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FLEX(1)								       FLEX(1)

       flex - fast lexical analyzer generator

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

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

	       a brief overview of the tool

	   Some Simple Examples

	   Format Of The Input File

	       the extended regular expressions used by flex

	   How The Input Is Matched
	       the rules for determining what has been matched

	       how to specify what to do when a pattern is matched

	   The Generated Scanner
	       details regarding the scanner that flex produces;
	       how to control the input source

	   Start Conditions
	       introducing context into your scanners, and
	       managing "mini-scanners"

	   Multiple Input Buffers
	       how to manipulate multiple input sources; how to
	       scan from strings instead of files

	   End-of-file Rules
	       special rules for matching the end of the input

	   Miscellaneous Macros
	       a summary of macros available to the actions

	   Values Available To The User
	       a summary of values available to the actions

	   Interfacing With Yacc
	       connecting flex scanners together with yacc parsers

	       flex command-line options, and the "%option"

	   Performance Considerations
	       how to make your scanner go as fast as possible

	   Generating C++ Scanners
	       the (experimental) facility for generating C++
	       scanner classes

	   Incompatibilities With Lex And POSIX
	       how flex differs from AT&T lex and the POSIX lex

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

	       files used by flex

	   Deficiencies / Bugs
	       known problems with flex

	   See Also
	       other documentation, related tools

	       includes contact information

       flex is a tool for generating scanners: programs which recognized lexi‐
       cal  patterns  in text.	flex reads the given input files, or its stan‐
       dard input if no file names are given, for a description of  a  scanner
       to  generate.   The  description	 is  in	 the  form of pairs of regular
       expressions and C code, called rules. flex  generates  as  output  a  C
       source  file,  lex.yy.c, which defines a routine yylex().  This file is
       compiled and linked with the -lfl library  to  produce  an  executable.
       When  the  executable  is run, it analyzes its input for occurrences of
       the regular expressions.	 Whenever it finds one, it executes the corre‐
       sponding C code.

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

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

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

       Here's another simple example:

		   int num_lines = 0, num_chars = 0;

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

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

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

       A somewhat more complicated example:

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

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

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


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

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

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

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

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

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

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

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


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


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

       The details of this example will be explained  in  the  following  sec‐

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

	   user code

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

       Name definitions have the form:

	   name definition

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

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

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


       is identical to


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

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

	   pattern   action

       where  the  pattern must be unindented and the action must begin on the
       same line.

       See below for a further description of patterns and actions.

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

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

       In  the	rules  section,	 any indented or %{} text appearing before the
       first rule may be used to declare variables  which  are	local  to  the
       scanning	 routine and (after the declarations) code which is to be exe‐
       cuted 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 fea‐

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

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

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

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

	   r|s	      either an r or an s

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

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

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

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

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


       is the same as


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


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


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

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

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

       For example, the following character classes are all equivalent:


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

       Some notes on patterns:

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

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

	      The following are illegal:


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

	      The  following will result in '$' or '^' being treated as a nor‐
	      mal character:


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

		  foo	   |
		  bar$	   /* action goes here */

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

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

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

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


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

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

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

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

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

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

	   "zap me"

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

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

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

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

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

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

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

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

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

       -      ECHO copies yytext to the scanner's output.

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

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

			  int word_count = 0;

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

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

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

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

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

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

		  mega-	   ECHO; yymore();
		  kludge   ECHO;

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

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

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

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

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

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

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

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

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

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

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

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

		  "/*"	      {
			      register int c;

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       When the scanner receives an end-of-file indication from	 YY_INPUT,  it
       then  checks  the yywrap() function.  If yywrap() returns false (zero),
       then it is assumed that the function has gone ahead and set up yyin  to
       point  to  another  input  file, and scanning continues.	 If it returns
       true (non-zero), then the scanner terminates, returning 0 to its	 call‐
       er.   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, std‐
       out), which may be redefined by the user simply by assigning it to some
       other FILE pointer.

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

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

       will  be	 active	 only when the scanner is in the "STRING" start condi‐
       tion, and

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

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

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

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

	   %s example

	   <example>foo	  do_something();

	   bar		  something_else();

       is equivalent to

	   %x example

	   <example>foo	  do_something();

	   <INITIAL,example>bar	   something_else();

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

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

	   %x example

	   <example>foo	  do_something();

	   <*>bar    something_else();

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

	   <*>.|\n     ECHO;

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

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

		   int enter_special;

	   %x SPECIAL
		   if ( enter_special )

	   ...more rules follow...

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

	   #include <math.h>
	   %s expect

	   expect-floats	BEGIN(expect);

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

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

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

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

	   %x comment
		   int line_num = 1;

	   "/*"		BEGIN(comment);

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

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

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

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

	   "/*"		{
			comment_caller = INITIAL;


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

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

       Furthermore, you can access the current start condition using the inte‐
       ger-valued  YY_START macro.  For example, the above assignments to com‐
       ment_caller could instead be written

	   comment_caller = YY_START;

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

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

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

	   %x str

		   char string_buf[MAX_STR_CONST];
		   char *string_buf_ptr;

	   \"	   string_buf_ptr = string_buf; BEGIN(str);

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

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

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

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

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

		   *string_buf_ptr++ = result;

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

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

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

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

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

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


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

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

       is equivalent to:

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

       Start condition scopes may be nested.

       Three  routines	are  available for manipulating stacks of start condi‐

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

       void yy_pop_state()
	      pops the top of the stack and switches to it via BEGIN.

       int yy_top_state()
	      returns  the  top of the stack without altering the stack's con‐

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

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

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

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

	   YY_BUFFER_STATE yy_create_buffer( FILE *file, int size )

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

	   void yy_switch_to_buffer( YY_BUFFER_STATE new_buffer )

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

	   void yy_delete_buffer( YY_BUFFER_STATE buffer )

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

	   void yy_flush_buffer( YY_BUFFER_STATE buffer )

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

       yy_new_buffer() is an alias for yy_create_buffer(), provided  for  com‐
       patibility with the C++ use of new and delete for creating and destroy‐
       ing dynamic objects.

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

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

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

	   #define MAX_INCLUDE_DEPTH 10
	   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_BUF‐
       FER_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_BUF‐
	      FER_CHAR	(ASCII	NUL).	These  last two bytes are not scanned;
	      thus, scanning consists of base[0] through base[size-2],	inclu‐

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

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

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

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

       -      executing a return statement;

       -      executing the special yyterminate() action;

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

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


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

	   %x quote

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

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

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

	   #define YY_USER_ACTION ++ctr[yy_act]

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

	   int ctr[YY_NUM_RULES];

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

       The  macro  yy_set_interactive(is_interactive)  can  be used to control
       whether the current buffer is considered interactive.   An  interactive
       buffer  is  processed  more slowly, but must be used when the scanner's
       input source is indeed interactive to avoid problems due to waiting  to
       fill  buffers  (see  the	 discussion of the -I flag below).  A non-zero
       value in the macro invocation marks the buffer as interactive,  a  zero
       value  as  non-interactive.   Note  that	 use  of  this macro overrides
       %option 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  buf‐
	      fers  its	 input; use yyrestart() instead.  Once scanning termi‐
	      nates because an end-of-file has been seen, you can assign  yyin
	      at  the  new  input file and then call the scanner again to con‐
	      tinue scanning.

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

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

       -      YY_CURRENT_BUFFER	 returns  a YY_BUFFER_STATE handle to the cur‐
	      rent buffer.

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

       One of the main uses of flex is as a companion to the yacc  parser-gen‐
       erator.	 yacc  parsers	expect to call a routine named yylex() to find
       the next input token.  The routine is supposed to return	 the  type  of
       the  next  token	 as well as putting any associated value in the global
       yylval.	To use flex with yacc, one specifies the -d option to yacc  to
       instruct	 it to generate the file 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:

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

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

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

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

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

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

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

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

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

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

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

	      Note  that  the  use  of	REJECT, %option yylineno, and variable
	      trailing context (see the Deficiencies  /	 Bugs  section	below)
	      entails  a substantial performance penalty; use of yymore(), the
	      ^ operator, and the -I flag entail minor performance penalties.

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

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

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

       -w     suppresses warning messages.

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

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

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

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

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

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

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

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

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

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

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

       -7     instructs flex to generate a 7-bit scanner, i.e., one which  can
	      only 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 disad‐
	      vantage is that such scanners often hang or crash if their input
	      contains an 8-bit character.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

		  slowest & smallest
		  fastest & largest

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

	      -Cfe is often a good compromise between speed and size for  pro‐
	      duction scanners.

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

	      changes the default yy prefix used by flex for all globally-vis‐
	      ible  variable  and  function  names  to instead be prefix.  For
	      example, -Pfoo changes the name of yytext to footext.   It  also
	      changes  the  name  of  the default output file from lex.yy.c to
	      lex.foo.c.  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.

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

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

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

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

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

	   case-insensitive or
	   caseless	   -i option

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

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

       Some %option's provide features otherwise not available:

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

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

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

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

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

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

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

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

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

	   %option outfile="ABC"

       is equivalent to -oABC, and

	   %option prefix="XYZ"

       is equivalent to -PXYZ.	Finally,

	   %option yyclass="foo"

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

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

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

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

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

	   %option yylineno
	   arbitrary trailing context

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

	   '^' beginning-of-line operator

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

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

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

	   foo	      return TOK_KEYWORD;
	   foobar     return TOK_KEYWORD;

       the file looks like:

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

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

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

	   Compressed tables always back up.

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

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

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

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

	   foo	       return TOK_KEYWORD;
	   foobar      return TOK_KEYWORD;

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

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

	   foo	       return TOK_KEYWORD;
	   foobar      return TOK_KEYWORD;

	   [a-z]+      return TOK_ID;

       This is usually the best solution when appropriate.

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

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

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

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

       is better written:

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

       or as

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

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

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

	   %x comment
		   int line_num = 1;

	   "/*"		BEGIN(comment);

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

       This could be sped up by writing it as:

	   %x comment
		   int line_num = 1;

	   "/*"		BEGIN(comment);

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

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

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

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

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

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

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

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

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

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

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

       One has to be careful here, as we have now reintroduced backing up into
       the scanner.  In particular, while we know that there will never be any
       characters  in  the  input  stream other than letters or newlines, flex
       can't figure this out, and it will plan for possibly needing to back up
       when  it has scanned a token like "auto" and then the next character is
       something other than a newline or a letter.  Previously it  would  then
       just  match the "auto" rule and be done, but now it has no "auto" rule,
       only 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 accom‐
       modate huge tokens is a slow process because it presently requires that
       the  (huge) token be rescanned from the beginning.  Thus if performance
       is vital, you should attempt to match "large" quantities	 of  text  but
       not  "huge" quantities, where the cutoff between the two is at about 8K

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	   int mylineno = 0;

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

	   ws	   [ \t]+

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


	   {ws}	   /* skip blanks and tabs */

	   "/*"	   {
		   int c;

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

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

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

	   \n	     mylineno++;

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

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


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

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

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

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

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

       flex is a rewrite of the AT&T Unix lex tool (the two implementations do
       not share any code, though), with some  extensions  and	incompatibili‐
       ties,  both of which are of concern to those who wish to write scanners
       acceptable to either implementation.  Flex is fully compliant with  the
       POSIX lex specification, except that when using %pointer (the default),
       a call to unput() destroys the contents of yytext, which is counter  to
       the POSIX specification.

       In  this	 section  we discuss all of the known areas of incompatibility
       between flex, AT&T lex, and the POSIX specification.

       flex's -l option turns on maximum compatibility with the original  AT&T
       lex  implementation, at the cost of a major loss in the generated scan‐
       ner's performance.  We note below which incompatibilities can be	 over‐
       come using the -l option.

       flex is fully compatible with lex with the following exceptions:

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

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

	      yylineno is not part of the POSIX specification.

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

	      Input is instead controlled by defining the YY_INPUT macro.

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

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

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

		  fatal flex scanner internal error--end of buffer missed

	      To reenter the scanner, first use

		  yyrestart( yyin );

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

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

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

	      output() is not part of the POSIX specification.

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

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

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

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

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

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

	      The  POSIX  specification	 is that the definition be enclosed in

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

		  foo|bar<space here>
		    { foobar_action(); }

	      flex does not support this feature.

       -      The  lex %r (generate a Ratfor scanner) option is not supported.
	      It is not part of the POSIX specification.

       -      After a call to unput(), yytext  is  undefined  until  the  next
	      token  is	 matched,  unless  the scanner was built using %array.
	      This is not the case with lex or the POSIX  specification.   The
	      -l option does away with this incompatibility.

       -      The  precedence of the {} (numeric range) operator is different.
	      lex interprets "abc{1,3}" as "match one, two,  or	 three	occur‐
	      rences of 'abc'", whereas flex interprets it as "match 'ab' fol‐
	      lowed by one, two, or three occurrences of 'c'".	The latter  is
	      in agreement with the POSIX specification.

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

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

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

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

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

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

	   foo	  handle_foo(); ++num_foos_seen;

       is (rather surprisingly) truncated to

	   foo	  handle_foo();

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

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

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

       Using REJECT in a scanner suppresses this warning.

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

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

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

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

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

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

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

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

	   yyrestart( yyin );

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

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

       -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  sys‐

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

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

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

	   abc	    |

       Use  of unput() invalidates yytext and yyleng, unless the %array direc‐
       tive or the -l option has been used.

       Pattern-matching of NUL's is substantially slower than  matching	 other

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

       Due to both buffering of input  and  read-ahead,	 you  cannot  intermix
       calls to <stdio.h> routines, such as, for example, getchar(), with flex
       rules and expect it to work.  Call input() instead.

       The total table entries listed by the -v flag excludes  the  number  of
       table entries needed to determine what rule has been matched.  The num‐
       ber of entries is equal to the number of DFA states if the scanner does
       not  use	 REJECT,  and somewhat greater than the number of states if it

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

       The flex internal algorithms need documentation.

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

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

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

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

       Vern  Paxson, with the help of many ideas and much inspiration from Van
       Jacobson.  Original version by Jef Poskanzer.  The fast table represen‐
       tation  is  a  partial implementation of a design done by Van Jacobson.
       The implementation was done by Kevin Gong and Vern Paxson.

       Thanks to the many flex beta-testers,  feedbackers,  and	 contributors,
       especially Francois Pinard, Casey Leedom, Robert Abramovitz, Stan Ader‐
       mann, Terry Allen, David Barker-Plummer, John Basrai, Neal Becker, Nel‐
       son H.F. Beebe, benson@odi.com, Karl Berry, Peter A. Bigot, Simon Blan‐
       chard, Keith Bostic, Frederic  Brehm,  Ian  Brockbank,  Kin  Cho,  Nick
       Christopher,  Brian  Clapper,  J.T.  Conklin, Jason Coughlin, Bill Cox,
       Nick Cropper, Dave Curtis, Scott David  Daniels,	 Chris	G.  Demetriou,
       Theo  Deraadt,  Mike  Donahue,  Chuck Doucette, Tom Epperly, Leo Eskin,
       Chris Faylor, Chris Flatters, Jon Forrest, Jeffrey Friedl,  Joe	Gayda,
       Kaveh  R.  Ghazi,  Wolfgang  Glunz, Eric Goldman, Christopher M. Gould,
       Ulrich Grepel, Peer Griebel, Jan Hajic, Charles Hemphill,  NORO	Hideo,
       Jarkko  Hietaniemi, Scott Hofmann, Jeff Honig, Dana Hudes, Eric Hughes,
       John Interrante, Ceriel Jacobs, Michal  Jaegermann,  Sakari  Jalovaara,
       Jeffrey R. Jones, Henry Juengst, Klaus Kaempf, Jonathan I. Kamens, Ter‐
       rence O Kane, Amir  Katz,  ken@ken.hilco.com,  Kevin  B.	 Kenny,	 Steve
       Kirsch,	Winfried  Koenig, Marq Kole, Ronald Lamprecht, Greg Lee, Rohan
       Lenard, Craig Leres, John Levine, Steve Liddle,	David  Loffredo,  Mike
       Long,  Mohamed  el  Lozy,  Brian	 Madsen,  Malte,  Joe  Marshall, Bengt
       Martensson, Chris Metcalf, Luke Mewburn,	 Jim  Meyering,	 R.  Alexander
       Milowski,  Erik	Naggum,	 G.T.  Nicol,  Landon Noll, James Nordby, Marc
       Nozell, Richard Ohnemus, Karsten Pahnke, Sven Panne, Roland Pesch, Wal‐
       ter  Pelissero, Gaumond Pierre, Esmond Pitt, Jef Poskanzer, Joe Rahmeh,
       Jarmo Raiha, Frederic Raimbault, Pat  Rankin,  Rick  Richardson,	 Kevin
       Rodgers, Kai Uwe Rommel, Jim Roskind, Alberto Santini, Andreas Scherer,
       Darrell Schiebel, Raf Schietekat, Doug Schmidt,	Philippe  Schnoebelen,
       Andreas	Schwab, Larry Schwimmer, Alex Siegel, Eckehard Stolz, Jan-Erik
       Strvmquist, Mike Stump, Paul Stuart, Dave Tallman,  Ian	Lance  Taylor,
       Chris Thewalt, Richard M. Timoney, Jodi Tsai, Paul Tuinenga, Gary Weik,
       Frank Whaley, Gerhard Wilhelms, Kent Williams,  Ken  Yap,  Ron  Zellar,
       Nathan Zelle, David Zuhn, and those whose names have slipped my margin‐
       al mail-archiving skills but whose contributions	 are  appreciated  all
       the same.

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

       Thanks to Esmond Pitt and Earle Horton for 8-bit character support;  to
       Benson  Margulies  and Fred Burke for C++ support; to Kent Williams and
       Tom Epperly for C++ class support; to Ove Ewerlid for support of NUL's;
       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 vern@ee.lbl.gov.

Version 2.5			  April 1995			       FLEX(1)

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