a file in the middle of a scan and switch to a new file?

Just call yyrestart(newfile). Be sure to reset the start state if you want a “fresh start, since yyrestart does NOT reset the start state back to INITIAL.

How do I execute code only during initialization (only before the first scan)?

You can specify an initial action by defining the macro YY_USER_INIT (though note that yyout may not be available at the time this macro is executed). Or you can add to the beginning of your rules section:

    /* Must be indented! */
    static int did_init = 0;

    if ( ! did_init ){
        did_init = 1;

How do I execute code at termination?

You can specify an action for the <<EOF>> rule.

Where else can I find help?

You can find the flex homepage on the web at http://flex.sourceforge.net/. See that page for details about flex mailing lists as well.

Can I include comments in the "rules" section of the file?

Yes, just about anywhere you want to. See the manual for the specific syntax.

I get an error about undefined yywrap().

You must supply a yywrap() function of your own, or link to libfl.a (which provides one), or use

%option noyywrap

in your source to say you don’t want a yywrap() function.

How can I change the matching pattern at run time?

You can’t, it’s compiled into a static table when flex builds the scanner.

How can I expand macros in the input?

The best way to approach this problem is at a higher level, e.g., in the parser.

However, you can do this using multiple input buffers.

macro/[a-z]+	{
/* Saw the macro "macro" followed by extra stuff. */
main_buffer = YY_CURRENT_BUFFER;
expansion_buffer = yy_scan_string(expand(yytext));

<<EOF>>	{
if ( expansion_buffer )
// We were doing an expansion, return to where
// we were.
expansion_buffer = 0;

You probably will want a stack of expansion buffers to allow nested macros. From the above though hopefully the idea is clear.

How can I build a two-pass scanner?

One way to do it is to filter the first pass to a temporary file, then process the temporary file on the second pass. You will probably see a performance hit, due to all the disk I/O.

When you need to look ahead far forward like this, it almost always means that the right solution is to build a parse tree of the entire input, then walk it after the parse in order to generate the output. In a sense, this is a two-pass approach, once through the text and once through the parse tree, but the performance hit for the latter is usually an order of magnitude smaller, since everything is already classified, in binary format, and residing in memory.

How do I match any string not matched in the preceding rules?

One way to assign precedence, is to place the more specific rules first. If two rules would match the same input (same sequence of characters) then the first rule listed in the flex input wins, e.g.,

foo[a-zA-Z_]+    return FOO_ID;
bar[a-zA-Z_]+    return BAR_ID;
[a-zA-Z_]+       return GENERIC_ID;

Note that the rule [a-zA-Z_]+ must come *after* the others. It will match the same amount of text as the more specific rules, and in that case the flex scanner will pick the first rule listed in your scanner as the one to match.

I am trying to port code from AT&T lex that uses yysptr and yysbuf.

Those are internal variables pointing into the AT&T scanner’s input buffer. I imagine they’re being manipulated in user versions of the input() and unput() functions. If so, what you need to do is analyze those functions to figure out what they’re doing, and then replace input() with an appropriate definition of YY_INPUT. You shouldn’t need to (and must not) replace flex’s unput() function.

Is there a way to make flex treat NULL like a regular character?

Yes, ‘\0’ and ‘\x00’ should both do the trick. Perhaps you have an ancient version of flex. The latest release is version 2.6.4.

Whenever flex can not match the input it says "flex scanner jammed".

You need to add a rule that matches the otherwise-unmatched text, e.g.,

%option yylineno
[[a bunch of rules here]]

.	printf("bad input character '%s' at line %d\n", yytext, yylineno);

See %option default for more information.

Why doesn’t flex have non-greedy operators like perl does?

A DFA can do a non-greedy match by stopping the first time it enters an accepting state, instead of consuming input until it determines that no further matching is possible (a “jam” state). This is actually easier to implement than longest leftmost match (which flex does).

But it’s also much less useful than longest leftmost match. In general, when you find yourself wishing for non-greedy matching, that’s usually a sign that you’re trying to make the scanner do some parsing. That’s generally the wrong approach, since it lacks the power to do a decent job. Better is to either introduce a separate parser, or to split the scanner into multiple scanners using (exclusive) start conditions.

You might have a separate start state once you’ve seen the ‘BEGIN’. In that state, you might then have a regex that will match ‘END’ (to kick you out of the state), and perhaps ‘(.|\n)’ to get a single character within the chunk ...

This approach also has much better error-reporting properties.

Memory leak - 16386 bytes allocated by malloc.

UPDATED 2002-07-10: As of flex version 2.5.9, this leak means that you did not call yylex_destroy(). If you are using an earlier version of flex, then read on.

The leak is about 16426 bytes. That is, (8192 * 2 + 2) for the read-buffer, and about 40 for struct yy_buffer_state (depending upon alignment). The leak is in the non-reentrant C scanner only (NOT in the reentrant scanner, NOT in the C++ scanner). Since flex doesn’t know when you are done, the buffer is never freed.

However, the leak won’t multiply since the buffer is reused no matter how many times you call yylex().

If you want to reclaim the memory when you are completely done scanning, then you might try this:

/* For non-reentrant C scanner only. */
yy_init = 1;

Note: yy_init is an "internal variable", and hasn’t been tested in this situation. It is possible that some other globals may need resetting as well.

How do I track the byte offset for lseek()?

>   We thought that it would be possible to have this number through the
>   evaluation of the following expression:
>   seek_position = (no_buffers)*YY_READ_BUF_SIZE + yy_c_buf_p - YY_CURRENT_BUFFER->yy_ch_buf

While this is the right idea, it has two problems. The first is that it’s possible that flex will request less than YY_READ_BUF_SIZE during an invocation of YY_INPUT (or that your input source will return less even though YY_READ_BUF_SIZE bytes were requested). The second problem is that when refilling its internal buffer, flex keeps some characters from the previous buffer (because usually it’s in the middle of a match, and needs those characters to construct yytext for the match once it’s done). Because of this, yy_c_buf_p - YY_CURRENT_BUFFER->yy_ch_buf won’t be exactly the number of characters already read from the current buffer.

An alternative solution is to count the number of characters you’ve matched since starting to scan. This can be done by using YY_USER_ACTION. For example,

#define YY_USER_ACTION num_chars += yyleng;

(You need to be careful to update your bookkeeping if you use yymore(), yyless(), unput(), or input().)

How do I use my own I/O classes in a C++ scanner?

When the flex C++ scanning class rewrite finally happens, then this sort of thing should become much easier.

You can do this by passing the various functions (such as LexerInput() and LexerOutput()) NULL iostream*’s, and then dealing with your own I/O classes surreptitiously (i.e., stashing them in special member variables). This works because the only assumption about the lexer regarding what’s done with the iostream’s is that they’re ultimately passed to LexerInput() and LexerOutput, which then do whatever is necessary with them.

How do I skip as many chars as possible?

How do I skip as many chars as possible – without interfering with the other patterns?

In the example below, we want to skip over characters until we see the phrase "endskip". The following will NOT work correctly (do you see why not?)

<INITIAL>startskip   BEGIN(SKIP);
<SKIP>"endskip"       BEGIN(INITIAL);
<SKIP>.*             ;

The problem is that the pattern .* will eat up the word "endskip." The simplest (but slow) fix is:

<SKIP>"endskip"      BEGIN(INITIAL);
<SKIP>.              ;

The fix involves making the second rule match more, without making it match "endskip" plus something else. So for example:

<SKIP>"endskip"     BEGIN(INITIAL);
<SKIP>[^e]+         ;
<SKIP>.		        ;/* so you eat up e's, too */


When was flex born?

Vern Paxson took over
the Software Tools lex project from Jef Poskanzer in 1982.  At that point it
was written in Ratfor.  Around 1987 or so, Paxson translated it into C, and
a legend was born :-).

Next: , Previous: , Up: FAQ   [Contents][Index]

Are certain equivalent patterns faster than others?

To: Adoram Rogel <adoram@orna.hybridge.com>
Subject: Re: Flex 2.5.2 performance questions
In-reply-to: Your message of Wed, 18 Sep 96 11:12:17 EDT.
Date: Wed, 18 Sep 96 10:51:02 PDT
From: Vern Paxson <vern>

[Note, the most recent flex release is 2.5.4, which you can get from
ftp.ee.lbl.gov.  It has bug fixes over 2.5.2 and 2.5.3.]

> 1. Using the pattern
>    ([Ff](oot)?)?[Nn](ote)?(\.)?
>    instead of
>    (((F|f)oot(N|n)ote)|((N|n)ote)|((N|n)\.)|((F|f)(N|n)(\.)))
>    (in a very complicated flex program) caused the program to slow from
>    300K+/min to 100K/min (no other changes were done).

These two are not equivalent.  For example, the first can match "footnote."
but the second can only match "footnote".  This is almost certainly the
cause in the discrepancy - the slower scanner run is matching more tokens,
and/or having to do more backing up.

> 2. Which of these two are better: [Ff]oot or (F|f)oot ?

From a performance point of view, they're equivalent (modulo presumably
minor effects such as memory cache hit rates; and the presence of trailing
context, see below).  From a space point of view, the first is slightly

> 3. I have a pattern that look like this:
>    pats {p1}|{p2}|{p3}|...|{p50}     (50 patterns ORd)
>    running yet another complicated program that includes the following rule:
>    <snext>{and}/{no4}{bb}{pats}
>    gets me to "too complicated - over 32,000 states"...

I can't tell from this example whether the trailing context is variable-length
or fixed-length (it could be the latter if {and} is fixed-length).  If it's
variable length, which flex -p will tell you, then this reflects a basic
performance problem, and if you can eliminate it by restructuring your
scanner, you will see significant improvement.

>    so I divided {pats} to {pats1}, {pats2},..., {pats5} each consists of about
>    10 patterns and changed the rule to be 5 rules.
>    This did compile, but what is the rule of thumb here ?

The rule is to avoid trailing context other than fixed-length, in which for
a/b, either the 'a' pattern or the 'b' pattern have a fixed length.  Use
of the '|' operator automatically makes the pattern variable length, so in
this case '[Ff]oot' is preferred to '(F|f)oot'.

> 4. I changed a rule that looked like this:
>    <snext8>{and}{bb}/{ROMAN}[^A-Za-z] { BEGIN...
>    to the next 2 rules:
>    <snext8>{and}{bb}/{ROMAN}[A-Za-z] { ECHO;}
>    <snext8>{and}{bb}/{ROMAN}         { BEGIN...
>    Again, I understand the using [^...] will cause a great performance loss

Actually, it doesn't cause any sort of performance loss.  It's a surprising
fact about regular expressions that they always match in linear time
regardless of how complex they are.

>    but are there any specific rules about it ?

See the "Performance Considerations" section of the man page, and also
the example in MISC/fastwc/.


Is backing up a big deal?

To: Adoram Rogel <adoram@hybridge.com>
Subject: Re: Flex 2.5.2 performance questions
In-reply-to: Your message of Thu, 19 Sep 96 10:16:04 EDT.
Date: Thu, 19 Sep 96 09:58:00 PDT
From: Vern Paxson <vern>

> a lot about the backing up problem.
> I believe that there lies my biggest problem, and I'll try to improve
> it.

Since you have variable trailing context, this is a bigger performance
problem.  Fixing it is usually easier than fixing backing up, which in a
complicated scanner (yours seems to fit the bill) can be extremely
difficult to do correctly.

You also don't mention what flags you are using for your scanner.
-f makes a large speed difference, and -Cfe buys you nearly as much
speed but the resulting scanner is considerably smaller.

> I have an | operator in {and} and in {pats} so both of them are variable
> length.

-p should have reported this.

> Is changing one of them to fixed-length is enough ?


> Is it possible to change the 32,000 states limit ?

Yes.  I've appended instructions on how.  Before you make this change,
though, you should think about whether there are ways to fundamentally
simplify your scanner - those are certainly preferable!


To increase the 32K limit (on a machine with 32 bit integers), you increase
the magnitude of the following in flexdef.h:

#define JAMSTATE -32766 /* marks a reference to the state that always jams */
#define MAXIMUM_MNS 31999
#define BAD_SUBSCRIPT -32767
#define MAX_SHORT 32700

Adding a 0 or two after each should do the trick.

Next: , Previous: , Up: FAQ   [Contents][Index]

Can I fake multi-byte character support?

To: Heeman_Lee@hp.com
Subject: Re: flex - multi-byte support?
In-reply-to: Your message of Thu, 03 Oct 1996 17:24:04 PDT.
Date: Fri, 04 Oct 1996 11:42:18 PDT
From: Vern Paxson <vern>

>      I assume as long as my *.l file defines the
>      range of expected character code values (in octal format), flex will
>      scan the file and read multi-byte characters correctly. But I have no
>      confidence in this assumption.

Your lack of confidence is justified - this won't work.

Flex has in it a widespread assumption that the input is processed
one byte at a time.  Fixing this is on the to-do list, but is involved,
so it won't happen any time soon.  In the interim, the best I can suggest
(unless you want to try fixing it yourself) is to write your rules in
terms of pairs of bytes, using definitions in the first section:

	X	\xfe\xc2
	foo{X}bar	found_foo_fe_c2_bar();

etc.  Definitely a pain - sorry about that.

By the way, the email address you used for me is ancient, indicating you
have a very old version of flex.  You can get the most recent, 2.5.4, from



To: moleary@primus.com
Subject: Re: Flex / Unicode compatibility question
In-reply-to: Your message of Tue, 22 Oct 1996 10:15:42 PDT.
Date: Tue, 22 Oct 1996 11:06:13 PDT
From: Vern Paxson <vern>

Unfortunately flex at the moment has a widespread assumption within it
that characters are processed 8 bits at a time.  I don't see any easy
fix for this (other than writing your rules in terms of double characters -
a pain).  I also don't know of a wider lex, though you might try surfing
the Plan 9 stuff because I know it's a Unicode system, and also the PCCT
toolkit (try searching say Alta Vista for "Purdue Compiler Construction

Fixing flex to handle wider characters is on the long-term to-do list.
But since flex is a strictly spare-time project these days, this probably
won't happen for quite a while, unless someone else does it first.


Can you discuss some flex internals?

To: Johan Linde <jl@theophys.kth.se>
Subject: Re: translation of flex
In-reply-to: Your message of Sun, 10 Nov 1996 09:16:36 PST.
Date: Mon, 11 Nov 1996 10:33:50 PST
From: Vern Paxson <vern>

> I'm working for the Swedish team translating GNU program, and I'm currently
> working with flex. I have a few questions about some of the messages which
> I hope you can answer.

All of the things you're wondering about, by the way, concerning flex
internals - probably the only person who understands what they mean in
English is me!  So I wouldn't worry too much about getting them right.
That said ...

> #: main.c:545
> msgid "  %d protos created\n"
> Does proto mean prototype?

Yes - prototypes of state compression tables.

> #: main.c:539
> msgid "  %d/%d (peak %d) template nxt-chk entries created\n"
> Here I'm mainly puzzled by 'nxt-chk'. I guess it means 'next-check'. (?)
> However, 'template next-check entries' doesn't make much sense to me. To be
> able to find a good translation I need to know a little bit more about it.

There is a scheme in the Aho/Sethi/Ullman compiler book for compressing
scanner tables.  It involves creating two pairs of tables.  The first has
"base" and "default" entries, the second has "next" and "check" entries.
The "base" entry is indexed by the current state and yields an index into
the next/check table.  The "default" entry gives what to do if the state
transition isn't found in next/check.  The "next" entry gives the next
state to enter, but only if the "check" entry verifies that this entry is
correct for the current state.  Flex creates templates of series of
next/check entries and then encodes differences from these templates as a
way to compress the tables.

> #: main.c:533
> msgid "  %d/%d base-def entries created\n"
> The same problem here for 'base-def'.

See above.


unput() messes up yy_at_bol

To: Xinying Li <xli@npac.syr.edu>
Subject: Re: FLEX ?
In-reply-to: Your message of Wed, 13 Nov 1996 17:28:38 PST.
Date: Wed, 13 Nov 1996 19:51:54 PST
From: Vern Paxson <vern>

> "unput()" them to input flow, question occurs. If I do this after I scan
> a carriage, the variable "YY_CURRENT_BUFFER->yy_at_bol" is changed. That
> means the carriage flag has gone.

You can control this by calling yy_set_bol().  It's described in the manual.

>      And if in pre-reading it goes to the end of file, is anything done
> to control the end of curren buffer and end of file?

No, there's no way to put back an end-of-file.

>      By the way I am using flex 2.5.2 and using the "-l".

The latest release is 2.5.4, by the way.  It fixes some bugs in 2.5.2 and
2.5.3.  You can get it from ftp.ee.lbl.gov.


The | operator is not doing what I want

To: Alain.ISSARD@st.com
Subject: Re: Start condition with FLEX
In-reply-to: Your message of Mon, 18 Nov 1996 09:45:02 PST.
Date: Mon, 18 Nov 1996 10:41:34 PST
From: Vern Paxson <vern>

> I am not able to use the start condition scope and to use the | (OR) with
> rules having start conditions.

The problem is that if you use '|' as a regular expression operator, for
example "a|b" meaning "match either 'a' or 'b'", then it must *not* have
any blanks around it.  If you instead want the special '|' *action* (which
from your scanner appears to be the case), which is a way of giving two
different rules the same action:

	foo	|
	bar	matched_foo_or_bar();

then '|' *must* be separated from the first rule by whitespace and *must*
be followed by a new line.  You *cannot* write it as:

	foo | bar	matched_foo_or_bar();

even though you might think you could because yacc supports this syntax.
The reason for this unfortunately incompatibility is historical, but it's
unlikely to be changed.

Your problems with start condition scope are simply due to syntax errors
from your use of '|' later confusing flex.

Let me know if you still have problems.


Why can’t flex understand this variable trailing context pattern?

To: Gregory Margo <gmargo@newton.vip.best.com>
Subject: Re: flex-2.5.3 bug report
In-reply-to: Your message of Sat, 23 Nov 1996 16:50:09 PST.
Date: Sat, 23 Nov 1996 17:07:32 PST
From: Vern Paxson <vern>

> Enclosed is a lex file that "real" lex will process, but I cannot get
> flex to process it.  Could you try it and maybe point me in the right direction?

Your problem is that some of the definitions in the scanner use the '/'
trailing context operator, and have it enclosed in ()'s.  Flex does not
allow this operator to be enclosed in ()'s because doing so allows undefined
regular expressions such as "(a/b)+".  So the solution is to remove the
parentheses.  Note that you must also be building the scanner with the -l
option for AT&T lex compatibility.  Without this option, flex automatically
encloses the definitions in parentheses.


The ^ operator isn’t working

To: Thomas Hadig <hadig@toots.physik.rwth-aachen.de>
Subject: Re: Flex Bug ?
In-reply-to: Your message of Tue, 26 Nov 1996 14:35:01 PST.
Date: Tue, 26 Nov 1996 11:15:05 PST
From: Vern Paxson <vern>

> In my lexer code, i have the line :
> ^\*.*          { }
> Thus all lines starting with an astrix (*) are comment lines.
> This does not work !

I can't get this problem to reproduce - it works fine for me.  Note
though that if what you have is slightly different:

	COMMENT	^\*.*
	{COMMENT}	{ }

then it won't work, because flex pushes back macro definitions enclosed
in ()'s, so the rule becomes

	(^\*.*)		{ }

and now that the '^' operator is not at the immediate beginning of the
line, it's interpreted as just a regular character.  You can avoid this
behavior by using the "-l" lex-compatibility flag, or "%option lex-compat".


Trailing context is getting confused with trailing optional patterns

To: Adoram Rogel <adoram@hybridge.com>
Subject: Re: Flex 2.5.4 BOF ???
In-reply-to: Your message of Tue, 26 Nov 1996 16:10:41 PST.
Date: Wed, 27 Nov 1996 10:56:25 PST
From: Vern Paxson <vern>

>     Organization(s)?/[a-z]
> This matched "Organizations" (looking in debug mode, the trailing s
> was matched with trailing context instead of the optional (s) in the
> end of the word.

That should only happen with lex.  Flex can properly match this pattern.
(That might be what you're saying, I'm just not sure.)

> Is there a way to avoid this dangerous trailing context problem ?

Unfortunately, there's no easy way.  On the other hand, I don't see why
it should be a problem.  Lex's matching is clearly wrong, and I'd hope
that usually the intent remains the same as expressed with the pattern,
so flex's matching will be correct.


Is flex GNU or not?

To: Cameron MacKinnon <mackin@interlog.com>
Subject: Re: Flex documentation bug
In-reply-to: Your message of Mon, 02 Dec 1996 00:07:08 PST.
Date: Sun, 01 Dec 1996 22:29:39 PST
From: Vern Paxson <vern>

> I'm not sure how or where to submit bug reports (documentation or
> otherwise) for the GNU project stuff ...

Well, strictly speaking flex isn't part of the GNU project.  They just
distribute it because no one's written a decent GPL'd lex replacement.
So you should send bugs directly to me.  Those sent to the GNU folks
sometimes find there way to me, but some may drop between the cracks.

> In GNU Info, under the section 'Start Conditions', and also in the man
> page (mine's dated April '95) is a nice little snippet showing how to
> parse C quoted strings into a buffer, defined to be MAX_STR_CONST in
> size. Unfortunately, no overflow checking is ever done ...

This is already mentioned in the manual:

Finally, here's an example of how to  match  C-style  quoted
strings using exclusive start conditions, including expanded
escape sequences (but not including checking  for  a  string
that's too long):

The reason for not doing the overflow checking is that it will needlessly
clutter up an example whose main purpose is just to demonstrate how to
use flex.

The latest release is 2.5.4, by the way, available from ftp.ee.lbl.gov.



To: tsv@cs.UManitoba.CA
Subject: Re: Flex (reg)..
In-reply-to: Your message of Thu, 06 Mar 1997 23:50:16 PST.
Date: Thu, 06 Mar 1997 15:54:19 PST
From: Vern Paxson <vern>

> [:alpha:] ([:alnum:] | \\_)*

If your rule really has embedded blanks as shown above, then it won't
work, as the first blank delimits the rule from the action.  (It wouldn't
even compile ...)  You need instead:


and that should work fine - there's no restriction on what can go inside
of ()'s except for the trailing context operator, '/'.


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I need to scan if-then-else blocks and while loops

To: "Mike Stolnicki" <mstolnic@ford.com>
Subject: Re: FLEX help
In-reply-to: Your message of Fri, 30 May 1997 13:33:27 PDT.
Date: Fri, 30 May 1997 10:46:35 PDT
From: Vern Paxson <vern>

> We'd like to add "if-then-else", "while", and "for" statements to our
> language ...
> We've investigated many possible solutions.  The one solution that seems
> the most reasonable involves knowing the position of a TOKEN in yyin.

I strongly advise you to instead build a parse tree (abstract syntax tree)
and loop over that instead.  You'll find this has major benefits in keeping
your interpreter simple and extensible.

That said, the functionality you mention for get_position and set_position
have been on the to-do list for a while.  As flex is a purely spare-time
project for me, no guarantees when this will be added (in particular, it
for sure won't be for many months to come).



To: Colin Paul Adams <colin@colina.demon.co.uk>
Subject: Re: Flex C++ classes and Bison
In-reply-to: Your message of 09 Aug 1997 17:11:41 PDT.
Date: Fri, 15 Aug 1997 10:48:19 PDT
From: Vern Paxson <vern>

> #define YY_DECL   int yylex (YYSTYPE *lvalp, struct parser_control
> *parm)
> I have been trying  to get this to work as a C++ scanner, but it does
> not appear to be possible (warning that it matches no declarations in
> yyFlexLexer, or something like that).
> Is this supposed to be possible, or is it being worked on (I DID
> notice the comment that scanner classes are still experimental, so I'm
> not too hopeful)?

What you need to do is derive a subclass from yyFlexLexer that provides
the above yylex() method, squirrels away lvalp and parm into member
variables, and then invokes yyFlexLexer::yylex() to do the regular scanning.


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To: Mikael.Latvala@lmf.ericsson.se
Subject: Re: Possible mistake in Flex v2.5 document
In-reply-to: Your message of Fri, 05 Sep 1997 16:07:24 PDT.
Date: Fri, 05 Sep 1997 10:01:54 PDT
From: Vern Paxson <vern>

> In that example you show how to count comment lines when using
> C style /* ... */ comments. My question is, shouldn't you take into
> account a scenario where end of a comment marker occurs inside
> character or string literals?

The scanner certainly needs to also scan character and string literals.
However it does that (there's an example in the man page for strings), the
lexer will recognize the beginning of the literal before it runs across the
embedded "/*".  Consequently, it will finish scanning the literal before it
even considers the possibility of matching "/*".



will match all the text between the ''s (inclusive).  So the lexer
considers this as a token beginning at the first ', and doesn't even
attempt to match other tokens inside it.

I thinnk this subtlety is not worth putting in the manual, as I suspect
it would confuse more people than it would enlighten.



To: "Marty Leisner" <leisner@sdsp.mc.xerox.com>
Subject: Re: flex limitations
In-reply-to: Your message of Sat, 06 Sep 1997 11:27:21 PDT.
Date: Mon, 08 Sep 1997 11:38:08 PDT
From: Vern Paxson <vern>

> %%
> [a-zA-Z]+       /* skip a line */
>                 {  printf("got %s\n", yytext); }
> %%

What version of flex are you using?  If I feed this to 2.5.4, it complains:

	"bug.l", line 5: EOF encountered inside an action
	"bug.l", line 5: unrecognized rule
	"bug.l", line 5: fatal parse error

Not the world's greatest error message, but it manages to flag the problem.

(With the introduction of start condition scopes, flex can't accommodate
an action on a separate line, since it's ambiguous with an indented rule.)

You can get 2.5.4 from ftp.ee.lbl.gov.


Is there a repository for flex scanners?

Not that we know of. You might try asking on comp.compilers.

How can I conditionally compile or preprocess my flex input file?

Flex doesn’t have a preprocessor like C does. You might try using m4, or the C preprocessor plus a sed script to clean up the result.

Where can I find grammars for lex and yacc?

In the sources for flex and bison.

I get an end-of-buffer message for each character scanned.

This will happen if your LexerInput() function returns only one character at a time, which can happen either if you’re scanner is "interactive", or if the streams library on your platform always returns 1 for yyin->gcount().

Solution: override LexerInput() with a version that returns whole buffers.


To: Georg.Rehm@CL-KI.Uni-Osnabrueck.DE
Subject: Re: Flex maximums
In-reply-to: Your message of Mon, 17 Nov 1997 17:16:06 PST.
Date: Mon, 17 Nov 1997 17:16:15 PST
From: Vern Paxson <vern>

> I took a quick look into the flex-sources and altered some #defines in
> flexdefs.h:
> 	#define INITIAL_MNS 64000
> 	#define MNS_INCREMENT 1024000
> 	#define MAXIMUM_MNS 64000

The things to fix are to add a couple of zeroes to:

#define JAMSTATE -32766 /* marks a reference to the state that always jams */
#define MAXIMUM_MNS 31999
#define BAD_SUBSCRIPT -32767
#define MAX_SHORT 32700

and, if you get complaints about too many rules, make the following change too:

	#define YY_TRAILING_MASK 0x200000
	#define YY_TRAILING_HEAD_MASK 0x400000

- Vern

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To: jimmey@lexis-nexis.com (Jimmey Todd)
Subject: Re: FLEX question regarding istream vs ifstream
In-reply-to: Your message of Mon, 08 Dec 1997 15:54:15 PST.
Date: Mon, 15 Dec 1997 13:21:35 PST
From: Vern Paxson <vern>

>         stdin_handle = YY_CURRENT_BUFFER;
>         ifstream fin( "aFile" );
>         yy_switch_to_buffer( yy_create_buffer( fin, YY_BUF_SIZE ) );
> What I'm wanting to do, is pass the contents of a file thru one set
> of rules and then pass stdin thru another set... It works great if, I
> don't use the C++ classes. But since everything else that I'm doing is
> in C++, I thought I'd be consistent.
> The problem is that 'yy_create_buffer' is expecting an istream* as it's
> first argument (as stated in the man page). However, fin is a ifstream
> object. Any ideas on what I might be doing wrong? Any help would be
> appreciated. Thanks!!

You need to pass &fin, to turn it into an ifstream* instead of an ifstream.
Then its type will be compatible with the expected istream*, because ifstream
is derived from istream.


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To: Enda Fadian <fadiane@piercom.ie>
Subject: Re: Question related to Flex man page?
In-reply-to: Your message of Tue, 16 Dec 1997 15:17:34 PST.
Date: Tue, 16 Dec 1997 14:17:09 PST
From: Vern Paxson <vern>

> Can you explain to me what is ment by a long-jump in relation to flex?

Using the longjmp() function while inside yylex() or a routine called by it.

> what is the flex activation frame.

Just yylex()'s stack frame.

> As far as I can see yyrestart will bring me back to the sart of the input
> file and using flex++ isnot really an option!

No, yyrestart() doesn't imply a rewind, even though its name might sound
like it does.  It tells the scanner to flush its internal buffers and
start reading from the given file at its present location.


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To: hassan@larc.info.uqam.ca (Hassan Alaoui)
Subject: Re: Need urgent Help
In-reply-to: Your message of Sat, 20 Dec 1997 19:38:19 PST.
Date: Sun, 21 Dec 1997 21:30:46 PST
From: Vern Paxson <vern>

> /usr/lib/yaccpar: In function `int yyparse()':
> /usr/lib/yaccpar:184: warning: implicit declaration of function `int yylex(...)'
> ld: Undefined symbol
>    _yylex
>    _yyparse
>    _yyin

This is a known problem with Solaris C++ (and/or Solaris yacc).  I believe
the fix is to explicitly insert some 'extern "C"' statements for the
corresponding routines/symbols.


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To: mc0307@mclink.it
Cc: gnu@prep.ai.mit.edu
Subject: Re: [mc0307@mclink.it: Help request]
In-reply-to: Your message of Fri, 12 Dec 1997 17:57:29 PST.
Date: Sun, 21 Dec 1997 22:33:37 PST
From: Vern Paxson <vern>

> This is my definition for float and integer types:
> . . .
> NZD          [1-9]
> ...
> I've tested my program on other lex version (on UNIX Sun Solaris an HP
> UNIX) and it work well, so I think that my definitions are correct.
> There are any differences between Lex and Flex?

There are indeed differences, as discussed in the man page.  The one
you are probably running into is that when flex expands a name definition,
it puts parentheses around the expansion, while lex does not.  There's
an example in the man page of how this can lead to different matching.
Flex's behavior complies with the POSIX standard (or at least with the
last POSIX draft I saw).


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To: hassan@larc.info.uqam.ca (Hassan Alaoui)
Subject: Re: Thanks
In-reply-to: Your message of Mon, 22 Dec 1997 16:06:35 PST.
Date: Mon, 22 Dec 1997 14:35:05 PST
From: Vern Paxson <vern>

> Thank you very much for your help. I compile and link well with C++ while
> declaring 'yylex ...' extern, But a little problem remains. I get a
> segmentation default when executing ( I linked with lfl library) while it
> works well when using LEX instead of flex. Do you have some ideas about the
> reason for this ?

The one possible reason for this that comes to mind is if you've defined
yytext as "extern char yytext[]" (which is what lex uses) instead of
"extern char *yytext" (which is what flex uses).  If it's not that, then
I'm afraid I don't know what the problem might be.


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To: "Bart Niswonger" <NISWONGR@almaden.ibm.com>
Subject: Re: flex 2.5: c++ scanners & start conditions
In-reply-to: Your message of Tue, 06 Jan 1998 10:34:21 PST.
Date: Tue, 06 Jan 1998 19:19:30 PST
From: Vern Paxson <vern>

> The problem is that when I do this (using %option c++) start
> conditions seem to not apply.

The BEGIN macro modifies the yy_start variable.  For C scanners, this
is a static with scope visible through the whole file.  For C++ scanners,
it's a member variable, so it only has visible scope within a member
function.  Your lexbegin() routine is not a member function when you
build a C++ scanner, so it's not modifying the correct yy_start.  The
diagnostic that indicates this is that you found you needed to add
a declaration of yy_start in order to get your scanner to compile when
using C++; instead, the correct fix is to make lexbegin() a member
function (by deriving from yyFlexLexer).


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To: "Boris Zinin" <boris@ippe.rssi.ru>
Subject: Re: current position in flex buffer
In-reply-to: Your message of Mon, 12 Jan 1998 18:58:23 PST.
Date: Mon, 12 Jan 1998 12:03:15 PST
From: Vern Paxson <vern>

> The problem is how to determine the current position in flex active
> buffer when a rule is matched....

You will need to keep track of this explicitly, such as by redefining
YY_USER_ACTION to count the number of characters matched.

The latest flex release, by the way, is 2.5.4, available from ftp.ee.lbl.gov.


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To: Bik.Dhaliwal@bis.org
Subject: Re: Flex question
In-reply-to: Your message of Mon, 26 Jan 1998 13:05:35 PST.
Date: Tue, 27 Jan 1998 22:41:52 PST
From: Vern Paxson <vern>

> That requirement involves knowing
> the character position at which a particular token was matched
> in the lexer.

The way you have to do this is by explicitly keeping track of where
you are in the file, by counting the number of characters scanned
for each token (available in yyleng).  It may prove convenient to
do this by redefining YY_USER_ACTION, as described in the manual.


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To: Vladimir Alexiev <vladimir@cs.ualberta.ca>
Subject: Re: flex: how to control start condition from parser?
In-reply-to: Your message of Mon, 26 Jan 1998 05:50:16 PST.
Date: Tue, 27 Jan 1998 22:45:37 PST
From: Vern Paxson <vern>

> It seems useful for the parser to be able to tell the lexer about such
> context dependencies, because then they don't have to be limited to
> local or sequential context.

One way to do this is to have the parser call a stub routine that's
included in the scanner's .l file, and consequently that has access ot
BEGIN.  The only ugliness is that the parser can't pass in the state
it wants, because those aren't visible - but if you don't have many
such states, then using a different set of names doesn't seem like
to much of a burden.

While generating a .h file like you suggests is certainly cleaner,
flex development has come to a virtual stand-still :-(, so a workaround
like the above is much more pragmatic than waiting for a new feature.


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To: Barbara Denny <denny@3com.com>
Subject: Re: freebsd flex bug?
In-reply-to: Your message of Fri, 30 Jan 1998 12:00:43 PST.
Date: Fri, 30 Jan 1998 12:42:32 PST
From: Vern Paxson <vern>

> lex.yy.c:1996: parse error before `='

This is the key, identifying this error.  (It may help to pinpoint
it by using flex -L, so it doesn't generate #line directives in its
output.)  I will bet you heavy money that you have a start condition
name that is also a variable name, or something like that; flex spits
out #define's for each start condition name, mapping them to a number,
so you can wind up with:

	%x foo
	void bar()
		int foo = 3;

and the penultimate will turn into "int 1 = 3" after C preprocessing,
since flex will put "#define foo 1" in the generated scanner.


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To: Maurice Petrie <mpetrie@infoscigroup.com>
Subject: Re: Lost flex .l file
In-reply-to: Your message of Mon, 02 Feb 1998 14:10:01 PST.
Date: Mon, 02 Feb 1998 11:15:12 PST
From: Vern Paxson <vern>

> I am curious as to
> whether there is a simple way to backtrack from the generated source to
> reproduce the lost list of tokens we are searching on.

In theory, it's straight-forward to go from the DFA representation
back to a regular-expression representation - the two are isomorphic.
In practice, a huge headache, because you have to unpack all the tables
back into a single DFA representation, and then write a program to munch
on that and translate it into an RE.

Sorry for the less-than-happy news ...


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To: jimmey@lexis-nexis.com (Jimmey Todd)
Subject: Re: Flex performance question
In-reply-to: Your message of Thu, 19 Feb 1998 11:01:17 PST.
Date: Thu, 19 Feb 1998 08:48:51 PST
From: Vern Paxson <vern>

> What I have found, is that the smaller the data chunk, the faster the
> program executes. This is the opposite of what I expected. Should this be
> happening this way?

This is exactly what will happen if your input file has embedded NULs.
From the man page:

A final note: flex is slow when matching NUL's, particularly
when  a  token  contains multiple NUL's.  It's best to write
rules which match short amounts of text if it's  anticipated
that the text will often include NUL's.

So that's the first thing to look for.


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To: jimmey@lexis-nexis.com (Jimmey Todd)
Subject: Re: Flex performance question
In-reply-to: Your message of Thu, 19 Feb 1998 11:01:17 PST.
Date: Thu, 19 Feb 1998 15:42:25 PST
From: Vern Paxson <vern>

So there are several problems.

First, to go fast, you want to match as much text as possible, which
your scanners don't in the case that what they're scanning is *not*
a <RN> tag.  So you want a rule like:


Second, C++ scanners are particularly slow if they're interactive,
which they are by default.  Using -B speeds it up by a factor of 3-4
on my workstation.

Third, C++ scanners that use the istream interface are slow, because
of how poorly implemented istream's are.  I built two versions of
the following scanner:


and the C version inhales a 2.5MB file on my workstation in 0.8 seconds.
The C++ istream version, using -B, takes 3.8 seconds.


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To: "Frescatore, David (CRD, TAD)" <frescatore@exc01crdge.crd.ge.com>
Subject: Re: FLEX 2.5 & THE YEAR 2000
In-reply-to: Your message of Wed, 03 Jun 1998 11:26:22 PDT.
Date: Wed, 03 Jun 1998 10:22:26 PDT
From: Vern Paxson <vern>

> I am researching the Y2K problem with General Electric R&D
> and need to know if there are any known issues concerning
> the above mentioned software and Y2K regardless of version.

There shouldn't be, all it ever does with the date is ask the system
for it and then print it out.


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To: "Hans Dermot Doran" <htd@ibhdoran.com>
Subject: Re: flex problem
In-reply-to: Your message of Wed, 15 Jul 1998 21:30:13 PDT.
Date: Tue, 21 Jul 1998 14:23:34 PDT
From: Vern Paxson <vern>

> To overcome this, I gets() the stdin into a string and lex the string. The
> string is lexed OK except that the end of string isn't lexed properly
> (yy_scan_string()), that is the lexer dosn't recognise the end of string.

Flex doesn't contain mechanisms for recognizing buffer endpoints.  But if
you use fgets instead (which you should anyway, to protect against buffer
overflows), then the final \n will be preserved in the string, and you can
scan that in order to find the end of the string.


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To: soumen@almaden.ibm.com
Subject: Re: Flex++ 2.5.3 instance member vs. static member
In-reply-to: Your message of Mon, 27 Jul 1998 02:10:04 PDT.
Date: Tue, 28 Jul 1998 01:10:34 PDT
From: Vern Paxson <vern>

> %{
> int mylineno = 0;
> %}
> ws      [ \t]+
> alpha   [A-Za-z]
> dig     [0-9]
> %%
> Now you'd expect mylineno to be a member of each instance of class
> yyFlexLexer, but is this the case?  A look at the lex.yy.cc file seems to
> indicate otherwise; unless I am missing something the declaration of
> mylineno seems to be outside any class scope.
> How will this work if I want to run a multi-threaded application with each
> thread creating a FlexLexer instance?

Derive your own subclass and make mylineno a member variable of it.


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To: Adoram Rogel <adoram@hybridge.com>
Subject: Re: More than 32K states change hangs
In-reply-to: Your message of Tue, 04 Aug 1998 16:55:39 PDT.
Date: Tue, 04 Aug 1998 22:28:45 PDT
From: Vern Paxson <vern>

> Vern Paxson,
> I followed your advice, posted on Usenet bu you, and emailed to me
> personally by you, on how to overcome the 32K states limit. I'm running
> on Linux machines.
> I took the full source of version 2.5.4 and did the following changes in
> flexdef.h:
> #define JAMSTATE -327660
> #define MAXIMUM_MNS 319990
> #define BAD_SUBSCRIPT -327670
> #define MAX_SHORT 327000
> and compiled.
> All looked fine, including check and bigcheck, so I installed.

Hmmm, you shouldn't increase MAX_SHORT, though looking through my email
archives I see that I did indeed recommend doing so.  Try setting it back
to 32700; that should suffice that you no longer need -Ca.  If it still
hangs, then the interesting question is - where?

> Compiling the same hanged program with a out-of-the-box (RedHat 4.2
> distribution of Linux)
> flex 2.5.4 binary works.

Since Linux comes with source code, you should diff it against what
you have to see what problems they missed.

> Should I always compile with the -Ca option now ? even short and simple
> filters ?

No, definitely not.  It's meant to be for those situations where you
absolutely must squeeze every last cycle out of your scanner.


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To: "Schmackpfeffer, Craig" <Craig.Schmackpfeffer@usa.xerox.com>
Subject: Re: flex output for static code portion
In-reply-to: Your message of Tue, 11 Aug 1998 11:55:30 PDT.
Date: Mon, 17 Aug 1998 23:57:42 PDT
From: Vern Paxson <vern>

> I would like to use flex under the hood to generate a binary file
> containing the data structures that control the parse.

This has been on the wish-list for a long time.  In principle it's
straight-forward - you redirect mkdata() et al's I/O to another file,
and modify the skeleton to have a start-up function that slurps these
into dynamic arrays.  The concerns are (1) the scanner generation code
is hairy and full of corner cases, so it's easy to get surprised when
going down this path :-( ; and (2) being careful about buffering so
that when the tables change you make sure the scanner starts in the
correct state and reading at the right point in the input file.

> I was wondering if you know of anyone who has used flex in this way.

I don't - but it seems like a reasonable project to undertake (unlike
numerous other flex tweaks :-).


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From: Georg Rehm <georg@hal.cl-ki.uni-osnabrueck.de>
Message-Id: <199808200747.JAA34834@hal.cl-ki.uni-osnabrueck.de>
Subject: "flex scanner push-back overflow"
To: vern@ee.lbl.gov
Date: Thu, 20 Aug 1998 09:47:54 +0200 (MEST)
Reply-To: Georg.Rehm@CL-KI.Uni-Osnabrueck.DE
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Hi Vern,

Yesterday, I encountered a strange problem: I use the macro processor m4
to include some lengthy lists into a .l file. Following is a flex macro
definition that causes some serious pain in my neck:

AUTHOR           ("A. Boucard / L. Boucard"|"A. Dastarac / M. Levent"|"A.Boucaud / L.Boucaud"|"Abderrahim Lamchichi"|"Achmat Dangor"|"Adeline Toullier"|"Adewale Maja-Pearce"|"Ahmed Ziri"|"Akram Ellyas"|"Alain Bihr"|"Alain Gresh"|"Alain Guillemoles"|"Alain Joxe"|"Alain Morice"|"Alain Renon"|"Alain Zecchini"|"Albert Memmi"|"Alberto Manguel"|"Alex De Waal"|"Alfonso Artico"| [...])

The complete list contains about 10kB. When I try to "flex" this file
(on a Solaris 2.6 machine, using a modified flex 2.5.4 (I only increased
some of the predefined values in flexdefs.h) I get the error:

myflex/flex -8  sentag.tmp.l
flex scanner push-back overflow

When I remove the slashes in the macro definition everything works fine.
As I understand it, the double quotes escape the slash-character so it
really means "/" and not "trailing context". Furthermore, I tried to
escape the slashes with backslashes, but with no use, the same error message
appeared when flexing the code.

Do you have an idea what's going on here?

Greetings from Germany,
Georg Rehm                                     georg@cl-ki.uni-osnabrueck.de
Institute for Semantic Information Processing, University of Osnabrueck, FRG

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To: Georg.Rehm@CL-KI.Uni-Osnabrueck.DE
Subject: Re: "flex scanner push-back overflow"
In-reply-to: Your message of Thu, 20 Aug 1998 09:47:54 PDT.
Date: Thu, 20 Aug 1998 07:05:35 PDT
From: Vern Paxson <vern>

> myflex/flex -8  sentag.tmp.l
> flex scanner push-back overflow

Flex itself uses a flex scanner.  That scanner is running out of buffer
space when it tries to unput() the humongous macro you've defined.  When
you remove the '/'s, you make it small enough so that it fits in the buffer;
removing spaces would do the same thing.

The fix is to either rethink how come you're using such a big macro and
perhaps there's another/better way to do it; or to rebuild flex's own
scan.c with a larger value for

	#define YY_BUF_SIZE 16384

- Vern

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To: Jan Kort <jan@research.techforce.nl>
Subject: Re: Flex
In-reply-to: Your message of Fri, 04 Sep 1998 12:18:43 +0200.
Date: Sat, 05 Sep 1998 00:59:49 PDT
From: Vern Paxson <vern>

> %%
> "TEST1\n"       { fprintf(stderr, "TEST1\n"); yyless(5); }
> ^\n             { fprintf(stderr, "empty line\n"); }
> .               { }
> \n              { fprintf(stderr, "new line\n"); }
> %%
> -- input ---------------------------------------
> -- output --------------------------------------
> empty line
> ------------------------------------------------

IMHO, it's not clear whether or not this is in fact a bug.  It depends
on whether you view yyless() as backing up in the input stream, or as
pushing new characters onto the beginning of the input stream.  Flex
interprets it as the latter (for implementation convenience, I'll admit),
and so considers the newline as in fact matching at the beginning of a
line, as after all the last token scanned an entire line and so the
scanner is now at the beginning of a new line.

I agree that this is counter-intuitive for yyless(), given its
functional description (it's less so for unput(), depending on whether
you're unput()'ing new text or scanned text).  But I don't plan to
change it any time soon, as it's a pain to do so.  Consequently,
you do indeed need to use yy_set_bol() and YY_AT_BOL() to tweak
your scanner into the behavior you desire.

Sorry for the less-than-completely-satisfactory answer.


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To: Patrick Krusenotto <krusenot@mac-info-link.de>
Subject: Re: Problems with restarting flex-2.5.2-generated scanner
In-reply-to: Your message of Thu, 24 Sep 1998 10:14:07 PDT.
Date: Thu, 24 Sep 1998 23:28:43 PDT
From: Vern Paxson <vern>

> I am using flex-2.5.2 and bison 1.25 for Solaris and I am desperately
> trying to make my scanner restart with a new file after my parser stops
> with a parse error. When my compiler restarts, the parser always
> receives the token after the token (in the old file!) that caused the
> parser error.

I suspect the problem is that your parser has read ahead in order
to attempt to resolve an ambiguity, and when it's restarted it picks
up with that token rather than reading a fresh one.  If you're using
yacc, then the special "error" production can sometimes be used to
consume tokens in an attempt to get the parser into a consistent state.


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To: Henric Jungheim <junghelh@pe-nelson.com>
Subject: Re: flex 2.5.4a
In-reply-to: Your message of Tue, 27 Oct 1998 16:41:42 PST.
Date: Tue, 27 Oct 1998 16:50:14 PST
From: Vern Paxson <vern>

> This brings up a feature request:  How about a command line
> option to specify the filename when reading from stdin?  That way one
> doesn't need to create a temporary file in order to get the "#line"
> directives to make sense.

Use -o combined with -t (per the man page description of -o).

> P.S., Is there any simple way to use non-blocking IO to parse multiple
> streams?

Simple, no.

One approach might be to return a magic character on EWOULDBLOCK and
have a rule

	.*<magic-character>	// put back .*, eat magic character

This is off the top of my head, not sure it'll work.


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To: "Repko, Billy D" <billy.d.repko@intel.com>
Subject: Re: Compiling scanners
In-reply-to: Your message of Wed, 13 Jan 1999 10:52:47 PST.
Date: Thu, 14 Jan 1999 00:25:30 PST
From: Vern Paxson <vern>

> It appears that maybe it cannot find the lfl library.

The Makefile in the distribution builds it, so you should have it.
It's exceedingly trivial, just a main() that calls yylex() and
a yyrap() that always returns 1.

> %%
>       \n      ++num_lines; ++num_chars;
>       .       ++num_chars;

You can't indent your rules like this - that's where the errors are coming
from.  Flex copies indented text to the output file, it's how you do things

	int num_lines_seen = 0;

to declare local variables.


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To: Erick Branderhorst <Erick.Branderhorst@asml.nl>
Subject: Re: flex input buffer
In-reply-to: Your message of Tue, 09 Feb 1999 13:53:46 PST.
Date: Tue, 09 Feb 1999 21:03:37 PST
From: Vern Paxson <vern>

> In the flex.skl file the size of the default input buffers is set.  Can you
> explain why this size is set and why it is such a high number.

It's large to optimize performance when scanning large files.  You can
safely make it a lot lower if needed.


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To: "Guido Minnen" <guidomi@cogs.susx.ac.uk>
Subject: Re: Flex error message
In-reply-to: Your message of Wed, 24 Feb 1999 15:31:46 PST.
Date: Thu, 25 Feb 1999 00:11:31 PST
From: Vern Paxson <vern>

> I'm extending a larger scanner written in Flex and I keep running into
> problems. More specifically, I get the error message:
> "flex: input rules are too complicated (>= 32000 NFA states)"

Increase the definitions in flexdef.h for:

#define JAMSTATE -32766 /* marks a reference to the state that always j
ams */
#define MAXIMUM_MNS 31999
#define BAD_SUBSCRIPT -32767

recompile everything, and it should all work.


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To: "Dmitriy Goldobin" <gold@ems.chel.su>
Subject: Re: FLEX trouble
In-reply-to: Your message of Mon, 31 May 1999 18:44:49 PDT.
Date: Tue, 01 Jun 1999 00:15:07 PDT
From: Vern Paxson <vern>

>   I have a trouble with FLEX. Why rule "/*".*"*/" work properly,=20
> but rule "/*"(.|\n)*"*/" don't work ?

The second of these will have to scan the entire input stream (because
"(.|\n)*" matches an arbitrary amount of any text) in order to see if
it ends with "*/", terminating the comment.  That potentially will overflow
the input buffer.

>   More complex rule "/*"([^*]|(\*/[^/]))*"*/ give an error
> 'unrecognized rule'.

You can't use the '/' operator inside parentheses.  It's not clear
what "(a/b)*" actually means.

>   I now use workaround with state <comment>, but single-rule is
> better, i think.

Single-rule is nice but will always have the problem of either setting
restrictions on comments (like not allowing multi-line comments) and/or
running the risk of consuming the entire input stream, as noted above.


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To: vern@ee.lbl.gov
Date: Tue, 15 Jun 1999 08:55:43 -0700
From: "Aki Niimura" <neko@my-deja.com>
Message-ID: <KNONDOHDOBGAEAAA@my-deja.com>
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Subject: A question on flex C++ scanner
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Dear Dr. Paxon,

I have been using flex for years.
It works very well on many projects.
Most case, I used it to generate a scanner on C language.
However, one project I needed to generate  a scanner
on C++ lanuage. Thanks to your enhancement, flex did
the job.

Currently, I'm working on enhancing my previous project.
I need to deal with multiple input streams (recursive
inclusion) in this scanner (C++).
I did similar thing for another scanner (C) as you
explained in your documentation.

The generated scanner (C++) has necessary methods:
- switch_to_buffer(struct yy_buffer_state *b)
- yy_create_buffer(istream *is, int sz)
- yy_delete_buffer(struct yy_buffer_state *b)

However, I couldn't figure out how to access current
buffer (yy_current_buffer).

yy_current_buffer is a protected member of yyFlexLexer.
I can't access it directly.
Then, I thought yy_create_buffer() with is = 0 might
return current stream buffer. But it seems not as far
as I checked the source. (flex 2.5.4)

I went through the Web in addition to Flex documentation.
However, it hasn't been successful, so far.

It is not my intention to bother you, but, can you
comment about how to obtain the current stream buffer?

Your response would be highly appreciated.

Best regards,
Aki Niimura

--== Sent via Deja.com http://www.deja.com/ ==--
Share what you know. Learn what you don't.

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To: neko@my-deja.com
Subject: Re: A question on flex C++ scanner
In-reply-to: Your message of Tue, 15 Jun 1999 08:55:43 PDT.
Date: Tue, 15 Jun 1999 09:04:24 PDT
From: Vern Paxson <vern>

> However, I couldn't figure out how to access current
> buffer (yy_current_buffer).

Derive your own subclass from yyFlexLexer.


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To: "Stones, Darren" <Darren.Stones@nectech.co.uk>
Subject: Re: You're the man to see?
In-reply-to: Your message of Wed, 23 Jun 1999 11:10:29 PDT.
Date: Wed, 23 Jun 1999 09:01:40 PDT
From: Vern Paxson <vern>

> I hope you can help me.  I am using Flex and Bison to produce an interpreted
> language.  However all goes well until I try to implement an IF statement or
> a WHILE.  I cannot get this to work as the parser parses all the conditions
> eg. the TRUE and FALSE conditons to check for a rule match.  So I cannot
> make a decision!!

You need to use the parser to build a parse tree (= abstract syntax trwee),
and when that's all done you recursively evaluate the tree, binding variables
to values at that time.


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To: Petr Danecek <petr@ics.cas.cz>
Subject: Re: flex - question
In-reply-to: Your message of Mon, 28 Jun 1999 19:21:41 PDT.
Date: Fri, 02 Jul 1999 16:52:13 PDT
From: Vern Paxson <vern>

> file, it takes an enormous amount of time. It is funny, because the
> source code has only 12 rules!!! I think it looks like an exponencial
> growth.

Right, that's the problem - some patterns (those with a lot of
ambiguity, where yours has because at any given time the scanner can
be in the middle of all sorts of combinations of the different
rules) blow up exponentially.

For your rules, there is an easy fix.  Change the ".*" that comes fater
the directory name to "[^ ]*".  With that in place, the rules are no
longer nearly so ambiguous, because then once one of the directories
has been matched, no other can be matched (since they all require a
leading blank).

If that's not an acceptable solution, then you can enter a start state
to pick up the .*\n after each directory is matched.

Also note that for speed, you'll want to add a ".*" rule at the end,
otherwise rules that don't match any of the patterns will be matched
very slowly, a character at a time.


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To: Tielman Koekemoer <tielman@spi.co.za>
Subject: Re: Please help.
In-reply-to: Your message of Thu, 08 Jul 1999 13:20:37 PDT.
Date: Thu, 08 Jul 1999 08:20:39 PDT
From: Vern Paxson <vern>

> I was hoping you could help me with my problem.
> I tried compiling (gnu)flex on a Solaris 2.4 machine
> but when I ran make (after configure) I got an error.
> --------------------------------------------------------------
> gcc -c -I. -I. -g -O parse.c
> ./flex -t -p  ./scan.l >scan.c
> sh: ./flex: not found
> *** Error code 1
> make: Fatal error: Command failed for target `scan.c'
> -------------------------------------------------------------
> What's strange to me is that I'm only
> trying to install flex now. I then edited the Makefile to
> and changed where it says "FLEX = flex" to "FLEX = lex"
> ( lex: the native Solaris one ) but then it complains about
> the "-p" option. Is there any way I can compile flex without
> using flex or lex?
> Thanks so much for your time.

You managed to step on the bootstrap sequence, which first copies
initscan.c to scan.c in order to build flex.  Try fetching a fresh
distribution from ftp.ee.lbl.gov.  (Or you can first try removing
".bootstrap" and doing a make again.)


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To: Tielman Koekemoer <tielman@spi.co.za>
Subject: Re: Please help.
In-reply-to: Your message of Fri, 09 Jul 1999 09:16:14 PDT.
Date: Fri, 09 Jul 1999 00:27:20 PDT
From: Vern Paxson <vern>

> First I removed .bootstrap (and ran make) - no luck. I downloaded the
> software but I still have the same problem. Is there anything else I
> could try.


	cp initscan.c scan.c
	touch scan.c
	make scan.o

If this last tries to first build scan.c from scan.l using ./flex, then
your "make" is broken, in which case compile scan.c to scan.o by hand.


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To: Sumanth Kamenani <skamenan@crl.nmsu.edu>
Subject: Re: Error
In-reply-to: Your message of Mon, 19 Jul 1999 23:08:41 PDT.
Date: Tue, 20 Jul 1999 00:18:26 PDT
From: Vern Paxson <vern>

> I am getting a compilation error. The error is given as "unknown symbol- yylex".

The parser relies on calling yylex(), but you're instead using the C++ scanning
class, so you need to supply a yylex() "glue" function that calls an instance
scanner of the scanner (e.g., "scanner->yylex()").


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To: daniel@synchrods.synchrods.COM (Daniel Senderowicz)
Subject: Re: lex
In-reply-to: Your message of Mon, 22 Nov 1999 11:19:04 PST.
Date: Tue, 23 Nov 1999 15:54:30 PST
From: Vern Paxson <vern>

Well, your problem is the

switch (yybgin-yysvec-1) {      /* witchcraft */

at the beginning of lex rules.  "witchcraft" == "non-portable".  It's
assuming knowledge of the AT&T lex's internal variables.

For flex, you can probably do the equivalent using a switch on YYSTATE.


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To: archow@hss.hns.com
Subject: Re: Regarding distribution of flex and yacc based grammars
In-reply-to: Your message of Sun, 19 Dec 1999 17:50:24 +0530.
Date: Wed, 22 Dec 1999 01:56:24 PST
From: Vern Paxson <vern>

> When we provide the customer with an object code distribution, is it
> necessary for us to provide source
> for the generated C files from flex and bison since they are generated by
> flex and bison ?

For flex, no.  I don't know what the current state of this is for bison.

> Also, is there any requrirement for us to neccessarily  provide source for
> the grammar files which are fed into flex and bison ?

Again, for flex, no.

See the file "COPYING" in the flex distribution for the legalese.


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To: Martin Gallwey <gallweym@hyperion.moe.ul.ie>
Subject: Re: Flex, and self referencing rules
In-reply-to: Your message of Sun, 20 Feb 2000 01:01:21 PST.
Date: Sat, 19 Feb 2000 18:33:16 PST
From: Vern Paxson <vern>

> However, I do not use unput anywhere. I do use self-referencing
> rules like this:
> UnaryExpr               ({UnionExpr})|("-"{UnaryExpr})

You can't do this - flex is *not* a parser like yacc (which does indeed
allow recursion), it is a scanner that's confined to regular expressions.



To: slg3@lehigh.edu (SAMUEL L. GULDEN)
Subject: Re: Flex problem
In-reply-to: Your message of Thu, 02 Mar 2000 12:29:04 PST.
Date: Thu, 02 Mar 2000 23:00:46 PST
From: Vern Paxson <vern>

If this is exactly your program:

> digit [0-9]
> digits {digit}+
> whitespace [ \t\n]+
> %%
> "[" { printf("open_brac\n");}
> "]" { printf("close_brac\n");}
> "+" { printf("addop\n");}
> "*" { printf("multop\n");}
> {digits} { printf("NUMBER = %s\n", yytext);}
> whitespace ;

then the problem is that the last rule needs to be "{whitespace}" !


What is the difference between YYLEX_PARAM and YY_DECL?

YYLEX_PARAM is not a flex symbol. It is for Bison. It tells Bison to pass extra params when it calls yylex() from the parser.

YY_DECL is the Flex declaration of yylex. The default is similar to this:

#define int yy_lex ()

Why do I get "conflicting types for yylex" error?

This is a compiler error regarding a generated Bison parser, not a Flex scanner. It means you need a prototype of yylex() in the top of the Bison file. Be sure the prototype matches YY_DECL.

How do I access the values set in a Flex action from within a Bison action?

With $1, $2, $3, etc. These are called "Semantic Values" in the Bison manual. See the GNU Bison Manual.

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Appendix A Appendices

A.1 Makefiles and Flex

In this appendix, we provide tips for writing Makefiles to build your scanners.

In a traditional build environment, we say that the .c files are the sources, and the .o files are the intermediate files. When using flex, however, the .l files are the sources, and the generated .c files (along with the .o files) are the intermediate files. This requires you to carefully plan your Makefile.

Modern make programs understand that foo.l is intended to generate lex.yy.c or foo.c, and will behave accordingly45. The following Makefile does not explicitly instruct make how to build foo.c from foo.l. Instead, it relies on the implicit rules of the make program to build the intermediate file, scan.c:

    # Basic Makefile -- relies on implicit rules
    # Creates "myprogram" from "scan.l" and "myprogram.c"
    myprogram: scan.o myprogram.o
    scan.o: scan.l

For simple cases, the above may be sufficient. For other cases, you may have to explicitly instruct make how to build your scanner. The following is an example of a Makefile containing explicit rules:

    # Basic Makefile -- provides explicit rules
    # Creates "myprogram" from "scan.l" and "myprogram.c"
    myprogram: scan.o myprogram.o
            $(CC) -o $@  $(LDFLAGS) $^

    myprogram.o: myprogram.c
            $(CC) $(CPPFLAGS) $(CFLAGS) -o $@ -c $^

    scan.o: scan.c
            $(CC) $(CPPFLAGS) $(CFLAGS) -o $@ -c $^

    scan.c: scan.l
            $(LEX) $(LFLAGS) -o $@ $^

            $(RM) *.o scan.c

Notice in the above example that scan.c is in the clean target. This is because we consider the file scan.c to be an intermediate file.

Finally, we provide a realistic example of a flex scanner used with a bison parser6. There is a tricky problem we have to deal with. Since a flex scanner will typically include a header file (e.g., y.tab.h) generated by the parser, we need to be sure that the header file is generated BEFORE the scanner is compiled. We handle this case in the following example:

    # Makefile example -- scanner and parser.
    # Creates "myprogram" from "scan.l", "parse.y", and "myprogram.c"
    LEX     = flex
    YACC    = bison -y
    YFLAGS  = -d
    objects = scan.o parse.o myprogram.o

    myprogram: $(objects)
    scan.o: scan.l parse.c
    parse.o: parse.y
    myprogram.o: myprogram.c

In the above example, notice the line,

    scan.o: scan.l parse.c

, which lists the file parse.c (the generated parser) as a dependency of scan.o. We want to ensure that the parser is created before the scanner is compiled, and the above line seems to do the trick. Feel free to experiment with your specific implementation of make.

For more details on writing Makefiles, see The GNU Make Manual.

A.2 C Scanners with Bison Parsers

This section describes the flex features useful when integrating flex with GNU bison7. Skip this section if you are not using bison with your scanner. Here we discuss only the flex half of the flex and bison pair. We do not discuss bison in any detail. For more information about generating bison parsers, see the GNU Bison Manual.

A compatible bison scanner is generated by declaring ‘%option bison-bridge’ or by supplying ‘--bison-bridge’ when invoking flex from the command line. This instructs flex that the macro yylval may be used. The data type for yylval, YYSTYPE, is typically defined in a header file, included in section 1 of the flex input file. For a list of functions and macros available, See bison-functions.

The declaration of yylex becomes,

      int yylex ( YYSTYPE * lvalp, yyscan_t scanner );

If %option bison-locations is specified, then the declaration becomes,

      int yylex ( YYSTYPE * lvalp, YYLTYPE * llocp, yyscan_t scanner );

Note that the macros yylval and yylloc evaluate to pointers. Support for yylloc is optional in bison, so it is optional in flex as well. The following is an example of a flex scanner that is compatible with bison.

    /* Scanner for "C" assignment statements... sort of. */
    #include "y.tab.h"  /* Generated by bison. */

    %option bison-bridge bison-locations

    [[:digit:]]+  { yylval->num = atoi(yytext);   return NUMBER;}
    [[:alnum:]]+  { yylval->str = strdup(yytext); return STRING;}
    "="|";"       { return yytext[0];}
    .  {}

As you can see, there really is no magic here. We just use yylval as we would any other variable. The data type of yylval is generated by bison, and included in the file y.tab.h. Here is the corresponding bison parser:

    /* Parser to convert "C" assignments to lisp. */
    /* Pass the argument to yyparse through to yylex. */
    #define YYPARSE_PARAM scanner
    #define YYLEX_PARAM   scanner
    %union {
        int num;
        char* str;
    %token <str> STRING
    %token <num> NUMBER
        STRING '=' NUMBER ';' {
            printf( "(setf %s %d)", $1, $3 );

A.3 M4 Dependency

The macro processor m48 must be installed wherever flex is installed. flex invokes ‘m4’, found by searching the directories in the PATH environment variable. Any code you place in section 1 or in the actions will be sent through m4. Please follow these rules to protect your code from unwanted m4 processing.

  • Do not use symbols that begin with, ‘m4_’, such as, ‘m4_define’, or ‘m4_include’, since those are reserved for m4 macro names. If for some reason you need m4_ as a prefix, use a preprocessor #define to get your symbol past m4 unmangled.
  • Do not use the strings ‘[[’ or ‘]]’ anywhere in your code. The former is not valid in C, except within comments and strings, but the latter is valid in code such as x[y[z]]. The solution is simple. To get the literal string "]]", use "]""]". To get the array notation x[y[z]], use x[y[z] ]. Flex will attempt to detect these sequences in user code, and escape them. However, it’s best to avoid this complexity where possible, by removing such sequences from your code.

m4 is only required at the time you run flex. The generated scanner is ordinary C or C++, and does not require m4.

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A.4 Common Patterns

This appendix provides examples of common regular expressions you might use in your scanner.

A.4.1 Numbers

C99 decimal constant


C99 hexadecimal constant


C99 octal constant


C99 floating point constant
 {dseq}      ([[:digit:]]+)
 {dseq_opt}  ([[:digit:]]*)
 {frac}      (({dseq_opt}"."{dseq})|{dseq}".")
 {exp}       ([eE][+-]?{dseq})
 {exp_opt}   ({exp}?)
 {fsuff}     [flFL]
 {fsuff_opt} ({fsuff}?)
 {hpref}     (0[xX])
 {hdseq}     ([[:xdigit:]]+)
 {hdseq_opt} ([[:xdigit:]]*)
 {hfrac}     (({hdseq_opt}"."{hdseq})|({hdseq}"."))
 {bexp}      ([pP][+-]?{dseq})
 {dfc}       (({frac}{exp_opt}{fsuff_opt})|({dseq}{exp}{fsuff_opt}))
 {hfc}       (({hpref}{hfrac}{bexp}{fsuff_opt})|({hpref}{hdseq}{bexp}{fsuff_opt}))

 {c99_floating_point_constant}  ({dfc}|{hfc})

See C99 section for the gory details.

A.4.2 Identifiers

C99 Identifier
ucn        ((\\u([[:xdigit:]]{4}))|(\\U([[:xdigit:]]{8})))
nondigit    [_[:alpha:]]
c99_id     ([_[:alpha:]]|{ucn})([_[:alnum:]]|{ucn})*

Technically, the above pattern does not encompass all possible C99 identifiers, since C99 allows for "implementation-defined" characters. In practice, C compilers follow the above pattern, with the addition of the ‘$’ character.

UTF-8 Encoded Unicode Code Point

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A.4.3 Quoted Constructs

C99 String Literal


C99 Comment


Note that in C99, a ‘//’-style comment may be split across lines, and, contrary to popular belief, does not include the trailing ‘\n’ character.

A better way to scan ‘/* */’ comments is by line, rather than matching possibly huge comments all at once. This will allow you to scan comments of unlimited length, as long as line breaks appear at sane intervals. This is also more efficient when used with automatic line number processing. See option-yylineno.

    "/*"      BEGIN(COMMENT);
    "*/"      BEGIN(0);
    [^*\n]+   ;
    "*"[^/]   ;
    \n        ;

A.4.4 Addresses

IPv4 Address
dec-octet     [0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5]
IPv4address   {dec-octet}\.{dec-octet}\.{dec-octet}\.{dec-octet}
IPv6 Address
h16           [0-9A-Fa-f]{1,4}
ls32          {h16}:{h16}|{IPv4address}
IPv6address   ({h16}:){6}{ls32}|

See RFC 2373 for details. Note that you have to fold the definition of IPv6address into one line and that it also matches the “unspecified address” “::”.



This pattern is nearly useless, since it allows just about any character to appear in a URI, including spaces and control characters. See RFC 2396 for details.

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Concept Index

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Index Entry  Section

$ as normal character in patterns: Patterns

%array, advantages of: Matching
%array, use of: Matching
%array, with C++: Matching
%option noyywrapp: Generated Scanner
%pointer, and unput(): Actions
%pointer, use of: Matching
%top: Definitions Section
%{ and %}, in Definitions Section: Definitions Section
%{ and %}, in Rules Section: Actions

<<EOF>>, use of: EOF

[] in patterns: Patterns

^ as non-special character in patterns: Patterns

|, in actions: Actions
|, use of: Actions

accessor functions, use of: Accessor Methods
actions: Actions
actions, embedded C strings: Actions
actions, redefining YY_BREAK: Misc Macros
actions, use of { and }: Actions
aliases, how to define: Definitions Section
arguments, command-line: Scanner Options
array, default size for yytext: User Values

backing up, eliminating: Performance
backing up, eliminating by adding error rules: Performance
backing up, eliminating with catch-all rule: Performance
backing up, example of eliminating: Performance
BEGIN: Actions
BEGIN, explanation: Start Conditions
beginning of line, in patterns: Patterns
bison, bridging with flex: Bison Bridge
bison, parser: Bison Bridge
bison, scanner to be called from bison: Bison Bridge
BOL, checking the BOL flag: Misc Macros
BOL, in patterns: Patterns
BOL, setting it: Misc Macros
braces in patterns: Patterns
bugs, reporting: Reporting Bugs

C code in flex input: Definitions Section
C++: Cxx
C++ and %array: User Values
C++ I/O, customizing: How do I use my own I/O classes in a C++ scanner?
C++ scanners, including multiple scanners: Cxx
C++ scanners, use of: Cxx
c++, experimental form of scanner class: Cxx
C++, multiple different scanners: Cxx
C-strings, in actions: Actions
case-insensitive, effect on character classes: Patterns
character classes in patterns: Patterns
character classes in patterns, syntax of: Patterns
character classes, equivalence of: Patterns
clearing an input buffer: Multiple Input Buffers
command-line options: Scanner Options
comments in flex input: Definitions Section
comments in the input: Comments in the Input
comments, discarding: Actions
comments, example of scanning C comments: Start Conditions
comments, in actions: Actions
comments, in rules section: Comments in the Input
comments, syntax of: Comments in the Input
comments, valid uses of: Comments in the Input
compressing whitespace: Actions
concatenation, in patterns: Patterns
copyright of flex: Copyright
counting characters and lines: Simple Examples
customizing I/O in C++ scanners: How do I use my own I/O classes in a C++ scanner?

default rule: Simple Examples
default rule: Matching
defining pattern aliases: Definitions Section
Definitions, in flex input: Definitions Section
deleting lines from input: Actions
discarding C comments: Actions
distributing flex: Copyright

ECHO: Actions
ECHO, and yyout: Generated Scanner
embedding C code in flex input: Definitions Section
end of file, in patterns: Patterns
end of line, in negated character classes: Patterns
end of line, in patterns: Patterns
end-of-file, and yyrestart(): Generated Scanner
EOF and yyrestart(): Generated Scanner
EOF in patterns, syntax of: Patterns
EOF, example using multiple input buffers: Multiple Input Buffers
EOF, explanation: EOF
EOF, pushing back: Actions
EOL, in negated character classes: Patterns
EOL, in patterns: Patterns
error messages, end of buffer missed: Lex and Posix
error reporting, diagnostic messages: Diagnostics
error reporting, in C++: Cxx
error rules, to eliminate backing up: Performance
escape sequences in patterns, syntax of: Patterns
exiting with yyterminate(): Actions
experimental form of c++ scanner class: Cxx
extended scope of start conditions: Start Conditions

file format: Format
file format, serialized tables: Tables File Format
flushing an input buffer: Multiple Input Buffers
flushing the internal buffer: Actions
format of flex input: Format
format of input file: Format
freeing tables: Loading and Unloading Serialized Tables

getting current start state with YY_START: Start Conditions

halting with yyterminate(): Actions
handling include files with multiple input buffers: Multiple Input Buffers
handling include files with multiple input buffers: Multiple Input Buffers
header files, with C++: Cxx

include files, with C++: Cxx
input file, Definitions section: Definitions Section
input file, Rules Section: Rules Section
input file, user code Section: User Code Section
input(): Actions
input(), and C++: Actions
input, format of: Format
input, matching: Matching

keywords, for performance: Performance

lex (traditional) and POSIX: Lex and Posix
LexerInput, overriding: How do I use my own I/O classes in a C++ scanner?
LexerOutput, overriding: How do I use my own I/O classes in a C++ scanner?
limitations of flex: Limitations
literal text in patterns, syntax of: Patterns
loading tables at runtime: Loading and Unloading Serialized Tables

m4: M4 Dependency
Makefile, example of implicit rules: Makefiles and Flex
Makefile, explicit example: Makefiles and Flex
Makefile, syntax: Makefiles and Flex
matching C-style double-quoted strings: Start Conditions
matching, and trailing context: Matching
matching, length of: Matching
matching, multiple matches: Matching
member functions, C++: Cxx
memory management: Memory Management
memory, allocating input buffers: Multiple Input Buffers
memory, considerations for reentrant scanners: Init and Destroy Functions
memory, deleting input buffers: Multiple Input Buffers
memory, for start condition stacks: Start Conditions
memory, serialized tables: Serialized Tables
memory, serialized tables: Loading and Unloading Serialized Tables
methods, c++: Cxx
minimal scanner: Matching
multiple input streams: Multiple Input Buffers

name definitions, not POSIX: Lex and Posix
negating ranges in patterns: Patterns
newline, matching in patterns: Patterns
non-POSIX features of flex: Lex and Posix
noyywrap, %option: Generated Scanner
NULL character in patterns, syntax of: Patterns

octal characters in patterns: Patterns
options, command-line: Scanner Options
overriding LexerInput: How do I use my own I/O classes in a C++ scanner?
overriding LexerOutput: How do I use my own I/O classes in a C++ scanner?
overriding the memory routines: Overriding The Default Memory Management

Pascal-like language: Simple Examples
pattern aliases, defining: Definitions Section
pattern aliases, expansion of: Patterns
pattern aliases, how to define: Definitions Section
pattern aliases, use of: Definitions Section
patterns and actions on different lines: Lex and Posix
patterns, character class equivalence: Patterns
patterns, common: Common Patterns
patterns, end of line: Patterns
patterns, grouping and precedence: Patterns
patterns, in rules section: Patterns
patterns, invalid trailing context: Patterns
patterns, matching: Matching
patterns, precedence of operators: Patterns
patterns, repetitions with grouping: Patterns
patterns, special characters treated as non-special: Patterns
patterns, syntax: Patterns
patterns, syntax: Patterns
patterns, tuning for performance: Performance
patterns, valid character classes: Patterns
performance optimization, matching longer tokens: Performance
performance optimization, recognizing keywords: Performance
performance, backing up: Performance
performance, considerations: Performance
performance, using keywords: Performance
popping an input buffer: Multiple Input Buffers
POSIX and lex: Lex and Posix
POSIX comp;compliance: Lex and Posix
POSIX, character classes in patterns, syntax of: Patterns
preprocessor macros, for use in actions: Actions
pushing an input buffer: Multiple Input Buffers
pushing back characters with unput: Actions
pushing back characters with unput(): Actions
pushing back characters with yyless: Actions
pushing back EOF: Actions

ranges in patterns: Patterns
ranges in patterns, negating: Patterns
recognizing C comments: Start Conditions
reentrant scanners, multiple interleaved scanners: Reentrant Uses
reentrant scanners, recursive invocation: Reentrant Uses
reentrant, accessing flex variables: Global Replacement
reentrant, accessor functions: Accessor Methods
reentrant, API explanation: Reentrant Overview
reentrant, calling functions: Extra Reentrant Argument
reentrant, example of: Reentrant Example
reentrant, explanation: Reentrant
reentrant, extra data: Extra Data
reentrant, initialization: Init and Destroy Functions
regular expressions, in patterns: Patterns
REJECT: Actions
REJECT, calling multiple times: Actions
REJECT, performance costs: Performance
reporting bugs: Reporting Bugs
restarting the scanner: Lex and Posix
RETURN, within actions: Generated Scanner
rules, default: Simple Examples
rules, in flex input: Rules Section

scanner, definition of: Introduction
sections of flex input: Format
serialization: Serialized Tables
serialization of tables: Creating Serialized Tables
serialized tables, multiple scanners: Creating Serialized Tables
stack, input buffer pop: Multiple Input Buffers
stack, input buffer push: Multiple Input Buffers
stacks, routines for manipulating: Start Conditions
start condition, applying to multiple patterns: Start Conditions
start conditions: Start Conditions
start conditions, behavior of default rule: Start Conditions
start conditions, exclusive: Start Conditions
start conditions, for different interpretations of same input: Start Conditions
start conditions, in patterns: Patterns
start conditions, inclusive: Start Conditions
start conditions, inclusive v.s. exclusive: Start Conditions
start conditions, integer values: Start Conditions
start conditions, multiple: Start Conditions
start conditions, special wildcard condition: Start Conditions
start conditions, use of a stack: Start Conditions
start conditions, use of wildcard condition (<*>): Start Conditions
start conditions, using BEGIN: Start Conditions
stdin, default for yyin: Generated Scanner
stdout, as default for yyout: Generated Scanner
strings, scanning strings instead of files: Multiple Input Buffers

tables, creating serialized: Creating Serialized Tables
tables, file format: Tables File Format
tables, freeing: Loading and Unloading Serialized Tables
tables, loading and unloading: Loading and Unloading Serialized Tables
terminating with yyterminate(): Actions
token: Matching
trailing context, in patterns: Patterns
trailing context, limits of: Patterns
trailing context, matching: Matching
trailing context, performance costs: Performance
trailing context, variable length: Performance

unput(): Actions
unput(), and %pointer: Actions
unput(), pushing back characters: Actions
user code, in flex input: User Code Section
username expansion: Simple Examples
using integer values of start condition names: Start Conditions

verbatim text in patterns, syntax of: Patterns

warning, dangerous trailing context: Limitations
warning, rule cannot be matched: Diagnostics
warnings, diagnostic messages: Diagnostics
whitespace, compressing: Actions

yacc interface: Yacc
yacc, interface: Yacc
yyalloc, overriding: Overriding The Default Memory Management
yyfree, overriding: Overriding The Default Memory Management
yyin: Generated Scanner
yyinput(): Actions
yyleng: Matching
yyleng, modification of: Actions
yyless(): Actions
yyless(), pushing back characters: Actions
yylex(), in generated scanner: Generated Scanner
yylex(), overriding: Generated Scanner
yylex, overriding the prototype of: Generated Scanner
yylineno, in a reentrant scanner: Reentrant Functions
yylineno, performance costs: Performance
yymore(): Actions
yymore() to append token to previous token: Actions
yymore(), mega-kludge: Actions
yymore, and yyleng: Actions
yymore, performance penalty of: Actions
yyout: Generated Scanner
yyrealloc, overriding: Overriding The Default Memory Management
yyrestart(): Generated Scanner
yyterminate(): Actions
yytext: Matching
yytext, default array size: User Values
yytext, memory considerations: A Note About yytext And Memory
yytext, modification of: Actions
yytext, two types of: Matching
yywrap(): Generated Scanner
yywrap, default for: Generated Scanner
YY_CURRENT_BUFFER, and multiple buffers Finally, the macro: Multiple Input Buffers
YY_EXTRA_TYPE, defining your own type: Extra Data
YY_INPUT: Generated Scanner
YY_INPUT, overriding: Generated Scanner
YY_START, example: Start Conditions
YY_USER_ACTION to track each time a rule is matched: Misc Macros

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Next: , Previous: , Up: Indices   [Contents][Index]

Index of Functions and Macros

This is an index of functions and preprocessor macros that look like functions. For macros that expand to variables or constants, see Index of Variables.

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Index Entry  Section

BEGIN: Start Conditions

debug (C++ only): Cxx

LexerError (C++ only): Cxx
LexerInput (C++ only): Cxx
LexerOutput (C++ only): Cxx
lineno (C++ only): Cxx

set_debug (C++ only): Cxx
switch_streams (C++ only): Cxx

yyFlexLexer constructor (C++ only): Cxx
yyget_debug: Reentrant Functions
yyget_extra: Extra Data
yyget_extra: Reentrant Functions
yyget_in: Reentrant Functions
yyget_leng: Reentrant Functions
yyget_lineno: Reentrant Functions
yyget_out: Reentrant Functions
yyget_text: Reentrant Functions
YYLeng (C++ only): Cxx
yylex (C++ version): Cxx
yylex (reentrant version): Bison Bridge
yylex (reentrant version): Bison Bridge
yylex_destroy: Init and Destroy Functions
yylex_init: Init and Destroy Functions
yypop_buffer_state: Multiple Input Buffers
yypush_buffer_state: Multiple Input Buffers
yyrestart: User Values
yyset_debug: Reentrant Functions
yyset_extra: Extra Data
yyset_extra: Reentrant Functions
yyset_in: Reentrant Functions
yyset_lineno: Reentrant Functions
yyset_out: Reentrant Functions
yytables_destroy: Loading and Unloading Serialized Tables
yytables_fload: Loading and Unloading Serialized Tables
YYText (C++ only): Cxx
YY_AT_BOL: Misc Macros
yy_create_buffer: Multiple Input Buffers
yy_delete_buffer: Multiple Input Buffers
yy_flush_buffer: Multiple Input Buffers
yy_new_buffer: Multiple Input Buffers
YY_NEW_FILE (now obsolete): EOF
yy_pop_state: Start Conditions
yy_push_state: Start Conditions
yy_scan_buffer: Multiple Input Buffers
yy_scan_bytes: Multiple Input Buffers
yy_scan_string: Multiple Input Buffers
yy_set_bol: Misc Macros
yy_set_interactive: Misc Macros
yy_switch_to_buffer: Multiple Input Buffers
yy_top_state: Start Conditions

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Index of Hooks

This is an index of "hooks" that the user may define. These hooks typically correspond to specific locations in the generated scanner, and may be used to insert arbitrary code.

Jump to:   Y  
Jump to:   Y  

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Index of Scanner Options

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Index Entry  Section

-+: Code-Level And API Options
—7bit: Options Affecting Scanner Behavior
—8bit: Options Affecting Scanner Behavior
—align: Options for Scanner Speed and Size
—always-interactive: Options Affecting Scanner Behavior
—array: Code-Level And API Options
—backup: Debugging Options
—batch: Options Affecting Scanner Behavior
—bison-bridge: Code-Level And API Options
—bison-locations: Code-Level And API Options
—c++: Code-Level And API Options
—case-insensitive: Options Affecting Scanner Behavior
—debug: Debugging Options
—default: Options Affecting Scanner Behavior
—ecs: Options for Scanner Speed and Size
—fast: Options for Scanner Speed and Size
—full: Options for Scanner Speed and Size
—header-file: Options for Specifying Filenames
—help: Miscellaneous Options
—interactive: Options Affecting Scanner Behavior
—lex-compat: Options Affecting Scanner Behavior
—main: Code-Level And API Options
—meta-ecs: Options for Scanner Speed and Size
—never-interactive: Options Affecting Scanner Behavior
—nodefault: Debugging Options
—noline: Code-Level And API Options
—nounistd: Code-Level And API Options
—nowarn: Debugging Options
—option-ansi-definitions: Code-Level And API Options
—option-ansi-prototypes: Code-Level And API Options
—outfile: Options for Specifying Filenames
—perf-report: Debugging Options
—pointer: Code-Level And API Options
—posix: Options Affecting Scanner Behavior
—prefix: Code-Level And API Options
—read: Options for Scanner Speed and Size
—reentrant: Code-Level And API Options
—skel: Options for Specifying Filenames
—stack: Options Affecting Scanner Behavior
—stdinit: Options Affecting Scanner Behavior
—stdout: Options for Specifying Filenames
—tables-file: Options for Specifying Filenames
—tables-verify: Options for Specifying Filenames
—trace: Debugging Options
—verbose: Debugging Options
—version: Miscellaneous Options
—warn: Debugging Options
—yyclass: Code-Level And API Options
—yylineno: Options Affecting Scanner Behavior
—yywrap: Options Affecting Scanner Behavior
-7: Options Affecting Scanner Behavior
-8: Options Affecting Scanner Behavior
-B: Options Affecting Scanner Behavior
-b: Debugging Options
-C: Options for Scanner Speed and Size
-c: Miscellaneous Options
-Ca: Options for Scanner Speed and Size
-Ce: Options for Scanner Speed and Size
-Cf: Options for Scanner Speed and Size
-CF: Options for Scanner Speed and Size
-Cm: Options for Scanner Speed and Size
-Cr: Options for Scanner Speed and Size
-d: Debugging Options
-f: Options for Scanner Speed and Size
-F: Options for Scanner Speed and Size
-h: Miscellaneous Options
-i: Options Affecting Scanner Behavior
-I: Options Affecting Scanner Behavior
-l: Options Affecting Scanner Behavior
-L: Code-Level And API Options
-n: Miscellaneous Options
-o: Options for Specifying Filenames
-P: Code-Level And API Options
-p: Debugging Options
-R: Code-Level And API Options
-s: Debugging Options
-t: Options for Specifying Filenames
-T: Debugging Options
-v: Debugging Options
-V: Miscellaneous Options
-w: Debugging Options
-X: Options Affecting Scanner Behavior

7bit: Options Affecting Scanner Behavior

8bit: Options Affecting Scanner Behavior

align: Options for Scanner Speed and Size
always-interactive: Options Affecting Scanner Behavior
ansi-definitions: Code-Level And API Options
ansi-prototypes: Code-Level And API Options
array: Code-Level And API Options

backup: Debugging Options
batch: Options Affecting Scanner Behavior
bison-bridge: Code-Level And API Options
bison-locations: Code-Level And API Options

c++: Code-Level And API Options
case-insensitive: Options Affecting Scanner Behavior

debug: Debugging Options
default: Options Affecting Scanner Behavior

ecs: Options for Scanner Speed and Size

fast: Options for Scanner Speed and Size
full: Options for Scanner Speed and Size

header-file: Options for Specifying Filenames

interactive: Options Affecting Scanner Behavior

lex-compat: Options Affecting Scanner Behavior

main: Code-Level And API Options
meta-ecs: Options for Scanner Speed and Size

nodefault: Debugging Options
noline: Code-Level And API Options
nounistd: Code-Level And API Options
nowarn: Debugging Options
noyyalloc: Overriding The Default Memory Management

outfile: Options for Specifying Filenames

perf-report: Debugging Options
pointer: Code-Level And API Options
posix: Options Affecting Scanner Behavior
prefix: Code-Level And API Options

read: Options for Scanner Speed and Size
reentrant: Code-Level And API Options

stack: Options Affecting Scanner Behavior
stdinit: Options Affecting Scanner Behavior
stdout: Options for Specifying Filenames

tables-file: Options for Specifying Filenames
tables-verify: Options for Specifying Filenames
trace: Debugging Options

verbose: Debugging Options

warn: Debugging Options

yyclass: Code-Level And API Options
yylineno: Options Affecting Scanner Behavior
yywrap: Options Affecting Scanner Behavior

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Actually, yyIN_HEADER is defined before the ‘%top’ block.


The quantities given here are approximate, and may vary due to host architecture, compiler configuration, or due to future enhancements to flex.


It is not necessary to override all (or any) of the memory management routines. You may, for example, override yyrealloc, but not yyfree or yyalloc.


GNU make and GNU automake are two such programs that provide implicit rules for flex-generated scanners.


GNU automake may generate code to execute flex in lex-compatible mode, or to stdout. If this is not what you want, then you should provide an explicit rule in your Makefile.am


This example also applies to yacc parsers.


The features described here are purely optional, and are by no means the only way to use flex with bison. We merely provide some glue to ease development of your parser-scanner pair.


The use of m4 is subject to change in future revisions of flex. It is not part of the public API of flex. Do not depend on it.