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FLEX(1)                     General Commands Manual                    FLEX(1)

       flex, lex - 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
       reference 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 ATT lex and the POSIX lex

               those error messages produced by flex (or scanners
               it generates) whose meanings might not be apparent

               files used by flex

           Deficiencies / Bugs
               known problems with flex

           See Also
               other documentation, related tools

               includes contact information

       flex is a tool  for  generating  scanners:  programs  which  recognized
       lexical  patterns  in  text.   flex reads the given input files, or its
       standard input if no file names are  given,  for  a  description  of  a
       scanner  to  generate.   The  description  is  in  the form of pairs of
       regular expressions and C code, called rules.  flex generates as output
       a  C source file, lex.yy.c, which defines a routine yylex().  This file
       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
       corresponding C code.

       First some simple examples to get the flavor of how one uses flex.  The
       following flex input specifies a scanner which whenever  it  encounters
       the string "username" will replace it with the user's login name:

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

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

       Here's another simple example:

                   int num_lines = 0, num_chars = 0;

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

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

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

       A somewhat more complicated example:

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

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

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


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

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

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

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

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

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

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

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


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


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

       The details  of  this  example  will  be  explained  in  the  following

       The  flex  input  file  consists of three sections, separated by a line
       with just %% in it:

           user code

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

       Name definitions have the form:

           name definition

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

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

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


       is identical to


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

       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
       scanner.  The presence of this section is optional; if it  is  missing,
       the second %% in the input file may be skipped, too.

       In  the  definitions  and  rules  sections,  any  indented text or text
       enclosed in %{ and %} is copied verbatim to the output (with the  %{}'s
       removed).  The %{}'s must appear unindented on lines by themselves.

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

       In  the  definitions  section  (but  not  in  the  rules  section),  an
       unindented  comment  (i.e.,  a line beginning with "/*") is also copied
       verbatim to the output up to the next "*/".

       The patterns in the input are written using an extended set of  regular
       expressions.  These are:

           x          match the character 'x'
           .          any character (byte) except newline
           [xyz]      a "character class"; in this case, the pattern
                        matches either an 'x', a 'y', or a 'z'
           [abj-oZ]   a "character class" with a range in it; matches
                        an 'a', a 'b', any letter from 'j' through 'o',
                        or a 'Z'
           [^A-Z]     a "negated character class", i.e., any character
                        but those in the class.  In this case, any
                        character EXCEPT an uppercase letter.
           [^A-Z\n]   any character EXCEPT an uppercase letter or
                        a newline
           r*         zero or more r's, where r is any regular expression
           r+         one or more r's
           r?         zero or one r's (that is, "an optional r")
           r{2,5}     anywhere from two to five r's
           r{2,}      two or more r's
           r{4}       exactly 4 r's
           {name}     the expansion of the "name" definition
                      (see above)
                      the literal string: [xyz]"foo
           \X         if X is an 'a', 'b', 'f', 'n', 'r', 't', or 'v',
                        then the ANSI-C interpretation of \x.
                        Otherwise, a literal 'X' (used to escape
                        operators such as '*')
           \0         a NUL character (ASCII code 0)
           \123       the character with octal value 123
           \x2a       the character with hexadecimal value 2a
           (r)        match an r; parentheses are used to override
                        precedence (see below)

           rs         the regular expression r followed by the
                        regular expression s; called "concatenation"

           r|s        either an r or an s

           r/s        an r but only if it is followed by an s.  The
                        text matched by s is included when determining
                        whether this rule is the "longest match",
                        but is then returned to the input before
                        the action is executed.  So the action only
                        sees the text matched by r.  This type
                        of pattern is called trailing context".
                        (There are some combinations of r/s that flex
                        cannot match correctly; see notes in the
                        Deficiencies / Bugs section below regarding
                        "dangerous trailing context".)
           ^r         an r, but only at the beginning of a line (i.e.,
                        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 particular, on some DOS
                      systems you must either filter out \r's in the
                      input yourself, or explicitly use r/\r\n for "r$".

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

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

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

       The   regular   expressions  listed  above  are  grouped  according  to
       precedence, from highest precedence at the top to lowest at the bottom.
       Those grouped together have equal precedence.  For example,


       is the same as


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


       and to match zero-or-more "foo"'s-or-"bar"'s:


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

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

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

       For example, the following character classes are all equivalent:


       If your scanner is case-insensitive (the -i flag), then  [:upper:]  and
       [:lower:] are equivalent to [:alpha:].

       Some notes on patterns:

       -      A  negated  character  class  such as the example "[^A-Z]" above
              will match a  newline  unless  "\n"  (or  an  equivalent  escape
              sequence)  is  one  of  the characters explicitly present in the
              negated character class (e.g., "[^A-Z\n]").  This is unlike  how
              many  other  regular  expression  tools  treat negated character
              classes, but unfortunately  the  inconsistency  is  historically
              entrenched.   Matching  newlines means that a pattern like [^"]*
              can match the entire input unless there's another quote  in  the

       -      A  rule  can  have at most one instance of trailing context (the
              '/' operator or the '$' operator).  The  start  condition,  '^',
              and "EOF" patterns can only occur at the beginning of a pattern,
              and, as well as with '/'  and  '$',  cannot  be  grouped  inside
              parentheses.   A  '^' which does not occur at the beginning of a
              rule or a '$' which does not occur at the end of  a  rule  loses
              its special properties and is treated as a normal character.

              The following are illegal:


              Note that the first of these, can be written "foo/bar\n".

              The  following  will  result  in  '$'  or '^' being treated as a
              normal character:


              If what's wanted is a "foo" or a bar-followed-by-a-newline,  the
              following  could  be  used  (the special '|' action is explained

                  foo      |
                  bar$     /* action goes here */

              A similar trick will work for matching a foo  or  a  bar-at-the-

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

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

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


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

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

       The  advantage  of  %array  is  that you can then modify yytext to your
       heart's content, and calls  to  unput()  do  not  destroy  yytext  (see
       below).   Furthermore,  existing  lex  programs sometimes access yytext
       externally using declarations of the form:
           extern char yytext[];
       This definition is erroneous when used with %pointer, but  correct  for

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

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

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

           "zap me"

       (It  will  copy  all  other characters in the input to the output since
       they will be matched by the default rule.)

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

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

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

       An action consisting solely of a vertical bar ('|') means "same as  the
       action for the next rule."  See below for an illustration.

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

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

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

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

       -      ECHO copies yytext to the scanner's output.

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

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

                          int word_count = 0;

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

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

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

              (The  first three rules share the fourth's action since they use
              the special '|' action.)  REJECT  is  a  particularly  expensive
              feature in terms of scanner performance; if it is used in any of
              the scanner's actions it will slow down  all  of  the  scanner's
              matching.   Furthermore,  REJECT  cannot be used with the -Cf or
              -CF options (see below).

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

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

                  mega-    ECHO; yymore();
                  kludge   ECHO;

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

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

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

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

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

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

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

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

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

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

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

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

                  "/*"        {
                              register int c;

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       A sample definition of YY_INPUT (in  the  definitions  section  of  the
       input file):

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

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

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

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

       Three routines are available for scanning from in-memory buffers rather
       than files: yy_scan_string(),  yy_scan_bytes(),  and  yy_scan_buffer().
       See the discussion of them below in the section Multiple Input Buffers.

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

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

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

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

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

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

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

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

           %s example

           examplefoo   do_something();

           bar            something_else();

       is equivalent to

           %x example

           examplefoo   do_something();

           INITIAL,examplebar    something_else();

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

           examplefoo   do_something();

           *bar    something_else();

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

           *.|\n     ECHO;

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

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

                   int enter_special;

           %x SPECIAL
                   if ( enter_special )

           ...more rules follow...

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

           #include math.h
           %s expect

           expect-floats        BEGIN(expect);

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

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

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

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

           %x comment
                   int line_num = 1;

           "/*"         BEGIN(comment);

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

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

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

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

           "/*"         {
                        comment_caller = INITIAL;


           foo"/*"    {
                        comment_caller = foo;

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

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

           comment_caller = YY_START;

       Flex  provides  YYSTATE  as an alias for YY_START (since that is what's
       used by ATT lex).

       Note that start conditions do not have their own name-space;  %s's  and
       %x's declare names in the same fashion as #define's.

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

           %x str

                   char string_buf[MAX_STR_CONST];
                   char *string_buf_ptr;

           \"      string_buf_ptr = string_buf; BEGIN(str);

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

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

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

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

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

                   *string_buf_ptr++ = result;

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

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

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

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

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

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


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

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

       is equivalent to:

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

       Start condition scopes may be nested.

       Three  routines  are  available  for  manipulating  stacks   of   start

       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

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

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

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

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

           YY_BUFFER_STATE yy_create_buffer( FILE *file, int size )

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

           void yy_switch_to_buffer( YY_BUFFER_STATE new_buffer )

       switches the scanner's input buffer so subsequent tokens will come from
       new_buffer.  Note that yy_switch_to_buffer() may be used by yywrap() to
       set things up for continued scanning, instead of opening a new file and
       pointing yyin at it.  Note also that switching input sources via either
       yy_switch_to_buffer() or yywrap() does not change the start condition.

           void yy_delete_buffer( YY_BUFFER_STATE buffer )

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

           void yy_flush_buffer( YY_BUFFER_STATE buffer )

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

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

       Finally,  the  YY_CURRENT_BUFFER macro returns a YY_BUFFER_STATE handle
       to the current buffer.

       Here is an example of using these features for writing a scanner  which
       expands include files (the EOF feature is discussed below):

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

           #define MAX_INCLUDE_DEPTH 10
           YY_BUFFER_STATE include_stack[MAX_INCLUDE_DEPTH];
           int include_stack_ptr = 0;

           include             BEGIN(incl);

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

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

                   include_stack[include_stack_ptr++] =

                   yyin = fopen( yytext, "r" );

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

                       yy_create_buffer( yyin, YY_BUF_SIZE ) );


           EOF {
                   if ( --include_stack_ptr  0 )

                       yy_delete_buffer( YY_CURRENT_BUFFER );
                            include_stack[include_stack_ptr] );

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

       yy_scan_string(const char *str)
              scans a NUL-terminated string.

       yy_scan_bytes(const char *bytes, int len)
              scans  len bytes (including possibly NUL's) starting at location

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

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

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

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

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

       -      assigning yyin to a new input  file  (in  previous  versions  of
              flex,  after  doing  the  assignment you had to call the special
              action YY_NEW_FILE; this is no longer necessary);

       -      executing a return statement;

       -      executing the special yyterminate() action;

       -      or, switching to a new  buffer  using  yy_switch_to_buffer()  as
              shown in the example above.

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


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

           %x quote

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

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

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

           #define YY_USER_ACTION ++ctr[yy_act]

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

           int ctr[YY_NUM_RULES];

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

       The macro yy_set_interactive(is_interactive) can  be  used  to  control
       whether  the  current buffer is considered interactive.  An interactive
       buffer is processed more slowly, but must be used  when  the  scanner's
       input  source is indeed interactive to avoid problems due to waiting to
       fill buffers (see the discussion of the -I  flag  below).   A  non-zero
       value  in  the macro invocation marks the buffer as interactive, a zero
       value as non-interactive.   Note  that  use  of  this  macro  overrides
       %option  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 the next token scanned  from  the
       current buffer will have '^' rules active, false otherwise.

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

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

       -      char  *yytext  holds  the  text of the current token.  It may be
              modified but not lengthened (you cannot append characters to the

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

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

       -      int yyleng holds the length of the current token.

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

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

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

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

           #include ""


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

       flex has the following options:

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

       -c     is   a   do-nothing,   deprecated   option  included  for  POSIX

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

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

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

       -f, --full
              specifies fast scanner.  No table compression is done and  stdio
              is  bypassed.   The  result  is  large but fast.  This option is
              equivalent to -Cfr (see below).

       -h, --help
              generates a "help" summary of flex's options to stdout and  then
              exits.  -?  and --help are synonyms for -h.

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

       -l, --lex-compat
              turns  on  maximum  compatibility  with  the  original  ATT  lex
              implementation.    Note   that   this   does   not   mean   full
              compatibility.   Use  of this option costs a considerable amount
              of performance, and it cannot be used with the -+, -f, -F,  -Cf,
              or -CF options.  For details on the compatibilities it provides,
              see the section "Incompatibilities With Lex  And  POSIX"  below.
              This  option  also  results in the name YY_FLEX_LEX_COMPAT being
              #define'd in the generated scanner.

       -n     is another do-nothing, deprecated option included only for POSIX

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

              Note  that  the  use  of  REJECT, %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, --no-default
              causes the default rule (that unmatched scanner input is  echoed
              to  stdout)  to  be suppressed.  If the scanner encounters input
              that does not match any of its rules, it aborts with  an  error.
              This option is useful for finding holes in a scanner's rule set.

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

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

       -w, --nowarn
              suppresses warning messages.

       -B, --batch
              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
              performance out of it.  If your goal is instead to squeeze out a
              lot more performance, you  should   be  using  the  -Cf  or  -CF
              options  (discussed  below),  which  turn  on  -B  automatically

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

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

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

              This option is equivalent to -CFr (see  below).   It  cannot  be
              used with -+.

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

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

              You  can  force a scanner to not be interactive by using -B (see

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

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

       -V, --version
              prints  the  version number to stdout and exits.  --version is a
              synonym for -V.

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

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

              Note that if you use  -Cfe  or  -CFe  (those  table  compression
              options,  but  also  using  equivalence classes as discussed see
              below), flex still defaults  to  generating  an  8-bit  scanner,
              since  usually  with these compression options full 8-bit tables
              are not much more expensive than 7-bit tables.

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

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

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

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

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

              -CF specifies that the alternative fast  scanner  representation
              (described above under the -F flag) should be used.  This option
              cannot be used with -+.

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

              -Cr, --read causes the generated scanner to bypass  use  of  the
              standard  I/O  library  (stdio)  for  input.  Instead of calling
              fread() or getc(), the scanner will use the read() system  call,
              resulting  in  a  performance  gain  which varies from system to
              system, but in general is probably  negligible  unless  you  are
              also using -Cf or -CF.  Using -Cr can cause strange behavior if,
              for example, you read from yyin using stdio prior to calling the
              scanner  (because  the  scanner  will  miss  whatever  text your
              previous reads left in the stdio input buffer).

              -Cr has no effect if you  define  YY_INPUT  (see  The  Generated
              Scanner above).

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

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

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

                  slowest  smallest
                  fastest  largest

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

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

       -ooutput, --outputfile=FILE
              directs  flex to write the scanner to the file output instead of
              lex.yy.c.  If you combine  -o  with  the  -t  option,  then  the
              scanner  is  written to stdout but its #line directives (see the
              -L option above) refer to the file output.

       -Pprefix, --prefix=STRING
              changes the default yy prefix used by  flex  for  all  globally-
              visible  variable  and function names to instead be prefix.  For
              example, -Pfoo changes the name of yytext to footext.   It  also
              changes  the  name  of  the default output file from lex.yy.c to
      Here are all of the names affected:


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

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

       -Sskeleton_file, --skel=FILE
              overrides  the  default skeleton file from which flex constructs
              its scanners.  You'll never need  this  option  unless  you  are
              doing flex maintenance or development.

       -X, --posix-compat
              maximal compatibility with POSIX lex.

              track line count in yylineno.

              name of C++ class.

              create a C header file in addition to the scanner.

              write tables to FILE.

              #define macro defn (default defn is '1').

       -R, --reentrant
              generate a reentrant C scanner

              scanner for bison pure parser.

              include yylloc support.

              initialize yyin/yyout to stdin/stdout.

       --noansi-definitions old-style function definitions.

              empty parameter list in prototypes.

              do not include unistd.h.

              do not generate a particular FUNCTION.

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

       Most options are given simply as names, optionally preceded by the word
       "no"  (with  no  intervening  whitespace)  to  negate their meaning.  A
       number are equivalent to flex flags or their negation:

           7bit            -7 option
           8bit            -8 option
           align           -Ca option
           backup          -b option
           batch           -B option
           c++             -+ option

           caseful or
           case-sensitive  opposite of -i (default)

           case-insensitive or
           caseless        -i option

           debug           -d option
           default         opposite of -s option
           ecs             -Ce option
           fast            -F option
           full            -f option
           interactive     -I option
           lex-compat      -l option
           meta-ecs        -Cm option
           perf-report     -p option
           read            -Cr option
           stdout          -t option
           verbose         -v option
           warn            opposite of -w option
                           (use "%option nowarn" for -w)

           array           equivalent to "%array"
           pointer         equivalent to "%pointer" (default)

       Some %option's provide features otherwise not available:

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

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

              instructs  flex  to generate a scanner which never considers its
              input "interactive" (again, no call made to isatty()).  This  is
              the opposite of always-interactive.

       stack  enables  the use of start condition stacks (see Start Conditions

              if set (i.e., %option stdinit) initializes  yyin  and  yyout  to
              stdin  and stdout, instead of the default of nil.  Some existing
              lex programs depend on this behavior,  even  though  it  is  not
              compliant  with  ANSI C, which does not require stdin and stdout
              to be compile-time constant.

              directs flex to generate a scanner that maintains the number  of
              the  current  line  read  from  its input in the global variable
              yylineno.  This option is implied by %option lex-compat.

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

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

       Three options take string-delimited values, offset with '=':

           %option outfile="ABC"

       is equivalent to -oABC, and

           %option prefix="XYZ"

       is equivalent to -PXYZ.  Finally,

           %option yyclass="foo"

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

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

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

       (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
       scanners.   It  has  been optimized for dealing well with large sets of
       rules.   Aside  from  the  effects  on  scanner  speed  of  the   table
       compression   -C   options  outlined  above,  there  are  a  number  of
       options/actions  which  degrade  performance.   These  are,  from  most
       expensive to least:

           %option yylineno
           arbitrary trailing context

           pattern sets that require backing up
           %option interactive
           %option always-interactive

           '^' beginning-of-line operator

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

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

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

           foo        return TOK_KEYWORD;
           foobar     return TOK_KEYWORD;

       the file looks like:

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

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

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

           Compressed tables always back up.

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

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

       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

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

           foo         return TOK_KEYWORD;
           foobar      return TOK_KEYWORD;

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

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

           foo         return TOK_KEYWORD;
           foobar      return TOK_KEYWORD;

           [a-z]+      return TOK_ID;

       This is usually the best solution when appropriate.

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

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

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

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

       is better written:

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

       or as

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

       Note that here the special '|' action does not provide any savings, and
       can even make things worse (see Deficiencies / Bugs below).

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

           %x comment
                   int line_num = 1;

           "/*"         BEGIN(comment);

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

       This could be sped up by writing it as:

           %x comment
                   int line_num = 1;

           "/*"         BEGIN(comment);

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       The first class, 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
              equivalent of yytext.

       int YYLeng()
              returns  the  length  of  the  most  recently matched token, the
              equivalent of yyleng.

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

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

       int debug() const
              returns the current setting of the debugging flag.

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

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

       yyFlexLexer( std::istream* arg_yyin = 0, std::ostream* arg_yyout = 0 )
              constructs a yyFlexLexer object  using  the  given  streams  for
              input  and output.  If not specified, the streams default to cin
              and cout, respectively.

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

       virtual void switch_streams(std::istream* new_in = 0,
              std::ostream* new_out = 0) reassigns yyin to new_in (if non-nil)
              and yyout to new_out (ditto), deleting the previous input buffer
              if yyin is reassigned.

       int yylex( std::istream* new_in, std::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
       functions  which  you  can  redefine  in  derived classes to tailor the

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

       virtual void LexerOutput( const char* buf, int size )
              writes  out  size  characters  from the buffer buf, which, while
              NUL-terminated,  may  also  contain  "internal"  NUL's  if   the
              scanner's rules can match text with NUL's in them.

       virtual void LexerError( const char* msg )
              reports  a  fatal  error  message.   The default version of this
              function writes the message to the stream cerr and exits.

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

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

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

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

           int mylineno = 0;

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

           ws      [ \t]+

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


           {ws}    /* skip blanks and tabs */

           "/*"    {
                   int c;

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

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

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

           \n        mylineno++;

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

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


           int main( int /* argc */, char** /* argv */ )
               FlexLexer* lexer = new yyFlexLexer;
               while(lexer-yylex() != 0)
               return 0;
       If  you  want to create multiple (different) lexer classes, you use the
       -P flag (or the prefix= option) to  rename  each  yyFlexLexer  to  some
       other  xxFlexLexer.   You  then  can  include FlexLexer.h in your other
       sources once per lexer class, first renaming yyFlexLexer as follows:

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

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

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

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

       flex is a rewrite of the ATT Unix lex tool (the two implementations  do
       not    share    any   code,   though),   with   some   extensions   and
       incompatibilities, both of which are of concern to those  who  wish  to
       write  scanners  acceptable  to  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, ATT lex, and the POSIX specification.

       flex's  -l  option turns on maximum compatibility with the original ATT
       lex implementation, at the cost  of  a  major  loss  in  the  generated
       scanner's  performance.   We  note below which incompatibilities can be
       overcome using the -l option.

       flex is fully compatible with lex with the following exceptions:

       -      The undocumented lex scanner internal variable yylineno  is  not
              supported unless -l or %option yylineno is used.

              yylineno should be maintained on a per-buffer basis, rather than
              a per-scanner (single global variable) basis.

              yylineno is not part of the POSIX specification.

       -      The input() routine is not redefinable, though it may be  called
              to  read  characters  following  whatever  has been matched by a
              rule.  If input() encounters an end-of-file the normal  yywrap()
              processing  is  done.   A  ``real''  end-of-file  is returned by
              input() as EOF.

              Input is instead controlled by defining the YY_INPUT macro.

              The flex restriction that input()  cannot  be  redefined  is  in
              accordance  with  the POSIX specification, which simply does not
              specify any way of controlling the scanner's input other than by
              making an initial assignment to yyin.

       -      The  unput() routine is not redefinable.  This restriction is in
              accordance with POSIX.

       -      flex  scanners  are  not  as  reentrant  as  lex  scanners.   In
              particular,  if you have an interactive scanner and an interrupt
              handler which long-jumps out of the scanner, and the scanner  is
              subsequently called again, you may get the following message:

                  fatal flex scanner internal error--end of buffer missed

              To reenter the scanner, first use

                  yyrestart( yyin );

              Note  that this call will throw away any buffered input; usually
              this isn't a problem with an interactive scanner.

              Also note that flex C++ scanner classes  are  reentrant,  so  if
              using  C++  is  an  option for you, you should use them instead.
              See "Generating C++ Scanners" above for details.

       -      output() is not supported.  Output from the ECHO macro  is  done
              to the file-pointer yyout (default stdout).

              output() is not part of the POSIX specification.

       -      lex  does  not  support  exclusive start conditions (%x), though
              they are in the POSIX specification.

       -      When  definitions  are   expanded,   flex   encloses   them   in
              parentheses.  With lex, the following:

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

              will  not  match  the  string  "foo"  because  when the macro is
              expanded the rule is equivalent to "foo[A-Z][A-Z0-9]*?"  and the
              precedence  is such that the '?' is associated with "[A-Z0-9]*".
              With flex, the rule will be expanded  to  "foo([A-Z][A-Z0-9]*)?"
              and so the string "foo" will match.

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

              Using  -l  results  in the lex behavior of no parentheses around
              the definition.

              The POSIX specification is that the definition  be  enclosed  in

       -      Some  implementations of lex allow a rule's action to begin on a
              separate line, if the rule's pattern has trailing whitespace:

                  foo|barspace 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
              occurrences of 'abc'", whereas flex interprets it as "match 'ab'
              followed  by one, two, or three occurrences of 'c'".  The latter
              is in agreement with the POSIX specification.

       -      The precedence of the ^ operator is different.   lex  interprets
              "^foo|bar" as "match either 'foo' at the beginning of a line, or
              'bar' anywhere", whereas flex interprets  it  as  "match  either
              'foo'  or  'bar'  if they come at the beginning of a line".  The
              latter is in agreement with the POSIX specification.

       -      The special table-size declarations such as %a supported by  lex
              are not required by flex scanners; flex ignores them.

       -      The  name  FLEX_SCANNER  is #define'd so scanners may be written
              for  use  with  either  flex  or  lex.   Scanners  also  include
              YY_FLEX_MAJOR_VERSION and YY_FLEX_MINOR_VERSION indicating which
              version of flex generated the scanner (for example, for the  2.5
              release, these defines would be 2 and 5 respectively).

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

           C++ scanners
           start condition scopes
           start condition stacks
           interactive/non-interactive scanners
           yy_scan_string() and friends
           #line directives
           %{}'s around actions
           multiple actions on a line

       plus almost all of the flex flags.  The last feature in the list refers
       to  the  fact  that  with flex you can put multiple actions on the same
       line, separated with semi-colons, while with lex, the following

           foo    handle_foo(); ++num_foos_seen;

       is (rather surprisingly) truncated to

           foo    handle_foo();

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

       warning, rule cannot be matched indicates that the given rule cannot be
       matched because it follows other rules that will always match the  same
       text  as  it.   For  example,  in the following "foo" cannot be matched
       because it comes after an identifier "catch-all" rule:

           [a-z]+    got_identifier();
           foo       got_foo();

       Using REJECT in a scanner suppresses this warning.

       warning, -s option given but default rule can be matched means that  it
       is  possible  (perhaps  only  in a particular start condition) that the
       default rule (match any single character) is the  only  one  that  will
       match  a  particular input.  Since -s was given, presumably this is not

       reject_used_but_not_detected undefined or  yymore_used_but_not_detected
       undefined - These errors can occur at compile time.  They indicate that
       the scanner uses REJECT or yymore() but that flex failed to notice  the
       fact,  meaning  that  flex  scanned  the first two sections looking for
       occurrences of these actions and failed to find any,  but  somehow  you
       snuck  some  in (via a #include file, for example).  Use %option reject
       or %option yymore to indicate to flex that  you  really  do  use  these

       flex  scanner  jammed  -  a scanner compiled with -s has encountered an
       input string which wasn't matched by any of its rules.  This error  can
       also occur due to internal problems.

       token  too  large, exceeds YYLMAX - your scanner uses %array and one of
       its rules matched a string longer than the YYLMAX constant (8K bytes by
       default).   You  can  increase  the  value by #define'ing YYLMAX in the
       definitions section of your flex input.

       scanner requires -8 flag to  use  the  character  'x'  -  Your  scanner
       specification  includes recognizing the 8-bit character 'x' and you did
       not specify the -8 flag, and your scanner defaulted  to  7-bit  because
       you  used the -Cf or -CF table compression options.  See the discussion
       of the -7 flag for details.

       flex scanner push-back overflow - you used unput() to push back so much
       text that the scanner's buffer could not hold both the pushed-back text
       and  the  current  token  in  yytext.   Ideally  the   scanner   should
       dynamically resize the buffer in this case, but at present it does not.

       input buffer overflow, can't enlarge buffer because scanner uses REJECT
       - the scanner was working on matching  an  extremely  large  token  and
       needed  to  expand  the  input buffer.  This doesn't work with scanners
       that use REJECT.

       fatal flex scanner internal error--end of  buffer  missed  -  This  can
       occur in an scanner which is reentered after a long-jump has jumped out
       (or over)  the  scanner's  activation  frame.   Before  reentering  the
       scanner, use:

           yyrestart( yyin );

       or, as noted above, switch to using the C++ scanner class.

       too  many  start  conditions  in  construct!  -  you  listed more start
       conditions in 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 defining the C++ scanner base class, FlexLexer, and
              its derived class, yyFlexLexer.

              skeleton scanner.  This file is only used  when  building  flex,
              not when flex executes.

              backing-up  information  for  -b  flag  (called  lex.bck on some

       Some trailing context patterns cannot be properly matched and  generate
       warning  messages  ("dangerous  trailing context").  These are patterns
       where the ending of the first part of the rule matches the beginning of
       the  second  part, such as "zx*/xy*", where the 'x*' matches the 'x' at
       the beginning of the trailing context.   (Note  that  the  POSIX  draft
       states that the text matched by such patterns is undefined.)

       For  some trailing context rules, parts which are actually fixed-length
       are not recognized as such, leading to the above mentioned  performance
       loss.   In  particular,  parts  using '|' or {n} (such as "foo{3}") are
       always considered variable-length.

       Combining trailing context with the special '|' action  can  result  in
       fixed  trailing  context  being turned into the more expensive variable
       trailing context.  For example, in the following:

           abc      |

       Use of  unput()  invalidates  yytext  and  yyleng,  unless  the  %array
       directive 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
       number  of  entries is equal to the number of DFA states if the scanner
       does not use REJECT, and somewhat greater than the number of states  if
       it does.

       REJECT cannot be used with the -f or -F options.

       The flex internal algorithms need documentation.

       lex(1), yacc(1), sed(1), awk(1).

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

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

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

       Vern Paxson, with the help of many ideas and much inspiration from  Van
       Jacobson.    Original   version  by  Jef  Poskanzer.   The  fast  table
       representation 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
       Adermann,  Terry Allen, David Barker-Plummer, John Basrai, Neal Becker,
       Nelson H.F. Beebe,, Karl Berry, Peter  A.  Bigot,  Simon
       Blanchard,  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 de Raadt, 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,
       Terrence  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,
       Walter Pelissero, Gaumond  Pierre,  Esmond  Pitt,  Jef  Poskanzer,  Joe
       Rahmeh,  Jarmo  Raiha, Frederic Raimbault, Pat Rankin, Rick Richardson,
       Kevin Rodgers, Kai Uwe Rommel, Jim Roskind,  Alberto  Santini,  Andreas
       Scherer,  Darrell  Schiebel,  Raf  Schietekat,  Doug  Schmidt, Philippe
       Schnoebelen, Andreas Schwab, Larry  Schwimmer,  Alex  Siegel,  Eckehard
       Stolz,  Jan-Erik Strvmquist, Mike Stump, Paul Stuart, Dave Tallman, Ian
       Lance Taylor, Chris  Thewalt,  Richard  M.  Timoney,  Jodi  Tsai,  Paul
       Tuinenga, Gary Weik, Frank Whaley, Gerhard Wilhelms, Kent Williams, Ken
       Yap, Ron Zellar, Nathan Zelle, David Zuhn, and those whose  names  have
       slipped  my  marginal mail-archiving skills but whose contributions are
       appreciated all the same.

       Thanks to Keith Bostic, Jon Forrest, Noah Friedman, John Gilmore, Craig
       Leres,  John  Levine,  Bob  Mulcahy, G.T.  Nicol, Francois Pinard, Rich
       Salz,  and  Richard  Stallman  for  help  with   various   distribution

       Thanks  to Esmond Pitt and Earle Horton for 8-bit character support; to
       Benson Margulies and Fred Burke for C++ support; to Kent  Williams  and
       Tom Epperly for C++ class support; to Ove Ewerlid for support of NUL's;
       and to Eric Hughes for support of multiple buffers.

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

       Send comments to

Version 2.5                      February 2008                         FLEX(1)