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FLEX(1)                                                                FLEX(1)

       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(8,n) 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 recognized lexi-
       cal  patterns  in(1,8) text.  flex reads the given input files, or its stan-
       dard input if(3,n) no file(1,n) names are given, for a description of  a  scanner
       to  generate.   The  description  is  in(1,8)  the  form of pairs of regular
       expressions and C code, called rules. flex  generates  as  output  a  C
       source  file(1,n),  lex.yy.c, which defines a routine yylex().  This file(1,n) is
       compiled and linked with the -lfl 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(3,n) "username" will replace it with the user's login(1,3,5) name:

           username    printf(1,3,1 builtins)( "%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(1,n)  to
       its output with each occurrence of "username" expanded.  In this input,
       there is just one rule.  "username" is the pattern and the "printf(1,3,1 builtins)"  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(1,3,1 builtins)( "# of lines = %d, # of chars = %d\n",
                           num_lines, num_chars );

       This scanner counts the number of characters and the number of lines in(1,8)
       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(1,8) 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(1,3,1 builtins)( "An integer: %s (%d)\n", yytext,
                               atoi( yytext ) );

           {DIGIT}+"."{DIGIT}*        {
                       printf(1,3,1 builtins)( "A float: %s (%g)\n", yytext,
                               atof( yytext ) );

           if(3,n)|then|begin|end|procedure|function        {
                       printf(1,3,1 builtins)( "A keyword: %s\n", yytext );

           {ID}        printf(1,3,1 builtins)( "An identifier: %s\n", yytext );

           "+"|"-"|"*"|"/"   printf(1,3,1 builtins)( "An operator: %s\n", yytext );

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

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

           .           printf(1,3,1 builtins)( "Unrecognized character: %s\n", yytext );


           main( argc, argv )
           int argc;
           char **argv;
               ++argv, --argc;  /* skip over program name */
               if(3,n) ( 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(1,8)  the  following  sec-

       The  flex  input  file(1,n)  consists of three sections, separated by a line
       with just %% in(1,8) 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(1,8) 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(3,n) it is  missing,  the
       second %% in(1,8) the input file(1,n) may be skipped, too.

       In  the  definitions  and  rules  sections,  any  indented text or text
       enclosed in(1,8) %{ 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 exe-
       cuted whenever the scanning routine is entered.  Other indented or  %{}
       text in(1,8) 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 fea-

       In the definitions section (but not in(1,8) 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(1,8) the input are written using an extended set(7,n,1 builtins) of  regular
       expressions.  These are:

           x          match the character 'x'
           .          any character (byte) except newline
           [xyz]      a "character class"; in(1,8) this case, the pattern
                        matches either an 'x', a 'y', or a 'z'
           [abj-oZ]   a "character class" with a range in(1,8) 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(1,8) 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(3,n) 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(3,n) 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(1,8) the
                        Deficiencies / Bugs section below regarding
                        "dangerous trailing context".)
           ^r         an r, but only at the beginning of a line (i.e.,
                        which 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(1,8) particular, on some DOS
                      systems you must either filter(1,3x,3x curs_util) out \r's in(1,8) the
                      input yourself, or explicitly use r/\r\n for "r$".

           <s>r       an r, but only in(1,8) start condition s (see
                        below for discussion of start conditions)
                      same, but in(1,8) any of start conditions s1,
                        s2, or s3
           <*>r       an r in(1,8) any start condition, even an exclusive one.

           <<EOF>>    an end-of-file
                      an end-of-file when in(1,8) 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(3,n) "foo" or the string(3,n) "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(7,n,1 builtins) 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(1,8) 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(1,8)  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(1,8) '$' 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(1,8) the  flex  input  file(1,n)  is
       chosen (unless you use %option subset-sort which is described below).

       Once  the  match  is  determined,  the  text corresponding to the match
       (called the token) is made available in(1,8) the  global  character  pointer
       yytext, and its length in(1,8) the global integer yyleng.  The action corre-
       sponding 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(1,8)  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(1,2,n)) to its output.

       Note  that  yytext  can  be  defined in(1,8) 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(1,8) the first (definitions) section  of  your  flex  input.   The
       default is %pointer, unless you use the -l lex compatibility option, in(1,8)
       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(1,8) 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(2,5)  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(1,8) 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(1,8) mind that each time(1,2,n) 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(3,n)  a  call  to  unput()  results  in(1,8)  too much text being pushed back;
       instead, a run-time error(8,n) results.

       Also note that you cannot use %array with C++ scanner classes (the  c++
       option; see below).

       Each  pattern  in(1,8)  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(1,8) 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(1,2,n) yylex()
       is called it continues processing tokens from where it  last  left  off
       until it either reaches the end of the file(1,n) 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(1,8)  the
       input  stream).   This  however  does  not apply when using %array (see
       above); in(1,8) that case, yytext may be freely modified in(1,8) any way.

       Actions are free to modify yyleng except they should not do so  if(3,n)  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(1,8) 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(1,8) "How the Input is Matched",
              and yytext and yyleng set(7,n,1 builtins) up appropriately.  It  may  either  be
              one which matched as much text as the originally chosen rule but
              came later in(1,8) the flex input file(1,n), or  one  which  matched  less(1,3)
              text.   For  example, the following will both count the words in(1,8)
              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(1,8)  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(1,2) "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(1,8) terms of scanner performance; if(3,n) it is used in(1,8) 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(1,8) the action will not  be

       -      yymore() tells the scanner that the next time(1,2,n) 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(1,2)  "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(1,2) "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(3,n) you  are  using  yymore().   Second,  the
       presence  of  yymore()  in(1,8) the scanner's action entails a minor perfor-
       mance penalty in(1,8) 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(1,2)
              out "foobarbar":

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

              An  argument  of 0 to yyless will cause the entire current input
              string(3,n) 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(1,8) an endless loop.

       Note that yyless is a macro and can only be  used  in(1,8)  the  flex  input
       file(1,n), not from other source files.

       -      unput(c)  puts(3,n)  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(1,8)

                  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(3,n) 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(3,n)  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(1,8) 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(3,n) ( c == '*' )
                                      while ( (c = input()) == '*' )
                                      if(3,n) ( c == '/' )
                                          break;    /* found the end */

                                  if(3,n) ( c == EOF )
                                      error(8,n)( "EOF in(1,8) comment" );

              (Note that if(3,n) the scanner is compiled using C++, then input() is
              instead referred to as yyinput(), in(1,8) 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(1,2,n)  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(1,8) the section  Mul-
              tiple Input Buffers.

       -      yyterminate()  can  be  used in(1,8) lieu of a return statement in(1,8) 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(1,n) 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(1,8) 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(3,n) 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(1,n)
       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(1,n) (in(1,8) which case scan-
       ning continues from that file(1,n)), or yyrestart() is called.   yyrestart()
       takes  one  argument, a FILE * pointer (which can be nil, if(3,n) you've set(7,n,1 builtins)
       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(1,n) 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(1,n) input files in(1,8) 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(1,7,1 tput) the
       start condition to INITIAL (see Start Conditions, below).

       If yylex() stops scanning due to executing a return statement in(1,8) 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(2,n,1 builtins) characters from yyin.
       The nature of how it gets(3,n) 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(1,8)  the character array buf and return in(1,8) the integer vari-
       able result either the  number  of  characters  read(2,n,1 builtins)  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(1,8)  the  definitions  section  of  the
       input file(1,n)):

           #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(7,n,1 builtins) up yyin  to
       point  to  another  input  file(1,n), and scanning continues.  If it returns
       true (non-zero), then  the  scanner  terminates,  returning  0  to  its
       caller.   Note  that  in(1,8)  either  case,  the  start  condition  remains
       unchanged; it does not revert to INITIAL.

       If you do not supply your own version(1,3,5) of yywrap(), then you must either
       use  %option  noyywrap  (in(1,8)  which  case  the scanner behaves as though
       yywrap() returned 1), or you must link(1,2) with -lfl to obtain the  default
       version(1,3,5) 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(1,8) 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(1,8) the start condition named(5,8) "sc".  For example,

           <STRING>[^"]*        { /* eat up the string(3,n) body ... */

       will be active only when the scanner is in(1,8) 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(1,8) 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(7,n,1 builtins) of rules contingent on the same exclusive start
       condition describe a scanner which is independent of any of  the  other
       rules  in(1,8)  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(7,n,1 builtins) 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(1,8) the  second
       example  wouldn't be active (i.e., couldn't match) when in(1,8) start condi-
       tion example.  If we just used <example> to qualify bar,  though,  then
       it  would  only  be  active in(1,8) example and not in(1,8) INITIAL, while in(1,8) the
       first example it's active in(1,8) both, because in(1,8)  the  first  example  the
       example startion 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(1,8)
       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(3,n) ( enter_special )

           ...more rules follow...

       To  illustrate  the  uses  of start conditions, here is a scanner which
       provides two different interpretations of a string(3,n) like "123.456".   By
       default  it  will  treat  it  as three tokens, the integer "123", a dot
       ('.'), and the integer "456".  But if(3,n) the string(3,n) is preceded earlier in(1,8)
       the  line  by  the  string(3,n) "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(1,3,1 builtins)( "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(1,3,1 builtins)( "found an integer, = %d\n",
                               atoi( yytext ) );

           "."         printf(1,3,1 builtins)( "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(1,2)  a  high-speed
       scanner  try to match as much possible in(1,8) 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(1,8) 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(2,5) 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(1,8) 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(3,n) 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(3,n) constant token type and
                    * value to parser

           <str>\n        {
                   /* error(8,n) - unterminated string(3,n) constant */
                   /* generate error(8,n) message */

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

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

                   if(3,n) ( result > 0xff )
                           /* error(8,n), constant is out-of-bounds */

                   *string_buf_ptr++ = result;

           <str>\\[0-9]+ {
                   /* generate error(8,n) - 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(1,8) 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(2,n,1 builtins) 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(1,2,n) 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(1,n), int size )

       which takes a FILE pointer and a size and creates a  buffer  associated
       with  the  given file(1,n) and large enough to hold size characters (when in(1,8)
       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(3,n) you wish, and also refer to the opaque  struc-
       ture  in(1,8) order to correctly declare input buffers in(1,8) source files other
       than that of your scanner.  Note that the FILE pointer in(1,8) the  call  to
       yy_create_buffer is only used as the value of yyin seen by YY_INPUT; if(3,n)
       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(2,7,2 select_tut) 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(7,n,1 builtins) things up for continued scanning, instead of opening a new file(1,n) 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(1,8) which case the routine does nothing.)  You  can  also  clear(1,3x,3x clrtobot)
       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(1,2,n) 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(1,n)
           %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(1,n) name */
                   if(3,n) ( include_stack_ptr >= MAX_INCLUDE_DEPTH )
                       fprintf( stderr, "Includes nested too deeply" );
                       exit(3,n,1 builtins)( 1 );

                   include_stack[include_stack_ptr++] =

                   yyin = fopen( yytext, "r" );

                   if(3,n) ( ! yyin )
                       error(8,n)( ... );

                       yy_create_buffer( yyin, YY_BUF_SIZE ) );


           <<EOF>> {
                   if(3,n) ( --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(3,n),   and  return  a  corresponding
       YY_BUFFER_STATE handle (which you should delete with yy_delete_buffer()
       when  done  with  it).   They  also  switch(1,n)  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(3,n)
       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(1,8) 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(7,n,1 builtins) up base in(1,8)  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(1,n)  (in(1,8)  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(1,8) 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(8,n)( "unterminated quote" );
           <<EOF>>  {
                    if(3,n) ( *++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(3,n) 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(2,n,1 builtins) in(1,8) a data table or open(2,3,n) 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(1,8)  the macro invocation marks the buffer as interactive, a zero
       value as non-interactive.   Note  that  use  of  this  macro  overrides
       %option  always-interactive  or  %option never-interactive (see Options
       below).  yy_set_interactive() must be invoked  prior  to  beginning  to
       scan the buffer that is (or is not) to be considered interactive.

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

       The macro YY_AT_BOL() returns true if(3,n) the next token scanned  from  the
       current buffer will have '^' rules active, false otherwise.

       In  the  generated  scanner,  the actions are all gathered in(1,8) one large
       switch(1,n) 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(1,5)  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(1,8) 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(1,8) 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(1,8) the first section if(3,n)  you  don't  like  the  default
              value  (generally 8KB).  Using %array results in(1,8) 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(1,n) 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(1,8) 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(1,n) 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(1,n) 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(1,n) 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(5,8) 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(1,8) the global
       yylval.  To use flex with yacc, one specifies the -d option to yacc  to
       instruct  it to generate the file(1,n) containing definitions of all
       the %tokens appearing in(1,8) the yacc input.  This file(1,n) is then included in(1,8)
       the  flex  scanner.  For example, if(3,n) one of the tokens is "TOK_NUMBER",
       part of the scanner might look(1,8,3 Search::Dict) 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(1,5) 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(1,8) 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(1,2) 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(1,8) the file(1,n)
              defining the scanner (i.e., the file(1,n)  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(1,8,3 Search::Dict) 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(1,8) the flex input patterns will be ignored, and
              tokens  in(1,8)  the  input  will be matched regardless of case.  The
              matched text given in(1,8) 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(1,8)  the  name YY_FLEX_LEX_COMPAT being #define'd in(1,8) 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(1,n) which will
              cause  a  serious  loss of performance in(1,8) 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(1,8) a scanner's rule set.

       -t     instructs  flex  to  write(1,2)  the scanner it generates to standard
              output instead of lex.yy.c.

       -v     specifies that flex should write(1,2) 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(1,3,5) 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(3,n) the pattern  set(7,n,1 builtins)  contains  both  "keywords"  and  a
              catch-all, "identifier" rule, such as in(1,8) the set:

                  "case"    return TOK_CASE;
                  "switch(1,n)"  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(3,n)  it  absolutely  must.  It turns out that
              always looking one extra character ahead, even  if(3,n)  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(1,8,3 Search::Dict) 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(1,8) another whole line.

              Flex scanners default to interactive unless you use the  -Cf  or
              -CF  table-compression  options  (see below).  That's because if(3,n)
              you're looking for high-performance you should be using  one  of
              these options, so if(3,n) 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(1,8) conjunction with
              -Cf or -CF.  Thus, this option is not really needed; it is on by
              default for all those cases in(1,8) which it is allowed.

              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(8,n) messages in(1,8) the actions will be correctly located  with
              respect  to  either  the original flex input file(1,n) (if(3,n) the errors
              are due to code in(1,8) the input file(1,n)), or lex.yy.c (if(3,n)  the  errors
              are  flex's  fault -- you should report these sorts of errors to
              the email address given below).

       -T     makes flex run in(1,8) trace(3x,n,3x _nc_tracebits) 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(1,8) maintaining flex.

       -V     prints  the  version(1,3,5) 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 recognized 7-bit characters in(1,8) 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(3,n) 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(1,3) portable.  Flex's default behavior is to generate an  8-bit
              scanner  unless  you  use  the  -Cf  or  -CF, in(1,8) 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(1,8) the -v output as described above.

              Note that if(3,n) 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(1,8)  the
              generated scanner for faster performance because the elements of
              the tables are better aligned for memory access(2,5) and computation.
              On  some RISC architectures, fetching and manipulating longwords
              is more efficient than with smaller-sized units(1,7) 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(3,n) the only appearance of digits in(1,8) the flex  input  is  in(1,8)  the
              character  class "[0-9]" then the digits '0', '1', ..., '9' will
              all be put in(1,8) the same equivalence class).  Equivalence  classes
              usually  give dramatic reductions in(1,8) the final table/object file(1,n)
              sizes (typically a factor(1,6) 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(3,n)  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(3,n)" 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(2,n,1 builtins)() system call, resulting
              in(1,8) a performance gain which varies from system to system, but in(1,8)
              general  is probably negligible unless you are also using -Cf or
              -CF.  Using -Cr can cause strange behavior if(3,n), for example,  you
              read(2,n,1 builtins) from yyin using stdio prior to calling the scanner (because
              the scanner will miss whatever text your previous reads left  in(1,8)
              the stdio input buffer).

              -Cr  has  no  effect  if(3,n)  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(3,n) 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(1,2) the scanner to the file(1,n) 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(1,n) 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(1,n)  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(1,3,5) of their name; but externally, they  have  the  modified

              This option lets you easily link(1,2) 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(1,3,5) of the routine for  your  scanner,
              or use %option noyywrap, as linking with -lfl no longer provides
              one for you by default.

              overrides the default skeleton file(1,n) 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(1,8) the  first  section  of  the
       scanner  specification.  You can specify multiple options with a single
       %option directive, and multiple directives in(1,8) 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(2,n,1 builtins)            -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(1,n) the scan-
              ner  calls isatty() in(1,8) an attempt to determine whether the scan-
              ner's input source is interactive and  thus  should  be  read(2,n,1 builtins)  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(3,n)  set(7,n,1 builtins)  (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.

              if(3,n)  set(7,n,1 builtins)  (i.e.,  %option  subset-sort) another method for disam-
              biguating rules is used if(3,n) two (or  more)  rules  apply  to  the
              longest  match.  A rule A is preferred over a rule B if(3,n) all text
              matched by A is also matched by B (together with at  least  some
              other  string(3,n)).   If  the  sets of possible matches of two rules
              have an intersection but neither is a proper subset of  another,
              the  rule listed first in(1,8) the input file(1,n) is chosen as usual.  In
              this way a kind of best-fit match is performed if(3,n) possible.   No
              performance  decrease in(1,8) the generated scanner is caused by this

              directs flex to generate a scanner that maintains the number  of
              the  current  line  read(2,n,1 builtins)  from  its input in(1,8) the global variable
              yylineno.  This option is implied by %option lex-compat.

       yywrap if(3,n) 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(1,n)
              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(1,8) the member function foo::yylex() instead  of
       yyFlexLexer::yylex().   It also generates a yyFlexLexer::yylex() member
       function that emits a run-time error(8,n) (by  invoking  yyFlexLexer::Lexer-
       Error()) if(3,n) 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(1,8) the generated scanner.  Each of
       the following, if(3,n) unset (e.g., %option nounput ), results in(1,8) the corre-
       sponding routine not appearing in(1,8) 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(3,n) 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(1,n) 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(1,8) which it
       can make a transition on an 'o' but not on  any  other  character,  and
       that  in(1,8) 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(1,8)  the  input  file.   If  the scanner is in(1,8) 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(1,8) when it has seen "fo".  When  this  has  hap-
       pened,  if(3,n)  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(2,8) "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(8,n)" rules:

           foo         return TOK_KEYWORD;
           foobar      return TOK_KEYWORD;

           fooba       |
           foob        |
           fo          {
                       /* false alarm(1,2), 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(7,n,1 builtins)  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(8,n) 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(1,8) mind that you gain the benefits of eliminat-
       ing backing up only if(3,n) 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(1,8)
       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(1,8) speeding up a scanner:  suppose  you  want  to  scan
       through  a  file(1,n)  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(5,8)     |
           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(5,8)     |
           break    |
           ... etc ...
           volatile |
           while    /* it's a keyword */

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

       Now, if(3,n) 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(1,8)  the
       recognition of newlines with that of the other tokens:

           asm\n    |
           auto(5,8)\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(1,8)  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(5,8)" and then the next character is
       something other than a newline or a letter.  Previously it  would  then
       just  match the "auto(5,8)" rule and be done, but now it has no "auto(5,8)" rule,
       only a "auto(5,8)\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(5,8)\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(1,2)  rules  which  match
       short  amounts  of  text  if(3,n)  it's anticipated that the text will often
       include NUL's.

       Another final note regarding performance: as  mentioned  above  in(1,8)  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(3,n) 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(1,8) the Author section below).  You can then use C++ code in(1,8)  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(3,n) the name of the flex executable ends in(1,8) a '+', such as flex++.
       When using this option, flex defaults to generating the scanner to  the
       file(1,n) instead of lex.yy.c.  The generated scanner includes the
       header file(1,n)  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(3,n) %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(1,8) 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(1,8)  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(5,8) 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(2,5) 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(3,n) called).

       virtual(5,8) void switch_streams(istream* new_in = 0,
              ostream*  new_out = 0) reassigns yyin to new_in (if(3,n) non-nil) and
              yyout to new_out (ditto), deleting the previous input buffer  if(3,n)
              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(5,8)  func-
       tions which you can redefine in(1,8) derived classes to tailor the scanner:

       virtual(5,8) 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(5,8) 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(3,n) the scan-
              ner's rules can match text with NUL's in(1,8) them.

       virtual(5,8) void LexerError( const char* msg )
              reports a fatal error(8,n) message.   The  default  version(1,3,5)  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(1,8) 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(3,n)  \"[^\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(3,n)(c == '\n')

                       else if(3,n)(c == '*')
                           if(3,n)((c = yyinput()) == '/')

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

           \n        mylineno++;

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

           {string(3,n)}  cout << "string(3,n) " << 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(1,2,n) each yyFlexLexer to some
       other xxFlexLexer.  You then can include <FlexLexer.h>  in(1,8)  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(3,n),  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(1,2) 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(1,8) 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(2,3)  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(2,n,1 builtins) 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(1,8)
              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(1,8)
              accordance with POSIX.

       -      flex scanners are not as reentrant as lex scanners.  In particu-
              lar, if(3,n) 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(3,n)
              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(1,8) the POSIX specification.

       -      When  definitions  are expanded, flex encloses them in(1,8) parenthe-
              ses.  With lex, the following:

                  NAME    [A-Z][A-Z0-9]*
                  foo{NAME}?      printf(1,3,1 builtins)( "Found it\n" );

              will not match the  string(3,n)  "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(3,n) "foo" will match.

              Note that if(3,n) the definition begins with ^ or ends with $ then it
              is  not  expanded  with parentheses, to allow these operators to
              appear in(1,8) definitions without  losing  their  special  meanings.
              But  the  <s>, /, and <<EOF>> operators cannot be used in(1,8) a flex

              Using -l results in(1,8) the lex behavior of  no  parentheses  around
              the definition.

              The  POSIX  specification  is that the definition be enclosed in(1,8)

       -      Some implementations of lex allow a rule's action to begin on  a
              separate line, if(3,n) 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(1,8) 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(3,n) they come at the beginning of  a  line".   The
              latter is in(1,8) 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(1,3,5)  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(1,8) lex or the POSIX speci-

           C++ scanners
           start condition scopes
           start condition stacks
           interactive/non-interactive scanners
           subset sorting for disambiguating rules
           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(1,8) 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(2,7) the action.  Actions that are not enclosed in(1,8)
       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 (or, in(1,8) the case of %option subset-sort,  other  rules  com-
       pletely  cover  all  of  its  possible  matches while each of them only
       matches a proper subset).  For example, in(1,8) 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(1,8) a scanner suppresses this warning.  Note that the above
       example works if(3,n) using %option subset-sort as the second rule  is  pre-
       ferred  over  the  first.  However, as the following example shows such
       problems can occur in(1,8) this case  as  well;  the  first  rule  is  never

           (foo|bar)   got_foo_or_bar();
           foo         got_foo();
           bar         got_bar();

       warning,  -s option given but default rule can be matched means that it
       is possible (perhaps only in(1,8) 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

       warning,  rule  is  in(1,8)  conflict with another rule indicates that while
       using %option subset-sort two rules had common matches but the language
       generated  by  neither  was  a  proper  subset of that generated by the
       other's.  This problem occurs in(1,8) the following example:

           ba.         got_ba();
           .ar         got_ar();

       The rule with which a rule is in(1,8) conflict is also shown as a (separate)
       warning.  In  this  case the rule listed first is chosen.  To eliminate
       the warning you would have to reformulate your rule set(7,n,1 builtins); e.g.,  if(3,n)  you
       wanted the string(3,n) "bar" to be matched by the second rule:

           ba.         got_ba();
           (ba.|.ar)   got_ar();

       Note  that  the default rule is often in(1,8) conflict with (multiple) other
       rules.  Thus, conflict warnings concerning the default rule are  always
       suppressed.   It  is only ever used as a last resort when no other rule
       applies (or if(3,n) REJECT is used and the default rule is not  suppressed).

       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(1,8) (via a #include file(1,n), 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(3,n) which wasn't matched by any of its rules.  This error(8,n) can
       also occur due to internal problems.

       token too large, exceeds YYLMAX - your scanner uses %array and  one  of
       its rules matched a string(3,n) longer than the YYLMAX constant (8K bytes by
       default).  You can increase the value by #define'ing YYLMAX in(1,8) 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(1,8) yytext.  Ideally the scanner should dynami-
       cally resize the buffer in(1,8) 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(1,8) an 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(1,n) to using the C++ scanner class.

       too many start conditions in(1,8) <> construct! - you listed more start con-
       ditions  in(1,8) a <> construct than exist (so you must have listed at least
       one of them twice).

       -lfl   library with which scanners must be linked.

              generated scanner (called lexyy.c on some systems).
              generated C++ scanner class, when using -+.

              header file(1,n) defining the C++ scanner base class, FlexLexer,  and
              its derived class, yyFlexLexer.

              skeleton  scanner.   This  file(1,n) 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 abovementioned performance
       loss.  In particular, parts using '|' or {n}  (such  as  "foo{3}")  are
       always considered variable-length.

       Combining  trailing  context  with the special '|' action can result in(1,8)
       fixed trailing context being turned into the  more  expensive  variable
       trailing context.  For example, in(1,8) 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(3,n) the scanner does
       not  use  REJECT,  and somewhat greater than the number of states if(3,n) 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(1,3,5) 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 margin-
       al 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;
       to  Eric Hughes for support of multiple buffers; and to Leif Kornstaedt
       for support of subset sorting.

       This work was primarily done when I was  with  the  Real  Time  Systems
       Group at the Lawrence Berkeley Laboratory in(1,8) Berkeley, CA.  Many thanks
       to all there for the support I received.

       Send comments to

Version 2.5                       April 1995                           FLEX(1)

References for this manual (incoming links)