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flex - fast lexical analyzer generator



        flex - fast lexical analyzer generator


        flex [-bcdfhilnpstvwBFILTV78+? -C[aefFmr] -ooutput -Pprefix -Sskeleton]
        [--help --version] [filename ...]


        This manual describes flex, a tool for generating programs that perform
        pattern-matching on text.  The manual includes both tutorial and refer‐
        ence sections:
                a brief overview of the tool
            Some Simple Examples
            Format Of The Input File
                the extended regular expressions used by flex
            How The Input Is Matched
                the rules for determining what has been matched
                how to specify what to do when a pattern is matched
            The Generated Scanner
                details regarding the scanner that flex produces;
                how to control the input source
            Start Conditions
                introducing context into your scanners, and
                managing "mini-scanners"
            Multiple Input Buffers
                how to manipulate multiple input sources; how to
                scan from strings instead of files
            End-of-file Rules
                special rules for matching the end of the input
            Miscellaneous Macros
                a summary of macros available to the actions
            Values Available To The User
                a summary of values available to the actions
            Interfacing With Yacc
                connecting flex scanners together with yacc parsers
                flex command-line options, and the "%option"
            Performance Considerations
                how to make your scanner go as fast as possible
            Generating C++ Scanners
                the (experimental) facility for generating C++
                scanner classes
            Incompatibilities With Lex And POSIX
                how flex differs from AT&T lex and the POSIX lex
                those error messages produced by flex (or scanners
                it generates) whose meanings might not be apparent
                files used by flex
            Deficiencies / Bugs
                known problems with flex
            See Also
                other documentation, related tools
                includes contact information


        flex is a tool for generating scanners: programs which recognize  lexi‐
        cal  patterns  in text.  flex reads the given input files, or its stan‐
        dard 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, which defines a routine yylex().  This file is
        compiled and linked with the -ll  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 corre‐
        sponding C code.
        First some simple examples to get the flavor of how one uses flex.  The
        following flex input specifies a scanner which whenever  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 "%%" 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;
                    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
        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 new‐
        line (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 );
            main( argc, argv )
            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
        The details of this example will be explained  in  the  following  sec‐
        The  flex  input  file  consists of three sections, separated by a line
        with just %% in it:
            user code
        The definitions section contains declarations of  simple  name  defini‐
        tions  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 def‐
        inition is taken to begin at the first non-white-space  character  fol‐
        lowing  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  fol‐
        lowed 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.
        The rules section of the flex input contains a series of rules  of  the
            pattern   action
        where  the  pattern must be unindented and the action must begin on the
        same line.
        See below for a further description of patterns and actions.
        Finally, 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 scan‐
        ner.  The presence of this section is optional; if it is  missing,  the
        second %% in the input file may be skipped, too.
        In  the  definitions  and  rules  sections,  any  indented text or text
        enclosed in %{ and %} is copied verbatim to the output (with the  %{}’s
        removed).  The %{}’s must appear unindented on lines by themselves.
        In  the  rules  section,  any indented or %{} 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 below for other such
        In the definitions section (but not in the  rules  section),  an  unin‐
        dented comment (i.e., a line beginning with "/*") is also copied verba‐
        tim to the output up to the next "*/".


        The patterns in the input are written using an extended set of  regular
        expressions.  These are:
            x          match the character ’x’
            .          any character (byte) except newline
            [xyz]      a "character class"; in this case, the pattern
                         matches either an ’x’, a ’y’, or a ’z’
            [abj-oZ]   a "character class" with a range in it; matches
                         an ’a’, a ’b’, any letter from ’j’ through ’o’,
                         or a ’Z’
            [^A-Z]     a "negated character class", i.e., any character
                         but those in the class.  In this case, any
                         character EXCEPT an uppercase letter.
            [^A-Z\n]   any character EXCEPT an uppercase letter or
                         a newline
            r*         zero or more r’s, where r is any regular expression
            r+         one or more r’s
            r?         zero or one r’s (that is, "an optional r")
            r{2,5}     anywhere from two to five r’s
            r{2,}      two or more r’s
            r{4}       exactly 4 r’s
            {name}     the expansion of the "name" definition
                       (see above)
                       the literal string: [xyz]"foo
            \X         if X is an ’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 ’*’)
            \0         a NUL character (ASCII code 0)
            \123       the character with octal value 123
            \x2a       the character with hexadecimal value 2a
            (r)        match an r; parentheses are used to override
                         precedence (see below)
            rs         the regular expression r followed by the
                         regular expression s; called "concatenation"
            r|s        either an r or an s
            r/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 notes in the
                         Deficiencies / Bugs section below regarding
                         "dangerous trailing context".)
            ^r         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).
            r$         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$".
            <s>r       an r, but only in start condition s (see
                         below for discussion of start conditions)
                       same, but in any of start conditions s1,
                         s2, or s3
            <*>r       an r in any start condition, even an exclusive one.
            <<EOF>>    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 prece‐
        dence, from highest precedence at the top  to  lowest  at  the  bottom.
        Those grouped together have equal precedence.  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 "foo" or the string "ba" followed by zero-or-
        more r’s.  To match "foo" or zero-or-more "bar"’s, use:
        and to match zero-or-more "foo"’s-or-"bar"’s:
        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:] desig‐
        nates those characters for which isalnum() returns  true  -  i.e.,  any
        alphabetic  or  numeric.  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:
        If your scanner is case-insensitive (the -i flag), then  [:upper:]  and
        [:lower:] are equivalent to [:alpha:].
        Some notes on patterns:
        -      A  negated  character  class  such as the example "[^A-Z]" above
               will match a  newline  unless  "\n"  (or  an  equivalent  escape
               sequence)  is  one  of  the characters explicitly present in the
               negated character class (e.g., "[^A-Z\n]").  This is unlike  how
               many  other  regular  expression  tools  treat negated character
               classes, but unfortunately  the  inconsistency  is  historically
               entrenched.   Matching  newlines means that a pattern like [^"]*
               can match the entire input unless there’s another quote  in  the
        -      A  rule  can  have at most one instance of trailing context (the
               ’/’ operator or the ’$’ operator).  The  start  condition,  ’^’,
               and "<<EOF>>" patterns can only occur at the beginning of a pat‐
               tern, and, as well as with ’/’ and ’$’, cannot be grouped inside
               parentheses.   A  ’^’ which does not occur at the beginning of a
               rule or a ’$’ which does not occur at the end of  a  rule  loses
               its special properties and is treated as a normal character.
               The following are illegal:
               Note that the first of these, can be written "foo/bar\n".
               The  following will result in ’$’ or ’^’ being treated as a nor‐
               mal character:
               If what’s wanted is a "foo" or a bar-followed-by-a-newline,  the
               following  could  be  used  (the special ’|’ action is explained
                   foo      |
                   bar$     /* action goes here */
               A similar trick will work for matching a foo  or  a  bar-at-the-
        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
        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 (a more  detailed
        description  of  actions  follows),  and  then  the  remaining input is
        scanned for another match.
        If no match is found, then the default rule is executed: the next char‐
        acter  in  the  input  is considered matched and copied to the standard
        output.  Thus, the simplest legal 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 defi‐
        nition 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
        the next section), 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
        below).  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 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 (the  c++
        option; see below).


        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"
        (It 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
        above); 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:
        -      ECHO copies yytext to the scanner’s output.
        -      BEGIN followed by the name of a start condition places the scan‐
               ner in the corresponding start condition (see below).
        -      REJECT  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
                           int word_count = 0;
                   frob        special(); REJECT;
                   [^ \t\n]+   ++word_count;
               Without  the  REJECT,  any  "frob"’s  in  the input would not be
               counted as words, since the scanner normally executes  only  one
               action per token.  Multiple REJECT     s are allowed, each one find‐
               ing 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 below).
               Note  also  that  unlike  the other special actions, REJECT is a
               branch; code immediately following it in the action will not  be
        -      yymore() 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 perfor‐
        mance 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 scan‐
               ner  will subsequently process its input (using BEGIN, for exam‐
               ple), 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
                   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:
                   "/*"        {
                               register 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  Scan‐
               ner,  below).  This action is a special case of the more general
               yy_flush_buffer() function, described below in the section  Mul‐
               tiple 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  scan‐
               ner’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.
        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 (;).
        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 scan‐
        ning continues from that file), or yyrestart() is called.   yyrestart()
        takes  one  argument, a FILE * pointer (which can be nil, 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 better  is  to  use
        YY_FLUSH_BUFFER  (see above).  Note that yyrestart() does not reset the
        start condition to INITIAL (see Start Conditions, below).
        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.       YY_INPUT’s      calling      sequence     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 vari‐
        able 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".
        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 -ll to obtain  the  default
        version of the routine, which always returns 1.
        Three routines are available for scanning from in-memory buffers rather
        than files: yy_scan_string(),  yy_scan_bytes(),  and  yy_scan_buffer().
        See the discussion of them below in the section Multiple Input Buffers.
        The scanner writes its ECHO output to the yyout global  (default,  std‐
        out), which may be redefined by the user simply by assigning it to some
        other FILE pointer.
        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  condi‐
        tion, and
            <INITIAL,STRING,QUOTE>\.        { /* handle an escape ... */
        will  be  active  only when the current start condition is either "INI‐
        TIAL", "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  inclu‐
        sive,  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 connec‐
        tion 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 condi‐
        tion 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
            %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 inte‐
        ger-valued  YY_START macro.  For example, the above assignments to com‐
        ment_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).
        Note  that  start conditions do not have their own name-space; %s’s and
        %x’s declare names in the same fashion as #define’s.
        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.
        Three  routines  are  available for manipulating stacks of start condi‐
        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
        void yy_pop_state()
               pops the top of the stack and switches to it via BEGIN.
        int yy_top_state()
               returns  the  top of the stack without altering the stack’s con‐
        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 Options below).
        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 con‐
        text.  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" 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:
            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  struc‐
        ture  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  nil FILE pointer to yy_create_buffer.  You select a particular
        buffer to scan from using:
            void yy_switch_to_buffer( YY_BUFFER_STATE new_buffer )
        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.  Note also that switching input sources via either
        yy_switch_to_buffer() or yywrap() does not change the start  condition.
            void yy_delete_buffer( YY_BUFFER_STATE buffer )
        is  used to reclaim the storage associated with a buffer.  ( buffer can
        be nil, in which case the routine does nothing.)  You  can  also  clear
        the current contents of a buffer using:
            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.
        yy_new_buffer()  is  an alias for yy_create_buffer(), provided for com‐
        patibility with the C++ use of new and delete for creating and destroy‐
        ing dynamic objects.
        Finally,  the  YY_CURRENT_BUFFER macro returns a YY_BUFFER_STATE handle
        to the current buffer.
        Here is an example of using these features for writing a scanner  which
        expands include files (the <<EOF>> feature is discussed below):
            /* the "incl" state is used for picking up the name
             * of an include file
            %x incl
            #define MAX_INCLUDE_DEPTH 10
            YY_BUFFER_STATE include_stack[MAX_INCLUDE_DEPTH];
            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] );
        Three  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.
        yy_scan_string(const char *str)
               scans a NUL-terminated string.
        yy_scan_bytes(const char *bytes, int len)
               scans  len bytes (including possibly NUL’s) starting at location
        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:
        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 nil pointer instead of creating a new input buffer.
               The type yy_size_t is an integral type to which you can cast  an
               integer expression reflecting the size of the buffer.
        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 four things:
        -      assigning yyin to a new input  file  (in  previous  versions  of
               flex,  after  doing  the  assignment you had to call the special
               action YY_NEW_FILE; this is no longer necessary);
        -      executing a return statement;
        -      executing the special yyterminate() action;
        -      or, switching to a new  buffer  using  yy_switch_to_buffer()  as
               shown in the example above.
        <<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
            %x quote
            ...other rules for dealing with quotes...
            <quote><<EOF>>   {
                     error( "unterminated quote" );
            <<EOF>>  {
                     if ( *++filelist )
                         yyin = fopen( *filelist, "r" );
        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
            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  inter‐
        nal 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  below).   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  interactive  ,  %option  always-interactive  or %option never-
        interactive (see Options below).  yy_set_interactive() must be  invoked
        prior to beginning to scan the buffer that is (or is not) to be consid‐
        ered 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 inac‐
        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.
        This section 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
               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
               input()  and  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  ter‐
               minates  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  immedi‐
               ate (any previously buffered-up input is lost).  Note that call‐
               ing 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.
        -      YY_CURRENT_BUFFER  returns  a YY_BUFFER_STATE handle to the cur‐
               rent buffer.
        -      YY_START returns an integer value corresponding to  the  current
               start condition.  You can subsequently use this value with BEGIN
               to return to that start condition.
        One of the main uses of flex is as a companion to the yacc  parser-gen‐
        erator.   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 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 ""
            [0-9]+        yylval = atoi( yytext ); return TOK_NUMBER;


        flex has the following options:
        -b     Generate backing-up information to lex.backup.  This is  a  list
               of scanner states which require backing up and the input charac‐
               ters on which they do so.  By adding rules one can remove  back‐
               ing-up  states.  If all backing-up states are eliminated and -Cf
               or -CF is used, the generated scanner will run faster  (see  the
               -p  flag).   Only users who wish to squeeze every last cycle out
               of their scanners need worry about this option.  (See  the  sec‐
               tion on Performance Considerations below.)
        -c     is  a  do-nothing,  deprecated option included for POSIX compli‐
        -d     makes the generated scanner run in debug mode.  Whenever a  pat‐
               tern  is  recognized  and  the  global 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.
        -f     specifies fast scanner.  No table compression is done and  stdio
               is  bypassed.   The  result  is  large but fast.  This option is
               equivalent to -Cfr (see below).
        -h     generates a "help" summary of flex’s options to stdout and  then
               exits.  -?  and --help are synonyms for -h.
        -i     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).
        -l     turns on maximum compatibility with the original AT&T lex imple‐
               mentation.  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 -+, -f, -F, -Cf, or -CF  options.
               For  details on the compatibilities it provides, see the section
               "Incompatibilities With Lex And POSIX" below.  This option  also
               results  in  the  name YY_FLEX_LEX_COMPAT being #define’d in the
               generated scanner.
        -n     is another do-nothing, deprecated option included only for POSIX
        -p     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, %option yylineno, and variable
               trailing context (see the Deficiencies  /  Bugs  section  below)
               entails  a substantial performance penalty; use of yymore(), the
               ^ operator, and the -I flag entail minor performance  penalties.
        -s     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     instructs  flex  to  write  the scanner it generates to standard
               output instead of lex.yy.c.
        -v     specifies that flex should write to stderr a summary of  statis‐
               tics 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 -V), and the
               next line the flags used when generating the scanner,  including
               those that are on by default.
        -w     suppresses warning messages.
        -B     instructs  flex  to  generate  a  batch scanner, the opposite of
               interactive scanners generated by -I (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  per‐
               formance  out  of  it.  If your goal is instead to squeeze out a
               lot more performance, you  should   be  using  the  -Cf  or  -CF
               options  (discussed  below), which turn on -B automatically any‐
        -F     specifies that the fast scanner table representation  should  be
               used (and stdio bypassed).  This representation is about as fast
               as the full table representation (-f), and for some sets of pat‐
               terns 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 -F.
               This option is equivalent to -CFr (see  below).   It  cannot  be
               used with -+.
        -I     instructs  flex to generate an interactive scanner.  An interac‐
               tive 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 per‐
               formance; 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 below).  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 -I 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.
               Note  that if isatty() returns false for the scanner input, flex
               will revert to batch mode, even if -I was specified.   To  force
               interactive  mode no matter what, use %option always-interactive
               (see Options below).
               You can force a scanner to not be interactive by using  -B  (see
        -L     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 below).
        -T     makes  flex  run  in trace mode.  It will generate a lot of mes‐
               sages to stderr concerning the form of the input and the  resul‐
               tant  non-deterministic and deterministic finite automata.  This
               option is mostly for use in maintaining flex.
        -V     prints the version number to stdout and exits.  --version  is  a
               synonym for -V.
        -7     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 -7 is that the scanner’s tables can be up to half the size
               of those generated using the -8 option (see below).  The  disad‐
               vantage 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 -7 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 gener‐
               ated a 7-bit or an 8-bit scanner by inspecting the flag  summary
               in the -v output as described above.
               Note  that  if  you  use  -Cfe  or -CFe (those table compression
               options, but also using equivalence  classes  as  discussed  see
               below),  flex  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     instructs flex to generate an 8-bit scanner, i.e., one which can
               recognize 8-bit characters.  This flag is only needed for  scan‐
               ners  generated  using -Cf or -CF, as otherwise flex defaults to
               generating an 8-bit scanner anyway.
               See the discussion of -7 above for flex’s default  behavior  and
               the tradeoffs between 7-bit and 8-bit scanners.
        -+     specifies  that  you  want flex to generate a C++ scanner class.
               See the section on Generating C++ Scanners below for details.
               controls the degree of table compression  and,  more  generally,
               trade-offs between small scanners and fast scanners.
               -Ca  ("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 short‐
               words.  This option can double the size of the  tables  used  by
               your scanner.
               -Ce 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 perfor‐
               mance-wise (one array look-up per character scanned).
               -Cf specifies that the full scanner tables should be generated -
               flex should not compress the tables by taking advantages of sim‐
               ilar transition functions for different states.
               -CF specifies that the  alternate  fast  scanner  representation
               (described above under the -F flag) should be used.  This option
               cannot be used with -+.
               -Cm 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 com‐
               pressed tables, but they have a moderate performance impact (one
               or  two "if" tests and one array look-up per character scanned).
               -Cr 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 above).
               A lone -C specifies that the scanner tables should be compressed
               but neither equivalence  classes  nor  meta-equivalence  classes
               should be used.
               The  options  -Cf  or  -CF  and -Cm do not make sense together -
               there is no opportunity for meta-equivalence classes if the  ta‐
               ble  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  gener‐
               ated  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  pro‐
               duction scanners.
               directs  flex to write the scanner to the file output instead of
               lex.yy.c.  If you combine -o with the -t option, then the  scan‐
               ner  is  written  to stdout but its #line directives (see the -L
               option above) refer to the file output.
               changes the default yy prefix used by flex for all globally-vis‐
               ible  variable  and  function  names  to instead be prefix.  For
               example, -Pfoo changes the name of yytext to footext.   It  also
               changes  the  name  of  the default output file from lex.yy.c to
       Here are all 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
               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 -ll no longer  provides
               one for you by default.
               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.
        flex also provides a mechanism for controlling options within the scan‐
        ner 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.  A num‐
        ber are equivalent to flex flags or their negation:
            7bit            -7 option
            8bit            -8 option
            align           -Ca option
            backup          -b option
            batch           -B option
            c++             -+ option
            caseful or
            case-sensitive  opposite of -i (default)
            case-insensitive or
            caseless        -i option
            debug           -d option
            default         opposite of -s option
            ecs             -Ce option
            fast            -F option
            full            -f option
            interactive     -I option
            lex-compat      -l option
            meta-ecs        -Cm option
            perf-report     -p option
            read            -Cr option
            stdout          -t option
            verbose         -v option
            warn            opposite of -w option
                            (use "%option nowarn" for -w)
            array           equivalent to "%array"
            pointer         equivalent to "%pointer" (default)
        Some %option     s provide features otherwise not available:
               instructs flex to generate a scanner which always considers  its
               input "interactive".  Normally, on each new input file the scan‐
               ner calls isatty() in an attempt to determine whether the  scan‐
               ner’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.
        main   directs  flex  to provide a default main() program for the scan‐
               ner, which simply calls yylex().  This option  implies  noyywrap
               (see below).
               instructs  flex  to generate a scanner which never considers its
               input "interactive" (again, no call made to isatty()).  This  is
               the opposite of always-interactive.
        stack  enables  the use of start condition stacks (see Start Conditions
               if set (i.e., %option stdinit) initializes  yyin  and  yyout  to
               stdin  and stdout, instead of the default of nil.  Some existing
               lex programs depend on this behavior, even though it is not com‐
               pliant  with  ANSI C, which does not require stdin and stdout to
               be compile-time constant.
               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.
        yywrap if unset (i.e., %option 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).
        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 set‐
        ting 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).
        Three options take string-delimited values, offset with ’=’:
            %option outfile="ABC"
        is equivalent to -oABC, and
            %option prefix="XYZ"
        is equivalent to -PXYZ.  Finally,
            %option yyclass="foo"
        only applies when generating a C++ scanner ( -+  option).   It  informs
        flex  that  you  have derived foo 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::Lexer     
        Error()) if called.  See Generating C++ Scanners, below, for additional
        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 corre‐
        sponding 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
        (though  yy_push_state() and friends won’t appear anyway unless you use
        %option stack).
        The main design goal of flex is that it generate high-performance scan‐
        ners.  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:
            %option yylineno
            arbitrary trailing context
            pattern sets that require backing up
            %option interactive
            %option always-interactive
            ’^’ beginning-of-line operator
        with the first three 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 just putting back some excess text you scanned, use
        REJECT should be avoided at all costs when  performance  is  important.
        It is a particularly expensive option.
        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  hap‐
        pened,  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
        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  scan‐
        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 eliminat‐
        ing 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 Deficiencies / Bugs below).
        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 an "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 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.
        Another final note regarding performance: as  mentioned  above  in  the
        section How the Input is Matched, dynamically resizing yytext to accom‐
        modate 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
        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
        compilations  errors  (please  report any you find to the email address
        given in the Author section below).  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  speci‐
        fied  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 instead of lex.yy.c.  The generated scanner includes the
        header file  FlexLexer.h,  which  defines  the  interface  to  two  C++
        The  first  class,  FlexLexer, provides an abstract base class defining
        the general scanner class interface.  It provides the following  member
        const char* YYText()
               returns the text of the most recently matched token, the equiva‐
               lent 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 the Options section above).  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
        pointer  and  not  a FILE*), yy_flush_buffer(), yy_delete_buffer(), and
        yyrestart() (again, the first argument is a istream* object pointer).
        The second class  defined  in  FlexLexer.h  is  yyFlexLexer,  which  is
        derived  from  FlexLexer.   It  defines the following additional member
        yyFlexLexer( istream* arg_yyin = 0, ostream* arg_yyout = 0 )
               constructs a yyFlexLexer object  using  the  given  streams  for
               input  and output.  If not specified, the streams default to cin
               and cout, respectively.
        virtual int yylex()
               performs the same role is yylex() does for ordinary  flex  scan‐
               ners:  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 vari‐
               ables 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 gener‐
               ates a dummy yyFlexLexer::yylex() that calls yyFlexLexer::Lexer     
               Error() if called).
        virtual void switch_streams(istream* new_in = 0,
               ostream*  new_out = 0) reassigns yyin to new_in (if non-nil) and
               yyout to new_out (ditto), deleting the previous input buffer  if
               yyin is reassigned.
        int yylex( istream* new_in, ostream* new_out = 0 )
               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  func‐
        tions 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  charac‐
               ters.   Note  that  "interactive"  scanners  (see  the -B and -I
               flags) define the macro YY_INTERACTIVE.  If  you  redefine  Lex     
               erInput()  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
        virtual void LexerOutput( const char* buf, int size )
               writes out size characters from the  buffer  buf,  which,  while
               NUL-terminated,  may  also contain "internal" NUL’s if the scan‐
               ner’s rules can match text with NUL’s 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.   You  can
        instantiate  multiple  instances of the same yyFlexLexer class, and you
        can also combine multiple C++ scanner classes together in the same pro‐
        gram 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.
            int mylineno = 0;
            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’;
            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.
        IMPORTANT: the present form of the scanning class is  experimental  and
        may change considerably between major releases.
        flex is a rewrite of the AT&T Unix lex tool (the two implementations do
        not share any code, though), with some  extensions  and  incompatibili‐
        ties,  both of which are of concern to those who wish to write scanners
        acceptable to either implementation.  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 scan‐
        ner’s performance.  We note below which incompatibilities can be  over‐
        come using the -l option.
        flex is fully compatible with lex with the following exceptions:
        -      The  undocumented  lex scanner internal variable yylineno is not
               supported unless -l or %option yylineno is used.
               yylineno should be maintained on a per-buffer basis, rather than
               a per-scanner (single global variable) basis.
               yylineno is not part of the POSIX specification.
        -      The  input() routine is not redefinable, though it may be called
               to read characters following whatever  has  been  matched  by  a
               rule.   If input() encounters an end-of-file the normal yywrap()
               processing is done.   A  ‘‘real’’  end-of-file  is  returned  by
               input() as EOF.
               Input is instead controlled by defining the YY_INPUT macro.
               The  flex  restriction  that  input()  cannot be redefined is in
               accordance with the POSIX specification, which simply  does  not
               specify any way of controlling the scanner’s input other than by
               making an initial assignment to yyin.
        -      The unput() routine is not redefinable.  This restriction is  in
               accordance with POSIX.
        -      flex scanners are not as reentrant as lex scanners.  In particu‐
               lar, if you have an interactive scanner and an interrupt handler
               which  long-jumps  out of the scanner, and the scanner is subse‐
               quently called again, you may get the following message:
                   fatal flex scanner internal error--end of buffer missed
               To reenter the scanner, first use
                   yyrestart( yyin );
               Note that this call will throw away any buffered input;  usually
               this isn’t a problem with an interactive scanner.
               Also  note  that  flex  C++ scanner classes are reentrant, so if
               using C++ is an option for you, you  should  use  them  instead.
               See "Generating C++ Scanners" above for details.
        -      output()  is  not supported.  Output from the ECHO macro is done
               to the file-pointer yyout (default stdout).
               output() is not part of the POSIX specification.
        -      lex does not support exclusive  start  conditions  (%x),  though
               they are in the POSIX specification.
        -      When  definitions  are expanded, flex encloses them in parenthe‐
               ses.  With lex, the following:
                   NAME    [A-Z][A-Z0-9]*
                   foo{NAME}?      printf( "Found it\n" );
               will not match the  string  "foo"  because  when  the  macro  is
               expanded the rule is equivalent to "foo[A-Z][A-Z0-9]*?"  and the
               precedence is such that the ’?’ is associated with  "[A-Z0-9]*".
               With  flex,  the rule will be expanded to "foo([A-Z][A-Z0-9]*)?"
               and so the string "foo" will match.
               Note that if the definition begins with ^ or ends with $ then it
               is  not  expanded  with parentheses, to allow these operators to
               appear in definitions without  losing  their  special  meanings.
               But  the  <s>, /, and <<EOF>> operators cannot be used in a flex
               Using -l results in the lex behavior of  no  parentheses  around
               the definition.
               The  POSIX  specification  is that the definition be enclosed in
        -      Some implementations of lex allow a rule’s action to begin on  a
               separate line, if the rule’s pattern has trailing whitespace:
                   foo|bar<space here>
                     { foobar_action(); }
               flex does not support this feature.
        -      The  lex %r (generate a Ratfor scanner) option is not supported.
               It is not part of the POSIX specification.
        -      After a call to unput(), yytext  is  undefined  until  the  next
               token  is  matched,  unless  the scanner was built using %array.
               This is not the case with lex or the POSIX  specification.   The
               -l option does away with this incompatibility.
        -      The  precedence of the {} (numeric range) operator is different.
               lex interprets "abc{1,3}" as "match one, two,  or  three  occur‐
               rences of ’abc’", whereas flex interprets it as "match ’ab’ fol‐
               lowed by one, two, or three occurrences of ’c’".  The latter  is
               in agreement with the POSIX specification.
        -      The  precedence  of the ^ operator is different.  lex interprets
               "^foo|bar" as "match either ’foo’ at the beginning of a line, or
               ’bar’  anywhere",  whereas  flex  interprets it as "match either
               ’foo’ or ’bar’ if they come at the beginning of  a  line".   The
               latter is in agreement with the POSIX specification.
        -      The  special table-size declarations such as %a supported by lex
               are not required by flex scanners; flex ignores them.
        -      The name FLEX_SCANNER is #define’d so scanners  may  be  written
               for  use  with  either  flex  or  lex.   Scanners  also  include
               YY_FLEX_MAJOR_VERSION and YY_FLEX_MINOR_VERSION indicating which
               version  of flex generated the scanner (for example, for the 2.5
               release, these defines would be 2 and 5 respectively).
        The following flex features are not included in lex or the POSIX speci‐
            C++ scanners
            start condition scopes
            start condition stacks
            interactive/non-interactive scanners
            yy_scan_string() and friends
            #line directives
            %{}’s around actions
            multiple actions on a line
        plus almost all of the flex flags.  The last feature in the list 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.


        warning, rule cannot be matched indicates that the given rule cannot be
        matched  because it follows other rules that will always match the same
        text as it.  For example, in the  following  "foo"  cannot  be  matched
        because it comes after an identifier "catch-all" rule:
            [a-z]+    got_identifier();
            foo       got_foo();
        Using REJECT in a scanner suppresses this warning.
        warning,  -s option given but default rule can be matched means that it
        is possible (perhaps only in a particular  start  condition)  that  the
        default  rule  (match  any  single character) is the only one that will
        match a particular input.  Since -s was given, presumably this  is  not
        reject_used_but_not_detected  undefined or yymore_used_but_not_detected
        undefined - These errors can occur at compile time.  They indicate that
        the  scanner uses REJECT or yymore() but that flex failed to notice the
        fact, meaning that flex scanned the  first  two  sections  looking  for
        occurrences  of  these  actions and failed to find any, but somehow you
        snuck some in (via a #include file, for example).  Use  %option  reject
        or %option yymore to indicate to flex that you really do use these fea‐
        flex scanner jammed - a scanner compiled with  -s  has  encountered  an
        input  string which wasn’t matched by any of its rules.  This error can
        also occur due to internal problems.
        token too large, exceeds YYLMAX - your scanner uses %array and  one  of
        its rules matched a string longer than the YYLMAX constant (8K bytes by
        default).  You can increase the value by #define’ing YYLMAX in the def‐
        initions section of your flex input.
        scanner requires -8 flag to use the character ’x’ - Your scanner speci‐
        fication includes recognizing the 8-bit character ’x’ and you  did  not
        specify  the  -8  flag, and your scanner defaulted to 7-bit because you
        used the -Cf or -CF table compression options.  See the  discussion  of
        the -7 flag for details.
        flex scanner push-back overflow - you used unput() to push back so much
        text that the scanner’s buffer could not hold both the pushed-back text
        and  the  current  token in yytext.  Ideally the scanner should dynami‐
        cally resize the buffer in this case, but at present it does not.
        input buffer overflow, can’t enlarge buffer because scanner uses REJECT
        -  the  scanner  was  working  on matching an extremely large token and
        needed to expand the input buffer.  This  doesn’t  work  with  scanners
        that use REJECT.
        fatal  flex  scanner  internal  error--end  of buffer missed - This can
        occur in a scanner which is reentered after a long-jump has jumped  out
        (or  over) the scanner’s activation frame.  Before reentering the scan‐
        ner, use:
            yyrestart( yyin );
        or, as noted above, switch to using the C++ scanner class.
        too many start conditions in <> construct! - you listed more start con‐
        ditions  in a <> construct than exist (so you must have listed at least
        one of them twice).


        -ll    library with which scanners must be linked.
               generated scanner (called lexyy.c on some systems).

               generated C++ scanner class, when using -+.
               header file defining the C++ scanner base class, FlexLexer,  and
               its derived class, yyFlexLexer.
               skeleton  scanner.   This  file is only used when building flex,
               not when flex executes.
               backing-up information for -b flag (called lex.bck on some  sys‐
        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 above mentioned 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 direc‐
        tive or the -l option has been used.
        Pattern-matching of NUL’s is substantially slower than  matching  other
        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, for example, 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 num‐
        ber 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
        REJECT cannot be used with the -f or -F options.
        The flex internal algorithms need documentation.
        lex(1), yacc(1), sed(1), awk(1).
        John Levine, Tony Mason, and Doug Brown, Lex & Yacc, O’Reilly and Asso‐
        ciates.  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, Tech‐
        niques and Tools, Addison-Wesley (1986).  Describes the  pattern-match‐
        ing techniques used by flex (deterministic finite automata).


        Vern  Paxson, with the help of many ideas and much inspiration from Van
        Jacobson.  Original version by Jef Poskanzer.  The fast table represen‐
        tation  is  a  partial implementation of a design done by Van Jacobson.
        The implementation was done by Kevin Gong and Vern Paxson.
        Thanks to the many flex beta-testers,  feedbackers,  and  contributors,
        especially Francois Pinard, Casey Leedom, Robert Abramovitz, Stan Ader‐
        mann, Terry Allen, David Barker-Plummer, John Basrai, Neal Becker, Nel‐
        son H.F. Beebe,, Karl Berry, Peter A. Bigot, Simon Blan‐
        chard, Keith Bostic, Frederic  Brehm,  Ian  Brockbank,  Kin  Cho,  Nick
        Christopher,  Brian  Clapper,  J.T.  Conklin, Jason Coughlin, Bill Cox,
        Nick Cropper, Dave Curtis, Scott David  Daniels,  Chris  G.  Demetriou,
        Theo  Deraadt,  Mike  Donahue,  Chuck Doucette, Tom Epperly, Leo Eskin,
        Chris Faylor, Chris Flatters, Jon Forrest, Jeffrey Friedl,  Joe  Gayda,
        Kaveh  R.  Ghazi,  Wolfgang  Glunz, Eric Goldman, Christopher M. Gould,
        Ulrich Grepel, Peer Griebel, Jan Hajic, Charles Hemphill,  NORO  Hideo,
        Jarkko  Hietaniemi, Scott Hofmann, Jeff Honig, Dana Hudes, Eric Hughes,
        John Interrante, Ceriel Jacobs, Michal  Jaegermann,  Sakari  Jalovaara,
        Jeffrey R. Jones, Henry Juengst, Klaus Kaempf, Jonathan I. Kamens, Ter‐
        rence O Kane, Amir  Katz,,  Kevin  B.  Kenny,  Steve
        Kirsch,  Winfried  Koenig, Marq Kole, Ronald Lamprecht, Greg Lee, Rohan
        Lenard, Craig Leres, John Levine, Steve Liddle,  David  Loffredo,  Mike
        Long,  Mohamed  el  Lozy,  Brian  Madsen,  Malte,  Joe  Marshall, Bengt
        Martensson, Chris Metcalf, Luke Mewburn,  Jim  Meyering,  R.  Alexander
        Milowski,  Erik  Naggum,  G.T.  Nicol,  Landon Noll, James Nordby, Marc
        Nozell, Richard Ohnemus, Karsten Pahnke, Sven Panne, Roland Pesch, Wal‐
        ter  Pelissero, Gaumond Pierre, Esmond Pitt, Jef Poskanzer, Joe Rahmeh,
        Jarmo Raiha, Frederic Raimbault, Pat  Rankin,  Rick  Richardson,  Kevin
        Rodgers, Kai Uwe Rommel, Jim Roskind, Alberto Santini, Andreas Scherer,
        Darrell Schiebel, Raf Schietekat, Doug Schmidt,  Philippe  Schnoebelen,
        Andreas  Schwab, Larry Schwimmer, Alex Siegel, Eckehard Stolz, Jan-Erik
        Strvmquist, Mike Stump, Paul Stuart, Dave Tallman,  Ian  Lance  Taylor,
        Chris Thewalt, Richard M. Timoney, Jodi Tsai, Paul Tuinenga, Gary Weik,
        Frank Whaley, Gerhard Wilhelms, Kent Williams,  Ken  Yap,  Ron  Zellar,
        Nathan  Zelle,  David  Zuhn,  and  those  whose  names  have slipped my
        marginal mail-archiving skills but whose contributions are  appreciated
        all the same.
        Thanks to Keith Bostic, Jon Forrest, Noah Friedman, John Gilmore, Craig
        Leres, John Levine, Bob Mulcahy, G.T.   Nicol,  Francois  Pinard,  Rich
        Salz,   and   Richard  Stallman  for  help  with  various  distribution
        Thanks to Esmond Pitt and Earle Horton for 8-bit character support;  to
        Benson  Margulies  and Fred Burke for C++ support; to Kent Williams and
        Tom Epperly for C++ class support; to Ove Ewerlid for support of NUL’s;
        and to Eric Hughes for support of multiple buffers.
        This  work  was  primarily  done  when I was with the Real Time Systems
        Group at the Lawrence Berkeley Laboratory in Berkeley, CA.  Many thanks
        to all there for the support I received.
        Send comments to


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