*nix Documentation Project
·  Home
 +   man pages
·  Linux HOWTOs
·  FreeBSD Tips
·  *niX Forums

  man pages->FreeBSD man pages -> lex++ (1)              



NAME    [Toc]    [Back]

       flex - fast lexical analyzer generator

SYNOPSIS    [Toc]    [Back]

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

OVERVIEW    [Toc]    [Back]

       This manual describes flex, a tool for generating programs that perform
       pattern-matching on text.  The manual includes both tutorial and reference

	       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

DESCRIPTION    [Toc]    [Back]

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


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

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

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

       Here's another simple example:

		   int num_lines = 0, num_chars = 0;

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

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

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

       A somewhat more complicated example:

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

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

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


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

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

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

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

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

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

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

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


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


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

       The details of this example will be explained  in  the  following  sections.


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

	   user code

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

       Name definitions have the form:

	   name definition

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

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

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


       is identical to


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

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

	   pattern   action

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

       See below for a further description of patterns and actions.

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

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

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

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

PATTERNS    [Toc]    [Back]

       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 precedence,
 from highest precedence at the top  to  lowest  at  the  bottom.
       Those grouped together have equal precedence.  For example,


       is the same as


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


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


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

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

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

       For example, the following character classes are all equivalent:


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

       Some notes on patterns:

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

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

	      The following are illegal:


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

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


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

		  foo	   |
		  bar$	   /* action goes here */

	      A similar trick will work for matching a foo  or	a  bar-at-thebeginning-of-a-line.


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

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

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


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

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

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

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

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

ACTIONS    [Toc]    [Back]

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

	   "zap me"

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

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

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

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

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

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

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

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

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

       -      ECHO copies yytext to the scanner's output.

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

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

			  int word_count = 0;

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

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

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

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

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

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

		  mega-    ECHO; yymore();
		  kludge   ECHO;

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

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

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

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

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

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

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

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

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

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

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

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

		  "/*"	      {
			      register int c;

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

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

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

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

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

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


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

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

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

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

       to give the scanning routine the name lexscan, returning a  float,  and
       taking two floats as arguments.	Note that if you give arguments to the
       scanning routine using a 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 scanning
 continues from that file), or yyrestart() is called.   yyrestart()
       takes  one  argument, a FILE * pointer (which can be nil, if you've set
       up YY_INPUT to scan from a source other	than  yyin),  and  initializes
       yyin  for  scanning from that file.  Essentially there is no difference
       between just assigning yyin to a new input file or using yyrestart() to
       do so; the latter is available for compatibility with previous versions
       of flex, and because it can be used to switch input files in the middle
       of  scanning.   It  can	also  be  used to throw away the current input
       buffer, by calling it with an argument of yyin; but better  is  to  use
       YY_FLUSH_BUFFER	(see above).  Note that yyrestart() does not reset the
       start condition to INITIAL (see Start Conditions, 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	blockreads
  rather  than  simple  getc() calls to read characters from yyin.
       The nature of how it gets its input can be controlled by  defining  the
       YY_INPUT      macro.	  YY_INPUT's	  calling      sequence     is
       "YY_INPUT(buf,result,max_size)".  Its action is to place up to max_size
       characters  in  the character array buf and return in the integer variable
 result either the  number  of  characters  read  or  the  constant
       YY_NULL	(0  on	Unix  systems)	to indicate EOF.  The default YY_INPUT
       reads from the global file-pointer "yyin".

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

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

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

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

       If you do not supply your own version of yywrap(), then you must either
       use  %option  noyywrap  (in  which  case  the scanner behaves as though
       yywrap() returned 1), or you must link with -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,  stdout),
 which may be redefined by the user simply by assigning it to some
       other FILE pointer.

START CONDITIONS    [Toc]    [Back]

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

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

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

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

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

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

       If the distinction between inclusive and exclusive start conditions  is
       still  a little vague, here's a simple example illustrating the connection
 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 condition
 example.  If we just used <example> to qualify bar,  though,  then
       it  would  only	be  active in example and not in INITIAL, while in the
       first example it's active in both, because in  the  first  example  the
       example 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
       token, the floating-point number 123.456:

	   #include <math.h>
	   %s expect

	   expect-floats	BEGIN(expect);

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

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

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

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

	   %x comment
		   int line_num = 1;

	   "/*" 	BEGIN(comment);

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

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

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

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

	   "/*" 	{
			comment_caller = INITIAL;


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

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

       Furthermore, you can access the current start condition using the integer-valued
  YY_START macro.  For example, the above assignments to 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 conditions:

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

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

       int yy_top_state()
	      returns  the  top of the stack without altering the stack's contents.

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

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


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

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

	   YY_BUFFER_STATE yy_create_buffer( FILE *file, int size )

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

	   void yy_switch_to_buffer( YY_BUFFER_STATE new_buffer )

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

	   void yy_delete_buffer( YY_BUFFER_STATE buffer )

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

	   void yy_flush_buffer( YY_BUFFER_STATE buffer )

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

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

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

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

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

	   #define MAX_INCLUDE_DEPTH 10
	   int include_stack_ptr = 0;

	   include	       BEGIN(incl);

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

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

		   include_stack[include_stack_ptr++] =

		   yyin = fopen( yytext, "r" );

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

		       yy_create_buffer( yyin, YY_BUF_SIZE ) );


	   <<EOF>> {
		   if ( --include_stack_ptr < 0 )

		       yy_delete_buffer( YY_CURRENT_BUFFER );
			    include_stack[include_stack_ptr] );

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

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

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

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

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

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

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

END-OF-FILE RULES    [Toc]    [Back]

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

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

       -      executing a return statement;

       -      executing the special yyterminate() action;

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

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


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

	   %x quote

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

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


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

	   #define YY_USER_ACTION ++ctr[yy_act]

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

	   int ctr[YY_NUM_RULES];

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

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

       The macro yy_set_bol(at_bol) can be used to control whether the current
       buffer's scanning context for the next token match is done as though at
       the beginning of  a  line.   A  non-zero  macro	argument  makes  rules
       anchored  with  '^' 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
	      input()  and  unput(),  which potentially destroy its value when
	      yytext is a  character  pointer.	 The  opposite	of  %array  is
	      %pointer, which is the default.

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

       -      int yyleng holds the length of the current token.

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

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

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

       -      YY_CURRENT_BUFFER  returns  a YY_BUFFER_STATE handle to the current

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


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

	   #include "y.tab.h"


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

OPTIONS    [Toc]    [Back]

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

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

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

		  --accepting rule at line 53 (

 Similar pages
Name OS Title
flex Tru64 Generates a C Language lexical analyzer
perllexwarn OpenBSD Perl Lexical Warnings
lex Tru64 Generates programs for lexical tasks
amesh IRIX audio spectrum analyzer
ssperf IRIX SpeedShop Performance Analyzer
cvbuild IRIX WorkShop Build Analyzer
lex IRIX generate programs for simple lexical tasks
pdffonts Linux Portable Document Format (PDF) font analyzer (version 1.00)
fru IRIX Field replacement unit analyzer for Challenge/Onyx systems
rcs2log OpenBSD RCS to ChangeLog generator
Copyright © 2004-2005 DeniX Solutions SRL
newsletter delivery service