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FLEX(1)			    General Commands Manual		       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 recognize	 lexi-
       cal  patterns  in text.	flex reads the given input files, or its stan-
       dard input if no	file names are given, for a description	of  a  scanner
       to  generate.   The  description	is in the form of pairs	of regular ex-
       pressions and C code, called rules. flex	generates as output a C	source
       file, lex.yy.c, which defines a routine yylex().	 This file is compiled
       and linked with the -ll library to produce an executable.  When the ex-
       ecutable	 is  run, it analyzes its input	for occurrences	of the regular
       expressions.  Whenever it finds one, it executes	 the  corresponding  C

       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 en-
       closed 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  be-
       tween  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 al-
       phabetic	or numeric.  Some systems don't	provide	isblank(), so flex de-
       fines [: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  se-
	      quence)  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 en-
	      trenched.	 Matching newlines means that a	pattern	like [^"]* can
	      match the	entire input unless there's another quote in  the  in-

       -      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 be-

		  foo	   |
		  bar$	   /* action goes here */

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

       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  de-
       scription  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  de-
       fault  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 be-
       low).  Furthermore, existing lex	programs sometimes access  yytext  ex-
       ternally	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  de-
       faults to a fairly large	value.	You can	change the size	by simply #de-
       fine'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 com-
       ments), 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 ar-
       bitrary C statement.  The pattern ends at the first non-escaped	white-
       space  character;  the remainder	of the line is its action.  If the ac-
       tion is empty, then when	the pattern is matched the input token is sim-
       ply  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 in-
       put  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  ex-
	      ample,  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 in-
       stead (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 ex-
	      ample, 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 de-
       clared 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,re-
       sult,max_size)".	 Its action is to place	up to max_size	characters  in
       the  character  array buf and return in the integer variable result ei-
       ther 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  in-
       put file):

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

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

       When the	scanner	receives an end-of-file	indication from	 YY_INPUT,  it
       then  checks  the yywrap() function.  If	yywrap() returns false (zero),
       then it is assumed that the function has	gone ahead and set up yyin  to
       point  to  another  input  file,	and scanning continues.	 If it returns
       true (non-zero),	then  the  scanner  terminates,	 returning  0  to  its
       caller.	 Note  that  in	 either	 case, the start condition remains un-
       changed;	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 yy-
       wrap() returned 1), or you must link with -ll  to  obtain  the  default
       version of the routine, which always returns 1.

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

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

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

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

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

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

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

       Start conditions	are declared in	the definitions	(first)	section	of the
       input using unindented lines beginning with either %s or	%x followed by
       a  list	of names.  The former declares inclusive start conditions, the
       latter exclusive	start conditions.  A start condition is	activated  us-
       ing  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 in-
       put 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 ex-
       ample start condition is	an inclusive (%s) start	condition.

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

	   %x example

	   <example>foo	  do_something();

	   <*>bar    something_else();

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

	   <*>.|\n     ECHO;

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

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

		   int enter_special;

	   %x SPECIAL
		   if (	enter_special )

	   ...more rules follow...

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

       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 fi-
	      nal two YY_END_OF_BUFFER_CHAR bytes), then yy_scan_buffer()  re-
	      turns 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., in-
       dicates 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 ac-
	      tion 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 al-
       ways  executed before the first scan (and before	the scanner's internal
       initializations are done).  For example,	it could be  used  to  call  a
       routine to read in a data table or open a logging file.

       The  macro  yy_set_interactive(is_interactive)  can  be used to control
       whether the current buffer is considered	interactive.   An  interactive
       buffer  is  processed  more slowly, but must be used when the scanner's
       input source is indeed interactive to avoid problems due	to waiting  to
       fill  buffers  (see  the	 discussion of the -I flag below).  A non-zero
       value in	the macro invocation marks the buffer as interactive,  a  zero
       value  as  non-interactive.  Note that use of this macro	overrides %op-
       tion interactive	, %option always-interactive or	%option	never-interac-
       tive  (see  Options below).  yy_set_interactive() must be invoked prior
       to beginning to scan the	buffer that is (or is not)  to	be  considered

       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 an-
       chored with '^' active, while a zero argument makes '^' rules inactive.

       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 in-
	      put()  and unput(), which	potentially destroy its	value when yy-
	      text  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 be-
	      gins 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	containing definitions of  all
       the %tokens appearing in	the yacc input.	 This file is then included in
       the flex	scanner.  For example, if one of the tokens  is	 "TOK_NUMBER",
       part of the scanner might look like:

	   #include ""


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

       flex has	the following options:

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

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

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

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

	      The line number refers to	the location of	the rule in  the  file
	      defining	the  scanner  (i.e.,  the  file	that was fed to	flex).
	      Messages are also	generated when the scanner backs  up,  accepts
	      the  default  rule,  reaches the end of its input	buffer (or en-
	      counters 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) en-
	      tails 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  in-
	      teractive	scanners generated by -I (see below).  In general, you
	      use -B when you are certain that your scanner will never be used
	      interactively, and you want to squeeze a little more performance
	      out of it.  If your goal is instead to squeeze out  a  lot  more
	      performance,  you	 should	 be using the -Cf or -CF options (dis-
	      cussed below), which turn	on -B automatically anyway.

       -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 us-
	      ing -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 al-
	      ways looking one extra character ahead, even if the scanner  has
	      already seen enough text to disambiguate the current token, is a
	      bit faster than only looking ahead when necessary.  But scanners
	      that  always  look  ahead	give dreadful interactive performance;
	      for example, when	a user types a newline,	it is  not  recognized
	      as  a  newline token until they enter another token, which often
	      means typing in another whole line.

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

	      Note  that if isatty() returns false for the scanner input, flex
	      will revert to batch mode, even if -I was	specified.   To	 force
	      interactive  mode	no matter what,	use %option always-interactive
	      (see Options below).

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

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

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

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

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

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

	      Note  that  if you use -Cfe or -CFe (those table compression op-
	      tions, but also using equivalence	classes	as discussed  see  be-
	      low),  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  ex-
	      ample,  -Pfoo  changes  the  name	of yytext to footext.  It also
	      changes the name of the default output  file  from  lex.yy.c  to  Here are all of the names affected:


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

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

	      overrides	the default skeleton file from which  flex  constructs
	      its  scanners.  You'll never need	this option unless you are do-
	      ing 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  yy-
	      lineno.  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., %op-
       tion 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 op-
       tions outlined above, there are a number	of options/actions  which  de-
       grade 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.,  yy-
       text) for the action.  Recall the scanner for C comments:

	   %x comment
		   int line_num	= 1;

	   "/*"		BEGIN(comment);

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

       This could be sped up by	writing	it as:

	   %x comment
		   int line_num	= 1;

	   "/*"		BEGIN(comment);

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       A final note: flex is slow when matching	NUL's, particularly when a to-
       ken  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  in-
       clude 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	-+ op-
       tion  (or, equivalently,	%option	c++), which is automatically specified
       if the name of the flex executable ends in a '+', such as flex++.  When
       using  this option, flex	defaults to generating the scanner to the file instead of lex.yy.c.	The  generated	scanner	 includes  the
       header  file  FlexLexer.h,  which  defines  the	interface  to  two C++

       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 de-
       rived from FlexLexer.  It defines the following additional member func-

       yyFlexLexer( istream* arg_yyin =	0, ostream* arg_yyout =	0 )
	      constructs  a yyFlexLexer	object using the given streams for in-
	      put 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 yy-
	      class="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 in-
	      put 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 in-
	      put() as EOF.

	      Input is instead controlled by defining the YY_INPUT macro.

	      The flex restriction that	input()	cannot be redefined is in  ac-
	      cordance	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 us-
	      ing  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 ex-
	      panded 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 to-
	      ken is matched, unless the scanner was built using %array.  This
	      is not the case with lex or the POSIX specification.  The	-l op-
	      tion 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 be-
       cause 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 de-
       fault rule (match any single character) is the only one that will match
       a  particular  input.   Since  -s was given, presumably this is not in-

       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 oc-
       currences 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 in-
       put 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  oc-
       cur  in	a  scanner which is reentered after a long-jump	has jumped out
       (or over) the scanner's activation frame.  Before reentering the	 scan-
       ner, use:

	   yyrestart( yyin );

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

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

       -ll    library with which scanners must be linked.

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

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

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

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

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

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

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

	   abc	    |

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

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

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

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

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

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

       The flex	internal algorithms need documentation.

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

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

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

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

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

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

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

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

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

       Send comments to

Version	2.5			  April	1995			       FLEX(1)


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