MULTIPLE INPUT BUFFERS

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

          #define YY_USER_ACTION ++ctr[yy_act]

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

          int ctr[YY_NUM_RULES];


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

      The macro yy_set_interactive(is_interactive) can be used to control
      whether the current buffer is considered interactive.  An interactive
      buffer is processed more slowly, but must be used when the scanner's
      input source is indeed interactive to avoid problems due to waiting to
      fill buffers (see the discussion of the -I flag 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
           end).

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

      -    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 y.tab.h containing
      definitions of all the %tokens appearing in the yacc input.  This file
      is then included in the flex scanner.  For example, if one of the
      tokens is "TOK_NUMBER", part of the scanner might look like:

          %{
          #include "y.tab.h"
          %}

          %%

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

flex has the following options:

      -bGenerate 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.)



      -cis a do-nothing, deprecated option included for POSIX compliance.

      -dmakes 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.

      -fspecifies 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).

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

      -iinstructs 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).

      -lturns on maximum compatibility with the original AT&T lex
            implementation.  Note that this does not mean full compatibility.
            Use of this option costs a considerable amount of performance,
            and it cannot be used with the -+, -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.

      -nis another do-nothing, deprecated option included only for POSIX
            compliance.

      -pgenerates 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.


      -scauses 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.

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

      -vspecifies 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.

      -wsuppresses warning messages.

      -Binstructs 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 anyway.

      -Fspecifies 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 -+.

      -Iinstructs 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
            above).

      -Linstructs 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).

      -Tmakes 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.

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

      -7instructs 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.

      -8instructs 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.

      -+specifies that you want flex to generate a C++ scanner class.
            See the section on Generating C++ Scanners below for details.

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

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

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

        -Cf specifies that the full scanner tables should be generated -
           flex should not compress the tables by taking advantages of
           similar transition functions for different states.
        -CF specifies that the alternate fast scanner representation
           (described above under the -F flag) should be used.  This option
           cannot be used with -+.

        -Cm directs flex to construct meta-equivalence classes, which are
           sets of equivalence classes (or characters, if equivalence
           classes are not being used) that are commonly used together.
           Meta-equivalence classes are often a big win when using
           compressed tables, but they have a moderate performance impact

           (one or two "if" tests and one array look-up per character
           scanned).

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

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

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

           The options -Cf or -CF and -Cm do not make sense together - there
           is no opportunity for meta-equivalence classes if the 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
                     -Cem
                     -Cm
                     -Ce
                     -C
                     -C{f,F}e
                     -C{f,F}
                     -C{f,F}a
               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
           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
           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
           lex.foo.c.  Here are all of the names affected:

               yy_create_buffer
               yy_delete_buffer
               yy_flex_debug
               yy_init_buffer
               yy_flush_buffer
               yy_load_buffer_state
               yy_switch_to_buffer
               yyin
               yyleng
               yylex
               yylineno
               yyout
               yyrestart
               yytext
               yywrap

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

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

      -Sskeleton_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.

      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:

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

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

   never-interactive
           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
           above).
  
      stdinit
           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.

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

   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:

          REJECT
          %option yylineno
          arbitrary trailing context

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

          '^' beginning-of-line operator
          yymore()

      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
      yyless().

      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:
                 3
           out-transitions: [ a ]
           jam-transitions: EOF [ \001-`  b-\177 ]

          State #9 is non-accepting -
           associated rule line numbers:
                 3
           out-transitions: [ r ]
           jam-transitions: EOF [ \001-q  s-\177 ]

          Compressed tables always back up.

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

      The comment regarding State #8 indicates there's a problem when "foob"
      has been scanned.  Indeed, on any character other than an 'a', the
      scanner will have to back up to accept "foo".  Similarly, the comment
      for State #9 concerns when "fooba" has been scanned and an 'r' does
      not follow.
      The final comment reminds us that there's no point going to all the
      trouble of removing backing up from the rules unless we're using -Cf
      or -CF, since there's no performance gain doing so with compressed
      scanners.

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

          %%
          foo         return TOK_KEYWORD;
          foobar      return TOK_KEYWORD;

          fooba       |
          foob        |
          fo          {


                      /* false alarm, not really a keyword */
                      return TOK_ID;
                      }


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

          %%
          foo         return TOK_KEYWORD;
          foobar      return TOK_KEYWORD;

          [a-z]+      return TOK_ID;

      This is usually the best solution when appropriate.

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

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

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

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

      is better written:

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

      or as

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

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

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

      This could be sped up by writing it as:

          %x comment
          %%
                  int line_num = 1;

          "/*"         BEGIN(comment);

          [^*\n]*
          [^*\n]*\n      ++line_num;
          "*"+[^*/\n]*
          "*"+[^*/\n]*\n ++line_num;
          "*"+"/"        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 lex.yy.cc instead of lex.yy.c.  The generated
      scanner includes the header file FlexLexer.h, which defines the
      interface to two C++ classes.

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

      const char* YYText()
           returns the text of the most recently matched token, the
           equivalent of yytext.

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

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

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

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

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

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

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

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

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

      int yylex( istream* new_in, ostream* new_out = 0 )
           first switches the input streams via switch_streams( new_in,
           new_out ) and then returns the value of yylex().

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

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

      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')
                          ++mylineno;

                      else if(c == '*')
                          {
                          if((c = yyinput()) == '/')
                              break;
                          else
                              unput(c);
                          }
                      }
                  }

          {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  in your other
      sources once per lexer class, first renaming yyFlexLexer as follows:

          #undef yyFlexLexer
          #define yyFlexLexer xxFlexLexer
          #include 

          #undef yyFlexLexer
          #define yyFlexLexer zzFlexLexer
          #include 

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




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

flex is a rewrite of the AT&T Unix lex tool (the two implementations
      do not share any code, though), with some extensions and
      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, AT&T lex, and the POSIX specification.
      flex's -l option turns on maximum compatibility with the original AT&T
      lex implementation, at the cost of a major loss in the generated
      scanner's performance.  We note below which incompatibilities can be
      overcome using the -l option.

      flex is fully compatible with lex with the following exceptions:

      -    The 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:

               

[A-Z][A-Z0-9]*
               %%
               foo{

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

           The POSIX specification is that the definition be enclosed in
           parentheses.

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

               %%

           is not expanded with parentheses, to allow these operators to
           appear in definitions without losing their special meanings.  But
           the, /, and <> operators cannot be used in a flex
           definition.
               foo|bar
                 { 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
      specification:

          C++ scanners
          %option
          start condition scopes
          start condition stacks
          interactive/non-interactive scanners
          yy_scan_string() and friends
          yyterminate()

          yy_set_interactive()
          yy_set_bol()
          YY_AT_BOL()
          <>
          <*>
          YY_DECL
          YY_START


          YY_USER_ACTION
          YY_USER_INIT
          #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
      intended.

      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 features.

      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 <> 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.

      lex.yy.c
           generated scanner (called lexyy.c on some systems).

      lex.yy.cc
           generated C++ scanner class, when using -+.

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

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

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

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

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

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

          %%
          abc      |
          xyz/def


      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
      characters.

      Dynamic resizing of the input buffer is slow, as it entails rescanning
      all the text matched so far by the current (generally huge) token.

      Due to both buffering of input and read-ahead, you cannot intermix
      calls to  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 Deraadt, Mike Donahue, Chuck Doucette, Tom Epperly, Leo Eskin,
      Chris Faylor, Chris Flatters, Jon Forrest, Jeffrey Friedl, Joe Gayda,
      Kaveh R. Ghazi, Wolfgang Glunz, Eric Goldman, Christopher M. Gould,
      Ulrich Grepel, Peer Griebel, Jan Hajic, Charles Hemphill, NORO Hideo,
      Jarkko Hietaniemi, Scott Hofmann, Jeff Honig, Dana Hudes, Eric Hughes,
      John Interrante, Ceriel Jacobs, Michal Jaegermann, Sakari Jalovaara,
      Jeffrey R. Jones, Henry Juengst, Klaus Kaempf, Jonathan I. Kamens,
      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


      headaches.

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

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

      Send comments to .

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