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4 FLEX(1) USER COMMANDS FLEX(1)
8 NAME
9 flex - fast lexical analyzer generator
11 SYNOPSIS
12 flex [-bcdfhilnpstvwBFILTV78+? -C[aefFmr] -ooutput -Pprefix
13 -Sskeleton] [--help --version] [filename ...]
15 OVERVIEW
16 This manual describes flex, a tool for generating programs
17 that perform pattern-matching on text. The manual includes
18 both tutorial and reference sections:
20 Description
21 a brief overview of the tool
23 Some Simple Examples
25 Format Of The Input File
27 Patterns
28 the extended regular expressions used by flex
30 How The Input Is Matched
31 the rules for determining what has been matched
33 Actions
34 how to specify what to do when a pattern is matched
36 The Generated Scanner
37 details regarding the scanner that flex produces;
38 how to control the input source
40 Start Conditions
41 introducing context into your scanners, and
42 managing "mini-scanners"
44 Multiple Input Buffers
45 how to manipulate multiple input sources; how to
46 scan from strings instead of files
48 End-of-file Rules
49 special rules for matching the end of the input
51 Miscellaneous Macros
52 a summary of macros available to the actions
54 Values Available To The User
55 a summary of values available to the actions
57 Interfacing With Yacc
58 connecting flex scanners together with yacc parsers
63 Version 2.5 Last change: April 1995 1
70 FLEX(1) USER COMMANDS FLEX(1)
74 Options
75 flex command-line options, and the "%option"
76 directive
78 Performance Considerations
79 how to make your scanner go as fast as possible
81 Generating C++ Scanners
82 the (experimental) facility for generating C++
83 scanner classes
85 Incompatibilities With Lex And POSIX
86 how flex differs from AT&T lex and the POSIX lex
87 standard
89 Diagnostics
90 those error messages produced by flex (or scanners
91 it generates) whose meanings might not be apparent
93 Files
94 files used by flex
96 Deficiencies / Bugs
97 known problems with flex
99 See Also
100 other documentation, related tools
102 Author
103 includes contact information
106 DESCRIPTION
107 flex is a tool for generating scanners: programs which
108 recognized lexical patterns in text. flex reads the given
109 input files, or its standard input if no file names are
110 given, for a description of a scanner to generate. The
111 description is in the form of pairs of regular expressions
112 and C code, called rules. flex generates as output a C
113 source file, lex.yy.c, which defines a routine yylex(). This
114 file is compiled and linked with the -lfl library to produce
115 an executable. When the executable is run, it analyzes its
116 input for occurrences of the regular expressions. Whenever
117 it finds one, it executes the corresponding C code.
119 SOME SIMPLE EXAMPLES
120 First some simple examples to get the flavor of how one uses
121 flex. The following flex input specifies a scanner which
122 whenever it encounters the string "username" will replace it
123 with the user's login name:
125 %%
129 Version 2.5 Last change: April 1995 2
136 FLEX(1) USER COMMANDS FLEX(1)
140 username printf( "%s", getlogin() );
142 By default, any text not matched by a flex scanner is copied
143 to the output, so the net effect of this scanner is to copy
144 its input file to its output with each occurrence of "user-
145 name" expanded. In this input, there is just one rule.
146 "username" is the pattern and the "printf" is the action.
147 The "%%" marks the beginning of the rules.
149 Here's another simple example:
151 int num_lines = 0, num_chars = 0;
153 %%
154 \n ++num_lines; ++num_chars;
155 . ++num_chars;
157 %%
158 main()
159 {
160 yylex();
161 printf( "# of lines = %d, # of chars = %d\n",
162 num_lines, num_chars );
163 }
165 This scanner counts the number of characters and the number
166 of lines in its input (it produces no output other than the
167 final report on the counts). The first line declares two
168 globals, "num_lines" and "num_chars", which are accessible
169 both inside yylex() and in the main() routine declared after
170 the second "%%". There are two rules, one which matches a
171 newline ("\n") and increments both the line count and the
172 character count, and one which matches any character other
173 than a newline (indicated by the "." regular expression).
175 A somewhat more complicated example:
177 /* scanner for a toy Pascal-like language */
179 %{
180 /* need this for the call to atof() below */
181 #include <math.h>
182 %}
184 DIGIT [0-9]
185 ID [a-z][a-z0-9]*
187 %%
189 {DIGIT}+ {
190 printf( "An integer: %s (%d)\n", yytext,
191 atoi( yytext ) );
195 Version 2.5 Last change: April 1995 3
202 FLEX(1) USER COMMANDS FLEX(1)
206 }
208 {DIGIT}+"."{DIGIT}* {
209 printf( "A float: %s (%g)\n", yytext,
210 atof( yytext ) );
211 }
213 if|then|begin|end|procedure|function {
214 printf( "A keyword: %s\n", yytext );
215 }
217 {ID} printf( "An identifier: %s\n", yytext );
219 "+"|"-"|"*"|"/" printf( "An operator: %s\n", yytext );
221 "{"[^}\n]*"}" /* eat up one-line comments */
223 [ \t\n]+ /* eat up whitespace */
225 . printf( "Unrecognized character: %s\n", yytext );
227 %%
229 main( argc, argv )
230 int argc;
231 char **argv;
232 {
233 ++argv, --argc; /* skip over program name */
234 if ( argc > 0 )
235 yyin = fopen( argv[0], "r" );
236 else
237 yyin = stdin;
239 yylex();
240 }
242 This is the beginnings of a simple scanner for a language
243 like Pascal. It identifies different types of tokens and
244 reports on what it has seen.
246 The details of this example will be explained in the follow-
247 ing sections.
249 FORMAT OF THE INPUT FILE
250 The flex input file consists of three sections, separated by
251 a line with just %% in it:
253 definitions
254 %%
255 rules
256 %%
257 user code
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268 FLEX(1) USER COMMANDS FLEX(1)
272 The definitions section contains declarations of simple name
273 definitions to simplify the scanner specification, and
274 declarations of start conditions, which are explained in a
275 later section.
277 Name definitions have the form:
279 name definition
281 The "name" is a word beginning with a letter or an under-
282 score ('_') followed by zero or more letters, digits, '_',
283 or '-' (dash). The definition is taken to begin at the
284 first non-white-space character following the name and con-
285 tinuing to the end of the line. The definition can subse-
286 quently be referred to using "{name}", which will expand to
287 "(definition)". For example,
289 DIGIT [0-9]
290 ID [a-z][a-z0-9]*
292 defines "DIGIT" to be a regular expression which matches a
293 single digit, and "ID" to be a regular expression which
294 matches a letter followed by zero-or-more letters-or-digits.
295 A subsequent reference to
297 {DIGIT}+"."{DIGIT}*
299 is identical to
301 ([0-9])+"."([0-9])*
303 and matches one-or-more digits followed by a '.' followed by
304 zero-or-more digits.
306 The rules section of the flex input contains a series of
307 rules of the form:
309 pattern action
311 where the pattern must be unindented and the action must
312 begin on the same line.
314 See below for a further description of patterns and actions.
316 Finally, the user code section is simply copied to lex.yy.c
317 verbatim. It is used for companion routines which call or
318 are called by the scanner. The presence of this section is
319 optional; if it is missing, the second %% in the input file
320 may be skipped, too.
322 In the definitions and rules sections, any indented text or
323 text enclosed in %{ and %} is copied verbatim to the output
327 Version 2.5 Last change: April 1995 5
334 FLEX(1) USER COMMANDS FLEX(1)
338 (with the %{}'s removed). The %{}'s must appear unindented
339 on lines by themselves.
341 In the rules section, any indented or %{} text appearing
342 before the first rule may be used to declare variables which
343 are local to the scanning routine and (after the declara-
344 tions) code which is to be executed whenever the scanning
345 routine is entered. Other indented or %{} text in the rule
346 section is still copied to the output, but its meaning is
347 not well-defined and it may well cause compile-time errors
348 (this feature is present for POSIX compliance; see below for
349 other such features).
351 In the definitions section (but not in the rules section),
352 an unindented comment (i.e., a line beginning with "/*") is
353 also copied verbatim to the output up to the next "*/".
355 PATTERNS
356 The patterns in the input are written using an extended set
357 of regular expressions. These are:
359 x match the character 'x'
360 . any character (byte) except newline
361 [xyz] a "character class"; in this case, the pattern
362 matches either an 'x', a 'y', or a 'z'
363 [abj-oZ] a "character class" with a range in it; matches
364 an 'a', a 'b', any letter from 'j' through 'o',
365 or a 'Z'
366 [^A-Z] a "negated character class", i.e., any character
367 but those in the class. In this case, any
368 character EXCEPT an uppercase letter.
369 [^A-Z\n] any character EXCEPT an uppercase letter or
370 a newline
371 r* zero or more r's, where r is any regular expression
372 r+ one or more r's
373 r? zero or one r's (that is, "an optional r")
374 r{2,5} anywhere from two to five r's
375 r{2,} two or more r's
376 r{4} exactly 4 r's
377 {name} the expansion of the "name" definition
378 (see above)
379 "[xyz]\"foo"
380 the literal string: [xyz]"foo
381 \X if X is an 'a', 'b', 'f', 'n', 'r', 't', or 'v',
382 then the ANSI-C interpretation of \x.
383 Otherwise, a literal 'X' (used to escape
384 operators such as '*')
385 \0 a NUL character (ASCII code 0)
386 \123 the character with octal value 123
387 \x2a the character with hexadecimal value 2a
388 (r) match an r; parentheses are used to override
389 precedence (see below)
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400 FLEX(1) USER COMMANDS FLEX(1)
404 rs the regular expression r followed by the
405 regular expression s; called "concatenation"
408 r|s either an r or an s
411 r/s an r but only if it is followed by an s. The
412 text matched by s is included when determining
413 whether this rule is the "longest match",
414 but is then returned to the input before
415 the action is executed. So the action only
416 sees the text matched by r. This type
417 of pattern is called trailing context".
418 (There are some combinations of r/s that flex
419 cannot match correctly; see notes in the
420 Deficiencies / Bugs section below regarding
421 "dangerous trailing context".)
422 ^r an r, but only at the beginning of a line (i.e.,
423 which just starting to scan, or right after a
424 newline has been scanned).
425 r$ an r, but only at the end of a line (i.e., just
426 before a newline). Equivalent to "r/\n".
428 Note that flex's notion of "newline" is exactly
429 whatever the C compiler used to compile flex
430 interprets '\n' as; in particular, on some DOS
431 systems you must either filter out \r's in the
432 input yourself, or explicitly use r/\r\n for "r$".
435 <s>r an r, but only in start condition s (see
436 below for discussion of start conditions)
437 <s1,s2,s3>r
438 same, but in any of start conditions s1,
439 s2, or s3
440 <*>r an r in any start condition, even an exclusive one.
443 <<EOF>> an end-of-file
444 <s1,s2><<EOF>>
445 an end-of-file when in start condition s1 or s2
447 Note that inside of a character class, all regular expres-
448 sion operators lose their special meaning except escape
449 ('\') and the character class operators, '-', ']', and, at
450 the beginning of the class, '^'.
452 The regular expressions listed above are grouped according
453 to precedence, from highest precedence at the top to lowest
454 at the bottom. Those grouped together have equal pre-
455 cedence. For example,
459 Version 2.5 Last change: April 1995 7
466 FLEX(1) USER COMMANDS FLEX(1)
470 foo|bar*
472 is the same as
474 (foo)|(ba(r*))
476 since the '*' operator has higher precedence than concatena-
477 tion, and concatenation higher than alternation ('|'). This
478 pattern therefore matches either the string "foo" or the
479 string "ba" followed by zero-or-more r's. To match "foo" or
480 zero-or-more "bar"'s, use:
482 foo|(bar)*
484 and to match zero-or-more "foo"'s-or-"bar"'s:
486 (foo|bar)*
489 In addition to characters and ranges of characters, charac-
490 ter classes can also contain character class expressions.
491 These are expressions enclosed inside [: and :] delimiters
492 (which themselves must appear between the '[' and ']' of the
493 character class; other elements may occur inside the charac-
494 ter class, too). The valid expressions are:
496 [:alnum:] [:alpha:] [:blank:]
497 [:cntrl:] [:digit:] [:graph:]
498 [:lower:] [:print:] [:punct:]
499 [:space:] [:upper:] [:xdigit:]
501 These expressions all designate a set of characters
502 equivalent to the corresponding standard C isXXX function.
503 For example, [:alnum:] designates those characters for which
504 isalnum() returns true - i.e., any alphabetic or numeric.
505 Some systems don't provide isblank(), so flex defines
506 [:blank:] as a blank or a tab.
508 For example, the following character classes are all
509 equivalent:
511 [[:alnum:]]
512 [[:alpha:][:digit:]
513 [[:alpha:]0-9]
514 [a-zA-Z0-9]
516 If your scanner is case-insensitive (the -i flag), then
517 [:upper:] and [:lower:] are equivalent to [:alpha:].
519 Some notes on patterns:
521 - A negated character class such as the example "[^A-Z]"
525 Version 2.5 Last change: April 1995 8
532 FLEX(1) USER COMMANDS FLEX(1)
536 above will match a newline unless "\n" (or an
537 equivalent escape sequence) is one of the characters
538 explicitly present in the negated character class
539 (e.g., "[^A-Z\n]"). This is unlike how many other reg-
540 ular expression tools treat negated character classes,
541 but unfortunately the inconsistency is historically
542 entrenched. Matching newlines means that a pattern
543 like [^"]* can match the entire input unless there's
544 another quote in the input.
546 - A rule can have at most one instance of trailing con-
547 text (the '/' operator or the '$' operator). The start
548 condition, '^', and "<<EOF>>" patterns can only occur
549 at the beginning of a pattern, and, as well as with '/'
550 and '$', cannot be grouped inside parentheses. A '^'
551 which does not occur at the beginning of a rule or a
552 '$' which does not occur at the end of a rule loses its
553 special properties and is treated as a normal charac-
554 ter.
556 The following are illegal:
558 foo/bar$
559 <sc1>foo<sc2>bar
561 Note that the first of these, can be written
562 "foo/bar\n".
564 The following will result in '$' or '^' being treated
565 as a normal character:
567 foo|(bar$)
568 foo|^bar
570 If what's wanted is a "foo" or a bar-followed-by-a-
571 newline, the following could be used (the special '|'
572 action is explained below):
574 foo |
575 bar$ /* action goes here */
577 A similar trick will work for matching a foo or a bar-
578 at-the-beginning-of-a-line.
580 HOW THE INPUT IS MATCHED
581 When the generated scanner is run, it analyzes its input
582 looking for strings which match any of its patterns. If it
583 finds more than one match, it takes the one matching the
584 most text (for trailing context rules, this includes the
585 length of the trailing part, even though it will then be
586 returned to the input). If it finds two or more matches of
587 the same length, the rule listed first in the flex input
591 Version 2.5 Last change: April 1995 9
598 FLEX(1) USER COMMANDS FLEX(1)
602 file is chosen.
604 Once the match is determined, the text corresponding to the
605 match (called the token) is made available in the global
606 character pointer yytext, and its length in the global
607 integer yyleng. The action corresponding to the matched pat-
608 tern is then executed (a more detailed description of
609 actions follows), and then the remaining input is scanned
610 for another match.
612 If no match is found, then the default rule is executed: the
613 next character in the input is considered matched and copied
614 to the standard output. Thus, the simplest legal flex input
615 is:
617 %%
619 which generates a scanner that simply copies its input (one
620 character at a time) to its output.
622 Note that yytext can be defined in two different ways:
623 either as a character pointer or as a character array. You
624 can control which definition flex uses by including one of
625 the special directives %pointer or %array in the first
626 (definitions) section of your flex input. The default is
627 %pointer, unless you use the -l lex compatibility option, in
628 which case yytext will be an array. The advantage of using
629 %pointer is substantially faster scanning and no buffer
630 overflow when matching very large tokens (unless you run out
631 of dynamic memory). The disadvantage is that you are res-
632 tricted in how your actions can modify yytext (see the next
633 section), and calls to the unput() function destroys the
634 present contents of yytext, which can be a considerable
635 porting headache when moving between different lex versions.
637 The advantage of %array is that you can then modify yytext
638 to your heart's content, and calls to unput() do not destroy
639 yytext (see below). Furthermore, existing lex programs
640 sometimes access yytext externally using declarations of the
641 form:
642 extern char yytext[];
643 This definition is erroneous when used with %pointer, but
644 correct for %array.
646 %array defines yytext to be an array of YYLMAX characters,
647 which defaults to a fairly large value. You can change the
648 size by simply #define'ing YYLMAX to a different value in
649 the first section of your flex input. As mentioned above,
650 with %pointer yytext grows dynamically to accommodate large
651 tokens. While this means your %pointer scanner can accommo-
652 date very large tokens (such as matching entire blocks of
653 comments), bear in mind that each time the scanner must
657 Version 2.5 Last change: April 1995 10
664 FLEX(1) USER COMMANDS FLEX(1)
668 resize yytext it also must rescan the entire token from the
669 beginning, so matching such tokens can prove slow. yytext
670 presently does not dynamically grow if a call to unput()
671 results in too much text being pushed back; instead, a run-
672 time error results.
674 Also note that you cannot use %array with C++ scanner
675 classes (the c++ option; see below).
677 ACTIONS
678 Each pattern in a rule has a corresponding action, which can
679 be any arbitrary C statement. The pattern ends at the first
680 non-escaped whitespace character; the remainder of the line
681 is its action. If the action is empty, then when the pat-
682 tern is matched the input token is simply discarded. For
683 example, here is the specification for a program which
684 deletes all occurrences of "zap me" from its input:
686 %%
687 "zap me"
689 (It will copy all other characters in the input to the out-
690 put since they will be matched by the default rule.)
692 Here is a program which compresses multiple blanks and tabs
693 down to a single blank, and throws away whitespace found at
694 the end of a line:
696 %%
697 [ \t]+ putchar( ' ' );
698 [ \t]+$ /* ignore this token */
701 If the action contains a '{', then the action spans till the
702 balancing '}' is found, and the action may cross multiple
703 lines. flex knows about C strings and comments and won't be
704 fooled by braces found within them, but also allows actions
705 to begin with %{ and will consider the action to be all the
706 text up to the next %} (regardless of ordinary braces inside
707 the action).
709 An action consisting solely of a vertical bar ('|') means
710 "same as the action for the next rule." See below for an
711 illustration.
713 Actions can include arbitrary C code, including return
714 statements to return a value to whatever routine called
715 yylex(). Each time yylex() is called it continues processing
716 tokens from where it last left off until it either reaches
717 the end of the file or executes a return.
723 Version 2.5 Last change: April 1995 11
730 FLEX(1) USER COMMANDS FLEX(1)
734 Actions are free to modify yytext except for lengthening it
735 (adding characters to its end--these will overwrite later
736 characters in the input stream). This however does not
737 apply when using %array (see above); in that case, yytext
738 may be freely modified in any way.
740 Actions are free to modify yyleng except they should not do
741 so if the action also includes use of yymore() (see below).
743 There are a number of special directives which can be
744 included within an action:
746 - ECHO copies yytext to the scanner's output.
748 - BEGIN followed by the name of a start condition places
749 the scanner in the corresponding start condition (see
750 below).
752 - REJECT directs the scanner to proceed on to the "second
753 best" rule which matched the input (or a prefix of the
754 input). The rule is chosen as described above in "How
755 the Input is Matched", and yytext and yyleng set up
756 appropriately. It may either be one which matched as
757 much text as the originally chosen rule but came later
758 in the flex input file, or one which matched less text.
759 For example, the following will both count the words in
760 the input and call the routine special() whenever
761 "frob" is seen:
763 int word_count = 0;
764 %%
766 frob special(); REJECT;
767 [^ \t\n]+ ++word_count;
769 Without the REJECT, any "frob"'s in the input would not
770 be counted as words, since the scanner normally exe-
771 cutes only one action per token. Multiple REJECT's are
772 allowed, each one finding the next best choice to the
773 currently active rule. For example, when the following
774 scanner scans the token "abcd", it will write "abcdab-
775 caba" to the output:
777 %%
778 a |
779 ab |
780 abc |
781 abcd ECHO; REJECT;
782 .|\n /* eat up any unmatched character */
784 (The first three rules share the fourth's action since
785 they use the special '|' action.) REJECT is a
789 Version 2.5 Last change: April 1995 12
796 FLEX(1) USER COMMANDS FLEX(1)
800 particularly expensive feature in terms of scanner per-
801 formance; if it is used in any of the scanner's actions
802 it will slow down all of the scanner's matching.
803 Furthermore, REJECT cannot be used with the -Cf or -CF
804 options (see below).
806 Note also that unlike the other special actions, REJECT
807 is a branch; code immediately following it in the
808 action will not be executed.
810 - yymore() tells the scanner that the next time it
811 matches a rule, the corresponding token should be
812 appended onto the current value of yytext rather than
813 replacing it. For example, given the input "mega-
814 kludge" the following will write "mega-mega-kludge" to
815 the output:
817 %%
818 mega- ECHO; yymore();
819 kludge ECHO;
821 First "mega-" is matched and echoed to the output.
822 Then "kludge" is matched, but the previous "mega-" is
823 still hanging around at the beginning of yytext so the
824 ECHO for the "kludge" rule will actually write "mega-
825 kludge".
827 Two notes regarding use of yymore(). First, yymore() depends
828 on the value of yyleng correctly reflecting the size of the
829 current token, so you must not modify yyleng if you are
830 using yymore(). Second, the presence of yymore() in the
831 scanner's action entails a minor performance penalty in the
832 scanner's matching speed.
834 - yyless(n) returns all but the first n characters of the
835 current token back to the input stream, where they will
836 be rescanned when the scanner looks for the next match.
837 yytext and yyleng are adjusted appropriately (e.g.,
838 yyleng will now be equal to n ). For example, on the
839 input "foobar" the following will write out "foobar-
840 bar":
842 %%
843 foobar ECHO; yyless(3);
844 [a-z]+ ECHO;
846 An argument of 0 to yyless will cause the entire
847 current input string to be scanned again. Unless
848 you've changed how the scanner will subsequently pro-
849 cess its input (using BEGIN, for example), this will
850 result in an endless loop.
855 Version 2.5 Last change: April 1995 13
862 FLEX(1) USER COMMANDS FLEX(1)
866 Note that yyless is a macro and can only be used in the flex
867 input file, not from other source files.
869 - unput(c) puts the character c back onto the input
870 stream. It will be the next character scanned. The
871 following action will take the current token and cause
872 it to be rescanned enclosed in parentheses.
874 {
875 int i;
876 /* Copy yytext because unput() trashes yytext */
877 char *yycopy = strdup( yytext );
878 unput( ')' );
879 for ( i = yyleng - 1; i >= 0; --i )
880 unput( yycopy[i] );
881 unput( '(' );
882 free( yycopy );
883 }
885 Note that since each unput() puts the given character
886 back at the beginning of the input stream, pushing back
887 strings must be done back-to-front.
889 An important potential problem when using unput() is that if
890 you are using %pointer (the default), a call to unput() des-
891 troys the contents of yytext, starting with its rightmost
892 character and devouring one character to the left with each
893 call. If you need the value of yytext preserved after a
894 call to unput() (as in the above example), you must either
895 first copy it elsewhere, or build your scanner using %array
896 instead (see How The Input Is Matched).
898 Finally, note that you cannot put back EOF to attempt to
899 mark the input stream with an end-of-file.
901 - input() reads the next character from the input stream.
902 For example, the following is one way to eat up C com-
903 ments:
905 %%
906 "/*" {
907 register int c;
909 for ( ; ; )
910 {
911 while ( (c = input()) != '*' &&
912 c != EOF )
913 ; /* eat up text of comment */
915 if ( c == '*' )
916 {
917 while ( (c = input()) == '*' )
921 Version 2.5 Last change: April 1995 14
928 FLEX(1) USER COMMANDS FLEX(1)
932 ;
933 if ( c == '/' )
934 break; /* found the end */
935 }
937 if ( c == EOF )
938 {
939 error( "EOF in comment" );
940 break;
941 }
942 }
943 }
945 (Note that if the scanner is compiled using C++, then
946 input() is instead referred to as yyinput(), in order
947 to avoid a name clash with the C++ stream by the name
948 of input.)
950 - YY_FLUSH_BUFFER flushes the scanner's internal buffer
951 so that the next time the scanner attempts to match a
952 token, it will first refill the buffer using YY_INPUT
953 (see The Generated Scanner, below). This action is a
954 special case of the more general yy_flush_buffer()
955 function, described below in the section Multiple Input
956 Buffers.
958 - yyterminate() can be used in lieu of a return statement
959 in an action. It terminates the scanner and returns a
960 0 to the scanner's caller, indicating "all done". By
961 default, yyterminate() is also called when an end-of-
962 file is encountered. It is a macro and may be rede-
963 fined.
965 THE GENERATED SCANNER
966 The output of flex is the file lex.yy.c, which contains the
967 scanning routine yylex(), a number of tables used by it for
968 matching tokens, and a number of auxiliary routines and mac-
969 ros. By default, yylex() is declared as follows:
971 int yylex()
972 {
973 ... various definitions and the actions in here ...
974 }
976 (If your environment supports function prototypes, then it
977 will be "int yylex( void )".) This definition may be
978 changed by defining the "YY_DECL" macro. For example, you
979 could use:
981 #define YY_DECL float lexscan( a, b ) float a, b;
983 to give the scanning routine the name lexscan, returning a
987 Version 2.5 Last change: April 1995 15
994 FLEX(1) USER COMMANDS FLEX(1)
998 float, and taking two floats as arguments. Note that if you
999 give arguments to the scanning routine using a K&R-
1000 style/non-prototyped function declaration, you must ter-
1001 minate the definition with a semi-colon (;).
1003 Whenever yylex() is called, it scans tokens from the global
1004 input file yyin (which defaults to stdin). It continues
1005 until it either reaches an end-of-file (at which point it
1006 returns the value 0) or one of its actions executes a return
1007 statement.
1009 If the scanner reaches an end-of-file, subsequent calls are
1010 undefined unless either yyin is pointed at a new input file
1011 (in which case scanning continues from that file), or yyres-
1012 tart() is called. yyrestart() takes one argument, a FILE *
1013 pointer (which can be nil, if you've set up YY_INPUT to scan
1014 from a source other than yyin), and initializes yyin for
1015 scanning from that file. Essentially there is no difference
1016 between just assigning yyin to a new input file or using
1017 yyrestart() to do so; the latter is available for compati-
1018 bility with previous versions of flex, and because it can be
1019 used to switch input files in the middle of scanning. It
1020 can also be used to throw away the current input buffer, by
1021 calling it with an argument of yyin; but better is to use
1022 YY_FLUSH_BUFFER (see above). Note that yyrestart() does not
1023 reset the start condition to INITIAL (see Start Conditions,
1024 below).
1026 If yylex() stops scanning due to executing a return state-
1027 ment in one of the actions, the scanner may then be called
1028 again and it will resume scanning where it left off.
1030 By default (and for purposes of efficiency), the scanner
1031 uses block-reads rather than simple getc() calls to read
1032 characters from yyin. The nature of how it gets its input
1033 can be controlled by defining the YY_INPUT macro.
1034 YY_INPUT's calling sequence is
1035 "YY_INPUT(buf,result,max_size)". Its action is to place up
1036 to max_size characters in the character array buf and return
1037 in the integer variable result either the number of charac-
1038 ters read or the constant YY_NULL (0 on Unix systems) to
1039 indicate EOF. The default YY_INPUT reads from the global
1040 file-pointer "yyin".
1042 A sample definition of YY_INPUT (in the definitions section
1043 of the input file):
1045 %{
1046 #define YY_INPUT(buf,result,max_size) \
1047 { \
1048 int c = getchar(); \
1049 result = (c == EOF) ? YY_NULL : (buf[0] = c, 1); \
1053 Version 2.5 Last change: April 1995 16
1060 FLEX(1) USER COMMANDS FLEX(1)
1064 }
1065 %}
1067 This definition will change the input processing to occur
1068 one character at a time.
1070 When the scanner receives an end-of-file indication from
1071 YY_INPUT, it then checks the yywrap() function. If yywrap()
1072 returns false (zero), then it is assumed that the function
1073 has gone ahead and set up yyin to point to another input
1074 file, and scanning continues. If it returns true (non-
1075 zero), then the scanner terminates, returning 0 to its
1076 caller. Note that in either case, the start condition
1077 remains unchanged; it does not revert to INITIAL.
1079 If you do not supply your own version of yywrap(), then you
1080 must either use %option noyywrap (in which case the scanner
1081 behaves as though yywrap() returned 1), or you must link
1082 with -lfl to obtain the default version of the routine,
1083 which always returns 1.
1085 Three routines are available for scanning from in-memory
1086 buffers rather than files: yy_scan_string(),
1087 yy_scan_bytes(), and yy_scan_buffer(). See the discussion of
1088 them below in the section Multiple Input Buffers.
1090 The scanner writes its ECHO output to the yyout global
1091 (default, stdout), which may be redefined by the user simply
1092 by assigning it to some other FILE pointer.
1094 START CONDITIONS
1095 flex provides a mechanism for conditionally activating
1096 rules. Any rule whose pattern is prefixed with "<sc>" will
1097 only be active when the scanner is in the start condition
1098 named "sc". For example,
1100 <STRING>[^"]* { /* eat up the string body ... */
1101 ...
1102 }
1104 will be active only when the scanner is in the "STRING"
1105 start condition, and
1107 <INITIAL,STRING,QUOTE>\. { /* handle an escape ... */
1108 ...
1109 }
1111 will be active only when the current start condition is
1112 either "INITIAL", "STRING", or "QUOTE".
1114 Start conditions are declared in the definitions (first)
1115 section of the input using unindented lines beginning with
1119 Version 2.5 Last change: April 1995 17
1126 FLEX(1) USER COMMANDS FLEX(1)
1130 either %s or %x followed by a list of names. The former
1131 declares inclusive start conditions, the latter exclusive
1132 start conditions. A start condition is activated using the
1133 BEGIN action. Until the next BEGIN action is executed,
1134 rules with the given start condition will be active and
1135 rules with other start conditions will be inactive. If the
1136 start condition is inclusive, then rules with no start con-
1137 ditions at all will also be active. If it is exclusive,
1138 then only rules qualified with the start condition will be
1139 active. A set of rules contingent on the same exclusive
1140 start condition describe a scanner which is independent of
1141 any of the other rules in the flex input. Because of this,
1142 exclusive start conditions make it easy to specify "mini-
1143 scanners" which scan portions of the input that are syntac-
1144 tically different from the rest (e.g., comments).
1146 If the distinction between inclusive and exclusive start
1147 conditions is still a little vague, here's a simple example
1148 illustrating the connection between the two. The set of
1149 rules:
1151 %s example
1152 %%
1154 <example>foo do_something();
1156 bar something_else();
1158 is equivalent to
1160 %x example
1161 %%
1163 <example>foo do_something();
1165 <INITIAL,example>bar something_else();
1167 Without the <INITIAL,example> qualifier, the bar pattern in
1168 the second example wouldn't be active (i.e., couldn't match)
1169 when in start condition example. If we just used <example>
1170 to qualify bar, though, then it would only be active in
1171 example and not in INITIAL, while in the first example it's
1172 active in both, because in the first example the example
1173 startion condition is an inclusive (%s) start condition.
1175 Also note that the special start-condition specifier <*>
1176 matches every start condition. Thus, the above example
1177 could also have been written;
1179 %x example
1180 %%
1185 Version 2.5 Last change: April 1995 18
1192 FLEX(1) USER COMMANDS FLEX(1)
1196 <example>foo do_something();
1198 <*>bar something_else();
1201 The default rule (to ECHO any unmatched character) remains
1202 active in start conditions. It is equivalent to:
1204 <*>.|\n ECHO;
1207 BEGIN(0) returns to the original state where only the rules
1208 with no start conditions are active. This state can also be
1209 referred to as the start-condition "INITIAL", so
1210 BEGIN(INITIAL) is equivalent to BEGIN(0). (The parentheses
1211 around the start condition name are not required but are
1212 considered good style.)
1214 BEGIN actions can also be given as indented code at the
1215 beginning of the rules section. For example, the following
1216 will cause the scanner to enter the "SPECIAL" start condi-
1217 tion whenever yylex() is called and the global variable
1218 enter_special is true:
1220 int enter_special;
1222 %x SPECIAL
1223 %%
1224 if ( enter_special )
1225 BEGIN(SPECIAL);
1227 <SPECIAL>blahblahblah
1228 ...more rules follow...
1231 To illustrate the uses of start conditions, here is a
1232 scanner which provides two different interpretations of a
1233 string like "123.456". By default it will treat it as three
1234 tokens, the integer "123", a dot ('.'), and the integer
1235 "456". But if the string is preceded earlier in the line by
1236 the string "expect-floats" it will treat it as a single
1237 token, the floating-point number 123.456:
1239 %{
1240 #include <math.h>
1241 %}
1242 %s expect
1244 %%
1245 expect-floats BEGIN(expect);
1247 <expect>[0-9]+"."[0-9]+ {
1251 Version 2.5 Last change: April 1995 19
1258 FLEX(1) USER COMMANDS FLEX(1)
1262 printf( "found a float, = %f\n",
1263 atof( yytext ) );
1264 }
1265 <expect>\n {
1266 /* that's the end of the line, so
1267 * we need another "expect-number"
1268 * before we'll recognize any more
1269 * numbers
1270 */
1271 BEGIN(INITIAL);
1272 }
1274 [0-9]+ {
1275 printf( "found an integer, = %d\n",
1276 atoi( yytext ) );
1277 }
1279 "." printf( "found a dot\n" );
1281 Here is a scanner which recognizes (and discards) C comments
1282 while maintaining a count of the current input line.
1284 %x comment
1285 %%
1286 int line_num = 1;
1288 "/*" BEGIN(comment);
1290 <comment>[^*\n]* /* eat anything that's not a '*' */
1291 <comment>"*"+[^*/\n]* /* eat up '*'s not followed by '/'s */
1292 <comment>\n ++line_num;
1293 <comment>"*"+"/" BEGIN(INITIAL);
1295 This scanner goes to a bit of trouble to match as much text
1296 as possible with each rule. In general, when attempting to
1297 write a high-speed scanner try to match as much possible in
1298 each rule, as it's a big win.
1300 Note that start-conditions names are really integer values
1301 and can be stored as such. Thus, the above could be
1302 extended in the following fashion:
1304 %x comment foo
1305 %%
1306 int line_num = 1;
1307 int comment_caller;
1309 "/*" {
1310 comment_caller = INITIAL;
1311 BEGIN(comment);
1312 }
1317 Version 2.5 Last change: April 1995 20
1324 FLEX(1) USER COMMANDS FLEX(1)
1328 ...
1330 <foo>"/*" {
1331 comment_caller = foo;
1332 BEGIN(comment);
1333 }
1335 <comment>[^*\n]* /* eat anything that's not a '*' */
1336 <comment>"*"+[^*/\n]* /* eat up '*'s not followed by '/'s */
1337 <comment>\n ++line_num;
1338 <comment>"*"+"/" BEGIN(comment_caller);
1340 Furthermore, you can access the current start condition
1341 using the integer-valued YY_START macro. For example, the
1342 above assignments to comment_caller could instead be written
1344 comment_caller = YY_START;
1346 Flex provides YYSTATE as an alias for YY_START (since that
1347 is what's used by AT&T lex).
1349 Note that start conditions do not have their own name-space;
1350 %s's and %x's declare names in the same fashion as
1351 #define's.
1353 Finally, here's an example of how to match C-style quoted
1354 strings using exclusive start conditions, including expanded
1355 escape sequences (but not including checking for a string
1356 that's too long):
1358 %x str
1360 %%
1361 char string_buf[MAX_STR_CONST];
1362 char *string_buf_ptr;
1365 \" string_buf_ptr = string_buf; BEGIN(str);
1367 <str>\" { /* saw closing quote - all done */
1368 BEGIN(INITIAL);
1369 *string_buf_ptr = '\0';
1370 /* return string constant token type and
1371 * value to parser
1372 */
1373 }
1375 <str>\n {
1376 /* error - unterminated string constant */
1377 /* generate error message */
1378 }
1383 Version 2.5 Last change: April 1995 21
1390 FLEX(1) USER COMMANDS FLEX(1)
1394 <str>\\[0-7]{1,3} {
1395 /* octal escape sequence */
1396 int result;
1398 (void) sscanf( yytext + 1, "%o", &result );
1400 if ( result > 0xff )
1401 /* error, constant is out-of-bounds */
1403 *string_buf_ptr++ = result;
1404 }
1406 <str>\\[0-9]+ {
1407 /* generate error - bad escape sequence; something
1408 * like '\48' or '\0777777'
1409 */
1410 }
1412 <str>\\n *string_buf_ptr++ = '\n';
1413 <str>\\t *string_buf_ptr++ = '\t';
1414 <str>\\r *string_buf_ptr++ = '\r';
1415 <str>\\b *string_buf_ptr++ = '\b';
1416 <str>\\f *string_buf_ptr++ = '\f';
1418 <str>\\(.|\n) *string_buf_ptr++ = yytext[1];
1420 <str>[^\\\n\"]+ {
1421 char *yptr = yytext;
1423 while ( *yptr )
1424 *string_buf_ptr++ = *yptr++;
1425 }
1428 Often, such as in some of the examples above, you wind up
1429 writing a whole bunch of rules all preceded by the same
1430 start condition(s). Flex makes this a little easier and
1431 cleaner by introducing a notion of start condition scope. A
1432 start condition scope is begun with:
1434 <SCs>{
1436 where SCs is a list of one or more start conditions. Inside
1437 the start condition scope, every rule automatically has the
1438 prefix <SCs> applied to it, until a '}' which matches the
1439 initial '{'. So, for example,
1441 <ESC>{
1442 "\\n" return '\n';
1443 "\\r" return '\r';
1444 "\\f" return '\f';
1445 "\\0" return '\0';
1449 Version 2.5 Last change: April 1995 22
1456 FLEX(1) USER COMMANDS FLEX(1)
1460 }
1462 is equivalent to:
1464 <ESC>"\\n" return '\n';
1465 <ESC>"\\r" return '\r';
1466 <ESC>"\\f" return '\f';
1467 <ESC>"\\0" return '\0';
1469 Start condition scopes may be nested.
1471 Three routines are available for manipulating stacks of
1472 start conditions:
1474 void yy_push_state(int new_state)
1475 pushes the current start condition onto the top of the
1476 start condition stack and switches to new_state as
1477 though you had used BEGIN new_state (recall that start
1478 condition names are also integers).
1480 void yy_pop_state()
1481 pops the top of the stack and switches to it via BEGIN.
1483 int yy_top_state()
1484 returns the top of the stack without altering the
1485 stack's contents.
1487 The start condition stack grows dynamically and so has no
1488 built-in size limitation. If memory is exhausted, program
1489 execution aborts.
1491 To use start condition stacks, your scanner must include a
1492 %option stack directive (see Options below).
1494 MULTIPLE INPUT BUFFERS
1495 Some scanners (such as those which support "include" files)
1496 require reading from several input streams. As flex
1497 scanners do a large amount of buffering, one cannot control
1498 where the next input will be read from by simply writing a
1499 YY_INPUT which is sensitive to the scanning context.
1500 YY_INPUT is only called when the scanner reaches the end of
1501 its buffer, which may be a long time after scanning a state-
1502 ment such as an "include" which requires switching the input
1503 source.
1505 To negotiate these sorts of problems, flex provides a
1506 mechanism for creating and switching between multiple input
1507 buffers. An input buffer is created by using:
1509 YY_BUFFER_STATE yy_create_buffer( FILE *file, int size )
1511 which takes a FILE pointer and a size and creates a buffer
1515 Version 2.5 Last change: April 1995 23
1522 FLEX(1) USER COMMANDS FLEX(1)
1526 associated with the given file and large enough to hold size
1527 characters (when in doubt, use YY_BUF_SIZE for the size).
1528 It returns a YY_BUFFER_STATE handle, which may then be
1529 passed to other routines (see below). The YY_BUFFER_STATE
1530 type is a pointer to an opaque struct yy_buffer_state struc-
1531 ture, so you may safely initialize YY_BUFFER_STATE variables
1532 to ((YY_BUFFER_STATE) 0) if you wish, and also refer to the
1533 opaque structure in order to correctly declare input buffers
1534 in source files other than that of your scanner. Note that
1535 the FILE pointer in the call to yy_create_buffer is only
1536 used as the value of yyin seen by YY_INPUT; if you redefine
1537 YY_INPUT so it no longer uses yyin, then you can safely pass
1538 a nil FILE pointer to yy_create_buffer. You select a partic-
1539 ular buffer to scan from using:
1541 void yy_switch_to_buffer( YY_BUFFER_STATE new_buffer )
1543 switches the scanner's input buffer so subsequent tokens
1544 will come from new_buffer. Note that yy_switch_to_buffer()
1545 may be used by yywrap() to set things up for continued scan-
1546 ning, instead of opening a new file and pointing yyin at it.
1547 Note also that switching input sources via either
1548 yy_switch_to_buffer() or yywrap() does not change the start
1549 condition.
1551 void yy_delete_buffer( YY_BUFFER_STATE buffer )
1553 is used to reclaim the storage associated with a buffer. (
1554 buffer can be nil, in which case the routine does nothing.)
1555 You can also clear the current contents of a buffer using:
1557 void yy_flush_buffer( YY_BUFFER_STATE buffer )
1559 This function discards the buffer's contents, so the next
1560 time the scanner attempts to match a token from the buffer,
1561 it will first fill the buffer anew using YY_INPUT.
1563 yy_new_buffer() is an alias for yy_create_buffer(), provided
1564 for compatibility with the C++ use of new and delete for
1565 creating and destroying dynamic objects.
1567 Finally, the YY_CURRENT_BUFFER macro returns a
1568 YY_BUFFER_STATE handle to the current buffer.
1570 Here is an example of using these features for writing a
1571 scanner which expands include files (the <<EOF>> feature is
1572 discussed below):
1574 /* the "incl" state is used for picking up the name
1575 * of an include file
1576 */
1577 %x incl
1581 Version 2.5 Last change: April 1995 24
1588 FLEX(1) USER COMMANDS FLEX(1)
1592 %{
1593 #define MAX_INCLUDE_DEPTH 10
1594 YY_BUFFER_STATE include_stack[MAX_INCLUDE_DEPTH];
1595 int include_stack_ptr = 0;
1596 %}
1598 %%
1599 include BEGIN(incl);
1601 [a-z]+ ECHO;
1602 [^a-z\n]*\n? ECHO;
1604 <incl>[ \t]* /* eat the whitespace */
1605 <incl>[^ \t\n]+ { /* got the include file name */
1606 if ( include_stack_ptr >= MAX_INCLUDE_DEPTH )
1607 {
1608 fprintf( stderr, "Includes nested too deeply" );
1609 exit( 1 );
1610 }
1612 include_stack[include_stack_ptr++] =
1613 YY_CURRENT_BUFFER;
1615 yyin = fopen( yytext, "r" );
1617 if ( ! yyin )
1618 error( ... );
1620 yy_switch_to_buffer(
1621 yy_create_buffer( yyin, YY_BUF_SIZE ) );
1623 BEGIN(INITIAL);
1624 }
1626 <<EOF>> {
1627 if ( --include_stack_ptr < 0 )
1628 {
1629 yyterminate();
1630 }
1632 else
1633 {
1634 yy_delete_buffer( YY_CURRENT_BUFFER );
1635 yy_switch_to_buffer(
1636 include_stack[include_stack_ptr] );
1637 }
1638 }
1640 Three routines are available for setting up input buffers
1641 for scanning in-memory strings instead of files. All of
1642 them create a new input buffer for scanning the string, and
1643 return a corresponding YY_BUFFER_STATE handle (which you
1647 Version 2.5 Last change: April 1995 25
1654 FLEX(1) USER COMMANDS FLEX(1)
1658 should delete with yy_delete_buffer() when done with it).
1659 They also switch to the new buffer using
1660 yy_switch_to_buffer(), so the next call to yylex() will
1661 start scanning the string.
1663 yy_scan_string(const char *str)
1664 scans a NUL-terminated string.
1666 yy_scan_bytes(const char *bytes, int len)
1667 scans len bytes (including possibly NUL's) starting at
1668 location bytes.
1670 Note that both of these functions create and scan a copy of
1671 the string or bytes. (This may be desirable, since yylex()
1672 modifies the contents of the buffer it is scanning.) You
1673 can avoid the copy by using:
1675 yy_scan_buffer(char *base, yy_size_t size)
1676 which scans in place the buffer starting at base, con-
1677 sisting of size bytes, the last two bytes of which must
1678 be YY_END_OF_BUFFER_CHAR (ASCII NUL). These last two
1679 bytes are not scanned; thus, scanning consists of
1680 base[0] through base[size-2], inclusive.
1682 If you fail to set up base in this manner (i.e., forget
1683 the final two YY_END_OF_BUFFER_CHAR bytes), then
1684 yy_scan_buffer() returns a nil pointer instead of
1685 creating a new input buffer.
1687 The type yy_size_t is an integral type to which you can
1688 cast an integer expression reflecting the size of the
1689 buffer.
1691 END-OF-FILE RULES
1692 The special rule "<<EOF>>" indicates actions which are to be
1693 taken when an end-of-file is encountered and yywrap()
1694 returns non-zero (i.e., indicates no further files to pro-
1695 cess). The action must finish by doing one of four things:
1697 - assigning yyin to a new input file (in previous ver-
1698 sions of flex, after doing the assignment you had to
1699 call the special action YY_NEW_FILE; this is no longer
1700 necessary);
1702 - executing a return statement;
1704 - executing the special yyterminate() action;
1706 - or, switching to a new buffer using
1707 yy_switch_to_buffer() as shown in the example above.
1713 Version 2.5 Last change: April 1995 26
1720 FLEX(1) USER COMMANDS FLEX(1)
1724 <<EOF>> rules may not be used with other patterns; they may
1725 only be qualified with a list of start conditions. If an
1726 unqualified <<EOF>> rule is given, it applies to all start
1727 conditions which do not already have <<EOF>> actions. To
1728 specify an <<EOF>> rule for only the initial start condi-
1729 tion, use
1731 <INITIAL><<EOF>>
1734 These rules are useful for catching things like unclosed
1735 comments. An example:
1737 %x quote
1738 %%
1740 ...other rules for dealing with quotes...
1742 <quote><<EOF>> {
1743 error( "unterminated quote" );
1744 yyterminate();
1745 }
1746 <<EOF>> {
1747 if ( *++filelist )
1748 yyin = fopen( *filelist, "r" );
1749 else
1750 yyterminate();
1751 }
1754 MISCELLANEOUS MACROS
1755 The macro YY_USER_ACTION can be defined to provide an action
1756 which is always executed prior to the matched rule's action.
1757 For example, it could be #define'd to call a routine to con-
1758 vert yytext to lower-case. When YY_USER_ACTION is invoked,
1759 the variable yy_act gives the number of the matched rule
1760 (rules are numbered starting with 1). Suppose you want to
1761 profile how often each of your rules is matched. The fol-
1762 lowing would do the trick:
1764 #define YY_USER_ACTION ++ctr[yy_act]
1766 where ctr is an array to hold the counts for the different
1767 rules. Note that the macro YY_NUM_RULES gives the total
1768 number of rules (including the default rule, even if you use
1769 -s), so a correct declaration for ctr is:
1771 int ctr[YY_NUM_RULES];
1774 The macro YY_USER_INIT may be defined to provide an action
1775 which is always executed before the first scan (and before
1779 Version 2.5 Last change: April 1995 27
1786 FLEX(1) USER COMMANDS FLEX(1)
1790 the scanner's internal initializations are done). For exam-
1791 ple, it could be used to call a routine to read in a data
1792 table or open a logging file.
1794 The macro yy_set_interactive(is_interactive) can be used to
1795 control whether the current buffer is considered interac-
1796 tive. An interactive buffer is processed more slowly, but
1797 must be used when the scanner's input source is indeed
1798 interactive to avoid problems due to waiting to fill buffers
1799 (see the discussion of the -I flag below). A non-zero value
1800 in the macro invocation marks the buffer as interactive, a
1801 zero value as non-interactive. Note that use of this macro
1802 overrides %option always-interactive or %option never-
1803 interactive (see Options below). yy_set_interactive() must
1804 be invoked prior to beginning to scan the buffer that is (or
1805 is not) to be considered interactive.
1807 The macro yy_set_bol(at_bol) can be used to control whether
1808 the current buffer's scanning context for the next token
1809 match is done as though at the beginning of a line. A non-
1810 zero macro argument makes rules anchored with
1812 The macro YY_AT_BOL() returns true if the next token scanned
1813 from the current buffer will have '^' rules active, false
1814 otherwise.
1816 In the generated scanner, the actions are all gathered in
1817 one large switch statement and separated using YY_BREAK,
1818 which may be redefined. By default, it is simply a "break",
1819 to separate each rule's action from the following rule's.
1820 Redefining YY_BREAK allows, for example, C++ users to
1821 #define YY_BREAK to do nothing (while being very careful
1822 that every rule ends with a "break" or a "return"!) to avoid
1823 suffering from unreachable statement warnings where because
1824 a rule's action ends with "return", the YY_BREAK is inacces-
1825 sible.
1827 VALUES AVAILABLE TO THE USER
1828 This section summarizes the various values available to the
1829 user in the rule actions.
1831 - char *yytext holds the text of the current token. It
1832 may be modified but not lengthened (you cannot append
1833 characters to the end).
1835 If the special directive %array appears in the first
1836 section of the scanner description, then yytext is
1837 instead declared char yytext[YYLMAX], where YYLMAX is a
1838 macro definition that you can redefine in the first
1839 section if you don't like the default value (generally
1840 8KB). Using %array results in somewhat slower
1841 scanners, but the value of yytext becomes immune to
1845 Version 2.5 Last change: April 1995 28
1852 FLEX(1) USER COMMANDS FLEX(1)
1856 calls to input() and unput(), which potentially destroy
1857 its value when yytext is a character pointer. The
1858 opposite of %array is %pointer, which is the default.
1860 You cannot use %array when generating C++ scanner
1861 classes (the -+ flag).
1863 - int yyleng holds the length of the current token.
1865 - FILE *yyin is the file which by default flex reads
1866 from. It may be redefined but doing so only makes
1867 sense before scanning begins or after an EOF has been
1868 encountered. Changing it in the midst of scanning will
1869 have unexpected results since flex buffers its input;
1870 use yyrestart() instead. Once scanning terminates
1871 because an end-of-file has been seen, you can assign
1872 yyin at the new input file and then call the scanner
1873 again to continue scanning.
1875 - void yyrestart( FILE *new_file ) may be called to point
1876 yyin at the new input file. The switch-over to the new
1877 file is immediate (any previously buffered-up input is
1878 lost). Note that calling yyrestart() with yyin as an
1879 argument thus throws away the current input buffer and
1880 continues scanning the same input file.
1882 - FILE *yyout is the file to which ECHO actions are done.
1883 It can be reassigned by the user.
1885 - YY_CURRENT_BUFFER returns a YY_BUFFER_STATE handle to
1886 the current buffer.
1888 - YY_START returns an integer value corresponding to the
1889 current start condition. You can subsequently use this
1890 value with BEGIN to return to that start condition.
1892 INTERFACING WITH YACC
1893 One of the main uses of flex is as a companion to the yacc
1894 parser-generator. yacc parsers expect to call a routine
1895 named yylex() to find the next input token. The routine is
1896 supposed to return the type of the next token as well as
1897 putting any associated value in the global yylval. To use
1898 flex with yacc, one specifies the -d option to yacc to
1899 instruct it to generate the file y.tab.h containing defini-
1900 tions of all the %tokens appearing in the yacc input. This
1901 file is then included in the flex scanner. For example, if
1902 one of the tokens is "TOK_NUMBER", part of the scanner might
1903 look like:
1905 %{
1906 #include "y.tab.h"
1907 %}
1911 Version 2.5 Last change: April 1995 29
1918 FLEX(1) USER COMMANDS FLEX(1)
1922 %%
1924 [0-9]+ yylval = atoi( yytext ); return TOK_NUMBER;
1927 OPTIONS
1928 flex has the following options:
1930 -b Generate backing-up information to lex.backup. This is
1931 a list of scanner states which require backing up and
1932 the input characters on which they do so. By adding
1933 rules one can remove backing-up states. If all
1934 backing-up states are eliminated and -Cf or -CF is
1935 used, the generated scanner will run faster (see the -p
1936 flag). Only users who wish to squeeze every last cycle
1937 out of their scanners need worry about this option.
1938 (See the section on Performance Considerations below.)
1940 -c is a do-nothing, deprecated option included for POSIX
1941 compliance.
1943 -d makes the generated scanner run in debug mode. When-
1944 ever a pattern is recognized and the global
1945 yy_flex_debug is non-zero (which is the default), the
1946 scanner will write to stderr a line of the form:
1948 --accepting rule at line 53 ("the matched text")
1950 The line number refers to the location of the rule in
1951 the file defining the scanner (i.e., the file that was
1952 fed to flex). Messages are also generated when the
1953 scanner backs up, accepts the default rule, reaches the
1954 end of its input buffer (or encounters a NUL; at this
1955 point, the two look the same as far as the scanner's
1956 concerned), or reaches an end-of-file.
1958 -f specifies fast scanner. No table compression is done
1959 and stdio is bypassed. The result is large but fast.
1960 This option is equivalent to -Cfr (see below).
1962 -h generates a "help" summary of flex's options to stdout
1963 and then exits. -? and --help are synonyms for -h.
1965 -i instructs flex to generate a case-insensitive scanner.
1966 The case of letters given in the flex input patterns
1967 will be ignored, and tokens in the input will be
1968 matched regardless of case. The matched text given in
1969 yytext will have the preserved case (i.e., it will not
1970 be folded).
1972 -l turns on maximum compatibility with the original AT&T
1973 lex implementation. Note that this does not mean full
1977 Version 2.5 Last change: April 1995 30
1984 FLEX(1) USER COMMANDS FLEX(1)
1988 compatibility. Use of this option costs a considerable
1989 amount of performance, and it cannot be used with the
1990 -+, -f, -F, -Cf, or -CF options. For details on the
1991 compatibilities it provides, see the section "Incompa-
1992 tibilities With Lex And POSIX" below. This option also
1993 results in the name YY_FLEX_LEX_COMPAT being #define'd
1994 in the generated scanner.
1996 -n is another do-nothing, deprecated option included only
1997 for POSIX compliance.
1999 -p generates a performance report to stderr. The report
2000 consists of comments regarding features of the flex
2001 input file which will cause a serious loss of perfor-
2002 mance in the resulting scanner. If you give the flag
2003 twice, you will also get comments regarding features
2004 that lead to minor performance losses.
2006 Note that the use of REJECT, %option yylineno, and
2007 variable trailing context (see the Deficiencies / Bugs
2008 section below) entails a substantial performance
2009 penalty; use of yymore(), the ^ operator, and the -I
2010 flag entail minor performance penalties.
2012 -s causes the default rule (that unmatched scanner input
2013 is echoed to stdout) to be suppressed. If the scanner
2014 encounters input that does not match any of its rules,
2015 it aborts with an error. This option is useful for
2016 finding holes in a scanner's rule set.
2018 -t instructs flex to write the scanner it generates to
2019 standard output instead of lex.yy.c.
2021 -v specifies that flex should write to stderr a summary of
2022 statistics regarding the scanner it generates. Most of
2023 the statistics are meaningless to the casual flex user,
2024 but the first line identifies the version of flex (same
2025 as reported by -V), and the next line the flags used
2026 when generating the scanner, including those that are
2027 on by default.
2029 -w suppresses warning messages.
2031 -B instructs flex to generate a batch scanner, the oppo-
2032 site of interactive scanners generated by -I (see
2033 below). In general, you use -B when you are certain
2034 that your scanner will never be used interactively, and
2035 you want to squeeze a little more performance out of
2036 it. If your goal is instead to squeeze out a lot more
2037 performance, you should be using the -Cf or -CF
2038 options (discussed below), which turn on -B automati-
2039 cally anyway.
2043 Version 2.5 Last change: April 1995 31
2050 FLEX(1) USER COMMANDS FLEX(1)
2054 -F specifies that the fast scanner table representation
2055 should be used (and stdio bypassed). This representa-
2056 tion is about as fast as the full table representation
2057 (-f), and for some sets of patterns will be consider-
2058 ably smaller (and for others, larger). In general, if
2059 the pattern set contains both "keywords" and a catch-
2060 all, "identifier" rule, such as in the set:
2062 "case" return TOK_CASE;
2063 "switch" return TOK_SWITCH;
2064 ...
2065 "default" return TOK_DEFAULT;
2066 [a-z]+ return TOK_ID;
2068 then you're better off using the full table representa-
2069 tion. If only the "identifier" rule is present and you
2070 then use a hash table or some such to detect the key-
2071 words, you're better off using -F.
2073 This option is equivalent to -CFr (see below). It can-
2074 not be used with -+.
2076 -I instructs flex to generate an interactive scanner. An
2077 interactive scanner is one that only looks ahead to
2078 decide what token has been matched if it absolutely
2079 must. It turns out that always looking one extra char-
2080 acter ahead, even if the scanner has already seen
2081 enough text to disambiguate the current token, is a bit
2082 faster than only looking ahead when necessary. But
2083 scanners that always look ahead give dreadful interac-
2084 tive performance; for example, when a user types a new-
2085 line, it is not recognized as a newline token until
2086 they enter another token, which often means typing in
2087 another whole line.
2089 Flex scanners default to interactive unless you use the
2090 -Cf or -CF table-compression options (see below).
2091 That's because if you're looking for high-performance
2092 you should be using one of these options, so if you
2093 didn't, flex assumes you'd rather trade off a bit of
2094 run-time performance for intuitive interactive
2095 behavior. Note also that you cannot use -I in conjunc-
2096 tion with -Cf or -CF. Thus, this option is not really
2097 needed; it is on by default for all those cases in
2098 which it is allowed.
2100 You can force a scanner to not be interactive by using
2101 -B (see above).
2103 -L instructs flex not to generate #line directives.
2104 Without this option, flex peppers the generated scanner
2105 with #line directives so error messages in the actions
2109 Version 2.5 Last change: April 1995 32
2116 FLEX(1) USER COMMANDS FLEX(1)
2120 will be correctly located with respect to either the
2121 original flex input file (if the errors are due to code
2122 in the input file), or lex.yy.c (if the errors are
2123 flex's fault -- you should report these sorts of errors
2124 to the email address given below).
2126 -T makes flex run in trace mode. It will generate a lot
2127 of messages to stderr concerning the form of the input
2128 and the resultant non-deterministic and deterministic
2129 finite automata. This option is mostly for use in
2130 maintaining flex.
2132 -V prints the version number to stdout and exits. --ver-
2133 sion is a synonym for -V.
2135 -7 instructs flex to generate a 7-bit scanner, i.e., one
2136 which can only recognized 7-bit characters in its
2137 input. The advantage of using -7 is that the scanner's
2138 tables can be up to half the size of those generated
2139 using the -8 option (see below). The disadvantage is
2140 that such scanners often hang or crash if their input
2141 contains an 8-bit character.
2143 Note, however, that unless you generate your scanner
2144 using the -Cf or -CF table compression options, use of
2145 -7 will save only a small amount of table space, and
2146 make your scanner considerably less portable. Flex's
2147 default behavior is to generate an 8-bit scanner unless
2148 you use the -Cf or -CF, in which case flex defaults to
2149 generating 7-bit scanners unless your site was always
2150 configured to generate 8-bit scanners (as will often be
2151 the case with non-USA sites). You can tell whether
2152 flex generated a 7-bit or an 8-bit scanner by inspect-
2153 ing the flag summary in the -v output as described
2154 above.
2156 Note that if you use -Cfe or -CFe (those table compres-
2157 sion options, but also using equivalence classes as
2158 discussed see below), flex still defaults to generating
2159 an 8-bit scanner, since usually with these compression
2160 options full 8-bit tables are not much more expensive
2161 than 7-bit tables.
2163 -8 instructs flex to generate an 8-bit scanner, i.e., one
2164 which can recognize 8-bit characters. This flag is
2165 only needed for scanners generated using -Cf or -CF, as
2166 otherwise flex defaults to generating an 8-bit scanner
2167 anyway.
2169 See the discussion of -7 above for flex's default
2170 behavior and the tradeoffs between 7-bit and 8-bit
2171 scanners.
2175 Version 2.5 Last change: April 1995 33
2182 FLEX(1) USER COMMANDS FLEX(1)
2186 -+ specifies that you want flex to generate a C++ scanner
2187 class. See the section on Generating C++ Scanners
2188 below for details.
2190 -C[aefFmr]
2191 controls the degree of table compression and, more gen-
2192 erally, trade-offs between small scanners and fast
2193 scanners.
2195 -Ca ("align") instructs flex to trade off larger tables
2196 in the generated scanner for faster performance because
2197 the elements of the tables are better aligned for
2198 memory access and computation. On some RISC architec-
2199 tures, fetching and manipulating longwords is more
2200 efficient than with smaller-sized units such as short-
2201 words. This option can double the size of the tables
2202 used by your scanner.
2204 -Ce directs flex to construct equivalence classes,
2205 i.e., sets of characters which have identical lexical
2206 properties (for example, if the only appearance of
2207 digits in the flex input is in the character class
2208 "[0-9]" then the digits '0', '1', ..., '9' will all be
2209 put in the same equivalence class). Equivalence
2210 classes usually give dramatic reductions in the final
2211 table/object file sizes (typically a factor of 2-5) and
2212 are pretty cheap performance-wise (one array look-up
2213 per character scanned).
2215 -Cf specifies that the full scanner tables should be
2216 generated - flex should not compress the tables by tak-
2217 ing advantages of similar transition functions for dif-
2218 ferent states.
2220 -CF specifies that the alternate fast scanner represen-
2221 tation (described above under the -F flag) should be
2222 used. This option cannot be used with -+.
2224 -Cm directs flex to construct meta-equivalence classes,
2225 which are sets of equivalence classes (or characters,
2226 if equivalence classes are not being used) that are
2227 commonly used together. Meta-equivalence classes are
2228 often a big win when using compressed tables, but they
2229 have a moderate performance impact (one or two "if"
2230 tests and one array look-up per character scanned).
2232 -Cr causes the generated scanner to bypass use of the
2233 standard I/O library (stdio) for input. Instead of
2234 calling fread() or getc(), the scanner will use the
2235 read() system call, resulting in a performance gain
2236 which varies from system to system, but in general is
2237 probably negligible unless you are also using -Cf or
2241 Version 2.5 Last change: April 1995 34
2248 FLEX(1) USER COMMANDS FLEX(1)
2252 -CF. Using -Cr can cause strange behavior if, for exam-
2253 ple, you read from yyin using stdio prior to calling
2254 the scanner (because the scanner will miss whatever
2255 text your previous reads left in the stdio input
2256 buffer).
2258 -Cr has no effect if you define YY_INPUT (see The Gen-
2259 erated Scanner above).
2261 A lone -C specifies that the scanner tables should be
2262 compressed but neither equivalence classes nor meta-
2263 equivalence classes should be used.
2265 The options -Cf or -CF and -Cm do not make sense
2266 together - there is no opportunity for meta-equivalence
2267 classes if the table is not being compressed. Other-
2268 wise the options may be freely mixed, and are cumula-
2269 tive.
2271 The default setting is -Cem, which specifies that flex
2272 should generate equivalence classes and meta-
2273 equivalence classes. This setting provides the highest
2274 degree of table compression. You can trade off
2275 faster-executing scanners at the cost of larger tables
2276 with the following generally being true:
2278 slowest & smallest
2279 -Cem
2280 -Cm
2281 -Ce
2282 -C
2283 -C{f,F}e
2284 -C{f,F}
2285 -C{f,F}a
2286 fastest & largest
2288 Note that scanners with the smallest tables are usually
2289 generated and compiled the quickest, so during develop-
2290 ment you will usually want to use the default, maximal
2291 compression.
2293 -Cfe is often a good compromise between speed and size
2294 for production scanners.
2296 -ooutput
2297 directs flex to write the scanner to the file output
2298 instead of lex.yy.c. If you combine -o with the -t
2299 option, then the scanner is written to stdout but its
2300 #line directives (see the -L option above) refer to the
2301 file output.
2303 -Pprefix
2307 Version 2.5 Last change: April 1995 35
2314 FLEX(1) USER COMMANDS FLEX(1)
2318 changes the default yy prefix used by flex for all
2319 globally-visible variable and function names to instead
2320 be prefix. For example, -Pfoo changes the name of
2321 yytext to footext. It also changes the name of the
2322 default output file from lex.yy.c to lex.foo.c. Here
2323 are all of the names affected:
2325 yy_create_buffer
2326 yy_delete_buffer
2327 yy_flex_debug
2328 yy_init_buffer
2329 yy_flush_buffer
2330 yy_load_buffer_state
2331 yy_switch_to_buffer
2332 yyin
2333 yyleng
2334 yylex
2335 yylineno
2336 yyout
2337 yyrestart
2338 yytext
2339 yywrap
2341 (If you are using a C++ scanner, then only yywrap and
2342 yyFlexLexer are affected.) Within your scanner itself,
2343 you can still refer to the global variables and func-
2344 tions using either version of their name; but exter-
2345 nally, they have the modified name.
2347 This option lets you easily link together multiple flex
2348 programs into the same executable. Note, though, that
2349 using this option also renames yywrap(), so you now
2350 must either provide your own (appropriately-named) ver-
2351 sion of the routine for your scanner, or use %option
2352 noyywrap, as linking with -lfl no longer provides one
2353 for you by default.
2355 -Sskeleton_file
2356 overrides the default skeleton file from which flex
2357 constructs its scanners. You'll never need this option
2358 unless you are doing flex maintenance or development.
2360 flex also provides a mechanism for controlling options
2361 within the scanner specification itself, rather than from
2362 the flex command-line. This is done by including %option
2363 directives in the first section of the scanner specifica-
2364 tion. You can specify multiple options with a single
2365 %option directive, and multiple directives in the first sec-
2366 tion of your flex input file.
2368 Most options are given simply as names, optionally preceded
2369 by the word "no" (with no intervening whitespace) to negate
2373 Version 2.5 Last change: April 1995 36
2380 FLEX(1) USER COMMANDS FLEX(1)
2384 their meaning. A number are equivalent to flex flags or
2385 their negation:
2387 7bit -7 option
2388 8bit -8 option
2389 align -Ca option
2390 backup -b option
2391 batch -B option
2392 c++ -+ option
2394 caseful or
2395 case-sensitive opposite of -i (default)
2397 case-insensitive or
2398 caseless -i option
2400 debug -d option
2401 default opposite of -s option
2402 ecs -Ce option
2403 fast -F option
2404 full -f option
2405 interactive -I option
2406 lex-compat -l option
2407 meta-ecs -Cm option
2408 perf-report -p option
2409 read -Cr option
2410 stdout -t option
2411 verbose -v option
2412 warn opposite of -w option
2413 (use "%option nowarn" for -w)
2415 array equivalent to "%array"
2416 pointer equivalent to "%pointer" (default)
2418 Some %option's provide features otherwise not available:
2420 always-interactive
2421 instructs flex to generate a scanner which always con-
2422 siders its input "interactive". Normally, on each new
2423 input file the scanner calls isatty() in an attempt to
2424 determine whether the scanner's input source is
2425 interactive and thus should be read a character at a
2426 time. When this option is used, however, then no such
2427 call is made.
2429 main directs flex to provide a default main() program for
2430 the scanner, which simply calls yylex(). This option
2431 implies noyywrap (see below).
2433 never-interactive
2434 instructs flex to generate a scanner which never con-
2435 siders its input "interactive" (again, no call made to
2439 Version 2.5 Last change: April 1995 37
2446 FLEX(1) USER COMMANDS FLEX(1)
2450 isatty()). This is the opposite of always-interactive.
2452 stack
2453 enables the use of start condition stacks (see Start
2454 Conditions above).
2456 stdinit
2457 if set (i.e., %option stdinit) initializes yyin and
2458 yyout to stdin and stdout, instead of the default of
2459 nil. Some existing lex programs depend on this
2460 behavior, even though it is not compliant with ANSI C,
2461 which does not require stdin and stdout to be compile-
2462 time constant.
2464 yylineno
2465 directs flex to generate a scanner that maintains the
2466 number of the current line read from its input in the
2467 global variable yylineno. This option is implied by
2468 %option lex-compat.
2470 yywrap
2471 if unset (i.e., %option noyywrap), makes the scanner
2472 not call yywrap() upon an end-of-file, but simply
2473 assume that there are no more files to scan (until the
2474 user points yyin at a new file and calls yylex()
2475 again).
2477 flex scans your rule actions to determine whether you use
2478 the REJECT or yymore() features. The reject and yymore
2479 options are available to override its decision as to whether
2480 you use the options, either by setting them (e.g., %option
2481 reject) to indicate the feature is indeed used, or unsetting
2482 them to indicate it actually is not used (e.g., %option
2483 noyymore).
2485 Three options take string-delimited values, offset with '=':
2487 %option outfile="ABC"
2489 is equivalent to -oABC, and
2491 %option prefix="XYZ"
2493 is equivalent to -PXYZ. Finally,
2495 %option yyclass="foo"
2497 only applies when generating a C++ scanner ( -+ option). It
2498 informs flex that you have derived foo as a subclass of
2499 yyFlexLexer, so flex will place your actions in the member
2500 function foo::yylex() instead of yyFlexLexer::yylex(). It
2501 also generates a yyFlexLexer::yylex() member function that
2505 Version 2.5 Last change: April 1995 38
2512 FLEX(1) USER COMMANDS FLEX(1)
2516 emits a run-time error (by invoking
2517 yyFlexLexer::LexerError()) if called. See Generating C++
2518 Scanners, below, for additional information.
2520 A number of options are available for lint purists who want
2521 to suppress the appearance of unneeded routines in the gen-
2522 erated scanner. Each of the following, if unset (e.g.,
2523 %option nounput ), results in the corresponding routine not
2524 appearing in the generated scanner:
2526 input, unput
2527 yy_push_state, yy_pop_state, yy_top_state
2528 yy_scan_buffer, yy_scan_bytes, yy_scan_string
2530 (though yy_push_state() and friends won't appear anyway
2531 unless you use %option stack).
2533 PERFORMANCE CONSIDERATIONS
2534 The main design goal of flex is that it generate high-
2535 performance scanners. It has been optimized for dealing
2536 well with large sets of rules. Aside from the effects on
2537 scanner speed of the table compression -C options outlined
2538 above, there are a number of options/actions which degrade
2539 performance. These are, from most expensive to least:
2541 REJECT
2542 %option yylineno
2543 arbitrary trailing context
2545 pattern sets that require backing up
2546 %array
2547 %option interactive
2548 %option always-interactive
2550 '^' beginning-of-line operator
2551 yymore()
2553 with the first three all being quite expensive and the last
2554 two being quite cheap. Note also that unput() is imple-
2555 mented as a routine call that potentially does quite a bit
2556 of work, while yyless() is a quite-cheap macro; so if just
2557 putting back some excess text you scanned, use yyless().
2559 REJECT should be avoided at all costs when performance is
2560 important. It is a particularly expensive option.
2562 Getting rid of backing up is messy and often may be an enor-
2563 mous amount of work for a complicated scanner. In princi-
2564 pal, one begins by using the -b flag to generate a
2565 lex.backup file. For example, on the input
2567 %%
2571 Version 2.5 Last change: April 1995 39
2578 FLEX(1) USER COMMANDS FLEX(1)
2582 foo return TOK_KEYWORD;
2583 foobar return TOK_KEYWORD;
2585 the file looks like:
2587 State #6 is non-accepting -
2588 associated rule line numbers:
2589 2 3
2590 out-transitions: [ o ]
2591 jam-transitions: EOF [ \001-n p-\177 ]
2593 State #8 is non-accepting -
2594 associated rule line numbers:
2595 3
2596 out-transitions: [ a ]
2597 jam-transitions: EOF [ \001-` b-\177 ]
2599 State #9 is non-accepting -
2600 associated rule line numbers:
2601 3
2602 out-transitions: [ r ]
2603 jam-transitions: EOF [ \001-q s-\177 ]
2605 Compressed tables always back up.
2607 The first few lines tell us that there's a scanner state in
2608 which it can make a transition on an 'o' but not on any
2609 other character, and that in that state the currently
2610 scanned text does not match any rule. The state occurs when
2611 trying to match the rules found at lines 2 and 3 in the
2612 input file. If the scanner is in that state and then reads
2613 something other than an 'o', it will have to back up to find
2614 a rule which is matched. With a bit of headscratching one
2615 can see that this must be the state it's in when it has seen
2616 "fo". When this has happened, if anything other than
2617 another 'o' is seen, the scanner will have to back up to
2618 simply match the 'f' (by the default rule).
2620 The comment regarding State #8 indicates there's a problem
2621 when "foob" has been scanned. Indeed, on any character
2622 other than an 'a', the scanner will have to back up to
2623 accept "foo". Similarly, the comment for State #9 concerns
2624 when "fooba" has been scanned and an 'r' does not follow.
2626 The final comment reminds us that there's no point going to
2627 all the trouble of removing backing up from the rules unless
2628 we're using -Cf or -CF, since there's no performance gain
2629 doing so with compressed scanners.
2631 The way to remove the backing up is to add "error" rules:
2633 %%
2637 Version 2.5 Last change: April 1995 40
2644 FLEX(1) USER COMMANDS FLEX(1)
2648 foo return TOK_KEYWORD;
2649 foobar return TOK_KEYWORD;
2651 fooba |
2652 foob |
2653 fo {
2654 /* false alarm, not really a keyword */
2655 return TOK_ID;
2656 }
2659 Eliminating backing up among a list of keywords can also be
2660 done using a "catch-all" rule:
2662 %%
2663 foo return TOK_KEYWORD;
2664 foobar return TOK_KEYWORD;
2666 [a-z]+ return TOK_ID;
2668 This is usually the best solution when appropriate.
2670 Backing up messages tend to cascade. With a complicated set
2671 of rules it's not uncommon to get hundreds of messages. If
2672 one can decipher them, though, it often only takes a dozen
2673 or so rules to eliminate the backing up (though it's easy to
2674 make a mistake and have an error rule accidentally match a
2675 valid token. A possible future flex feature will be to
2676 automatically add rules to eliminate backing up).
2678 It's important to keep in mind that you gain the benefits of
2679 eliminating backing up only if you eliminate every instance
2680 of backing up. Leaving just one means you gain nothing.
2682 Variable trailing context (where both the leading and trail-
2683 ing parts do not have a fixed length) entails almost the
2684 same performance loss as REJECT (i.e., substantial). So
2685 when possible a rule like:
2687 %%
2688 mouse|rat/(cat|dog) run();
2690 is better written:
2692 %%
2693 mouse/cat|dog run();
2694 rat/cat|dog run();
2696 or as
2698 %%
2699 mouse|rat/cat run();
2703 Version 2.5 Last change: April 1995 41
2710 FLEX(1) USER COMMANDS FLEX(1)
2714 mouse|rat/dog run();
2716 Note that here the special '|' action does not provide any
2717 savings, and can even make things worse (see Deficiencies /
2718 Bugs below).
2720 Another area where the user can increase a scanner's perfor-
2721 mance (and one that's easier to implement) arises from the
2722 fact that the longer the tokens matched, the faster the
2723 scanner will run. This is because with long tokens the pro-
2724 cessing of most input characters takes place in the (short)
2725 inner scanning loop, and does not often have to go through
2726 the additional work of setting up the scanning environment
2727 (e.g., yytext) for the action. Recall the scanner for C
2728 comments:
2730 %x comment
2731 %%
2732 int line_num = 1;
2734 "/*" BEGIN(comment);
2736 <comment>[^*\n]*
2737 <comment>"*"+[^*/\n]*
2738 <comment>\n ++line_num;
2739 <comment>"*"+"/" BEGIN(INITIAL);
2741 This could be sped up by writing it as:
2743 %x comment
2744 %%
2745 int line_num = 1;
2747 "/*" BEGIN(comment);
2749 <comment>[^*\n]*
2750 <comment>[^*\n]*\n ++line_num;
2751 <comment>"*"+[^*/\n]*
2752 <comment>"*"+[^*/\n]*\n ++line_num;
2753 <comment>"*"+"/" BEGIN(INITIAL);
2755 Now instead of each newline requiring the processing of
2756 another action, recognizing the newlines is "distributed"
2757 over the other rules to keep the matched text as long as
2758 possible. Note that adding rules does not slow down the
2759 scanner! The speed of the scanner is independent of the
2760 number of rules or (modulo the considerations given at the
2761 beginning of this section) how complicated the rules are
2762 with regard to operators such as '*' and '|'.
2764 A final example in speeding up a scanner: suppose you want
2765 to scan through a file containing identifiers and keywords,
2769 Version 2.5 Last change: April 1995 42
2776 FLEX(1) USER COMMANDS FLEX(1)
2780 one per line and with no other extraneous characters, and
2781 recognize all the keywords. A natural first approach is:
2783 %%
2784 asm |
2785 auto |
2786 break |
2787 ... etc ...
2788 volatile |
2789 while /* it's a keyword */
2791 .|\n /* it's not a keyword */
2793 To eliminate the back-tracking, introduce a catch-all rule:
2795 %%
2796 asm |
2797 auto |
2798 break |
2799 ... etc ...
2800 volatile |
2801 while /* it's a keyword */
2803 [a-z]+ |
2804 .|\n /* it's not a keyword */
2806 Now, if it's guaranteed that there's exactly one word per
2807 line, then we can reduce the total number of matches by a
2808 half by merging in the recognition of newlines with that of
2809 the other tokens:
2811 %%
2812 asm\n |
2813 auto\n |
2814 break\n |
2815 ... etc ...
2816 volatile\n |
2817 while\n /* it's a keyword */
2819 [a-z]+\n |
2820 .|\n /* it's not a keyword */
2822 One has to be careful here, as we have now reintroduced
2823 backing up into the scanner. In particular, while we know
2824 that there will never be any characters in the input stream
2825 other than letters or newlines, flex can't figure this out,
2826 and it will plan for possibly needing to back up when it has
2827 scanned a token like "auto" and then the next character is
2828 something other than a newline or a letter. Previously it
2829 would then just match the "auto" rule and be done, but now
2830 it has no "auto" rule, only a "auto\n" rule. To eliminate
2831 the possibility of backing up, we could either duplicate all
2835 Version 2.5 Last change: April 1995 43
2842 FLEX(1) USER COMMANDS FLEX(1)
2846 rules but without final newlines, or, since we never expect
2847 to encounter such an input and therefore don't how it's
2848 classified, we can introduce one more catch-all rule, this
2849 one which doesn't include a newline:
2851 %%
2852 asm\n |
2853 auto\n |
2854 break\n |
2855 ... etc ...
2856 volatile\n |
2857 while\n /* it's a keyword */
2859 [a-z]+\n |
2860 [a-z]+ |
2861 .|\n /* it's not a keyword */
2863 Compiled with -Cf, this is about as fast as one can get a
2864 flex scanner to go for this particular problem.
2866 A final note: flex is slow when matching NUL's, particularly
2867 when a token contains multiple NUL's. It's best to write
2868 rules which match short amounts of text if it's anticipated
2869 that the text will often include NUL's.
2871 Another final note regarding performance: as mentioned above
2872 in the section How the Input is Matched, dynamically resiz-
2873 ing yytext to accommodate huge tokens is a slow process
2874 because it presently requires that the (huge) token be res-
2875 canned from the beginning. Thus if performance is vital,
2876 you should attempt to match "large" quantities of text but
2877 not "huge" quantities, where the cutoff between the two is
2878 at about 8K characters/token.
2880 GENERATING C++ SCANNERS
2881 flex provides two different ways to generate scanners for
2882 use with C++. The first way is to simply compile a scanner
2883 generated by flex using a C++ compiler instead of a C com-
2884 piler. You should not encounter any compilations errors
2885 (please report any you find to the email address given in
2886 the Author section below). You can then use C++ code in
2887 your rule actions instead of C code. Note that the default
2888 input source for your scanner remains yyin, and default
2889 echoing is still done to yyout. Both of these remain FILE *
2890 variables and not C++ streams.
2892 You can also use flex to generate a C++ scanner class, using
2893 the -+ option (or, equivalently, %option c++), which is
2894 automatically specified if the name of the flex executable
2895 ends in a '+', such as flex++. When using this option, flex
2896 defaults to generating the scanner to the file lex.yy.cc
2897 instead of lex.yy.c. The generated scanner includes the
2901 Version 2.5 Last change: April 1995 44
2908 FLEX(1) USER COMMANDS FLEX(1)
2912 header file FlexLexer.h, which defines the interface to two
2913 C++ classes.
2915 The first class, FlexLexer, provides an abstract base class
2916 defining the general scanner class interface. It provides
2917 the following member functions:
2919 const char* YYText()
2920 returns the text of the most recently matched token,
2921 the equivalent of yytext.
2923 int YYLeng()
2924 returns the length of the most recently matched token,
2925 the equivalent of yyleng.
2927 int lineno() const
2928 returns the current input line number (see %option
2929 yylineno), or 1 if %option yylineno was not used.
2931 void set_debug( int flag )
2932 sets the debugging flag for the scanner, equivalent to
2933 assigning to yy_flex_debug (see the Options section
2934 above). Note that you must build the scanner using
2935 %option debug to include debugging information in it.
2937 int debug() const
2938 returns the current setting of the debugging flag.
2940 Also provided are member functions equivalent to
2941 yy_switch_to_buffer(), yy_create_buffer() (though the first
2942 argument is an istream* object pointer and not a FILE*),
2943 yy_flush_buffer(), yy_delete_buffer(), and yyrestart()
2944 (again, the first argument is a istream* object pointer).
2946 The second class defined in FlexLexer.h is yyFlexLexer,
2947 which is derived from FlexLexer. It defines the following
2948 additional member functions:
2950 yyFlexLexer( istream* arg_yyin = 0, ostream* arg_yyout = 0 )
2951 constructs a yyFlexLexer object using the given streams
2952 for input and output. If not specified, the streams
2953 default to cin and cout, respectively.
2955 virtual int yylex()
2956 performs the same role is yylex() does for ordinary
2957 flex scanners: it scans the input stream, consuming
2958 tokens, until a rule's action returns a value. If you
2959 derive a subclass S from yyFlexLexer and want to access
2960 the member functions and variables of S inside yylex(),
2961 then you need to use %option yyclass="S" to inform flex
2962 that you will be using that subclass instead of yyFlex-
2963 Lexer. In this case, rather than generating
2967 Version 2.5 Last change: April 1995 45
2974 FLEX(1) USER COMMANDS FLEX(1)
2978 yyFlexLexer::yylex(), flex generates S::yylex() (and
2979 also generates a dummy yyFlexLexer::yylex() that calls
2980 yyFlexLexer::LexerError() if called).
2982 virtual void switch_streams(istream* new_in = 0,
2983 ostream* new_out = 0) reassigns yyin to new_in (if
2984 non-nil) and yyout to new_out (ditto), deleting the
2985 previous input buffer if yyin is reassigned.
2987 int yylex( istream* new_in, ostream* new_out = 0 )
2988 first switches the input streams via switch_streams(
2989 new_in, new_out ) and then returns the value of
2990 yylex().
2992 In addition, yyFlexLexer defines the following protected
2993 virtual functions which you can redefine in derived classes
2994 to tailor the scanner:
2996 virtual int LexerInput( char* buf, int max_size )
2997 reads up to max_size characters into buf and returns
2998 the number of characters read. To indicate end-of-
2999 input, return 0 characters. Note that "interactive"
3000 scanners (see the -B and -I flags) define the macro
3001 YY_INTERACTIVE. If you redefine LexerInput() and need
3002 to take different actions depending on whether or not
3003 the scanner might be scanning an interactive input
3004 source, you can test for the presence of this name via
3005 #ifdef.
3007 virtual void LexerOutput( const char* buf, int size )
3008 writes out size characters from the buffer buf, which,
3009 while NUL-terminated, may also contain "internal" NUL's
3010 if the scanner's rules can match text with NUL's in
3011 them.
3013 virtual void LexerError( const char* msg )
3014 reports a fatal error message. The default version of
3015 this function writes the message to the stream cerr and
3016 exits.
3018 Note that a yyFlexLexer object contains its entire scanning
3019 state. Thus you can use such objects to create reentrant
3020 scanners. You can instantiate multiple instances of the
3021 same yyFlexLexer class, and you can also combine multiple
3022 C++ scanner classes together in the same program using the
3023 -P option discussed above.
3025 Finally, note that the %array feature is not available to
3026 C++ scanner classes; you must use %pointer (the default).
3028 Here is an example of a simple C++ scanner:
3033 Version 2.5 Last change: April 1995 46
3040 FLEX(1) USER COMMANDS FLEX(1)
3044 // An example of using the flex C++ scanner class.
3046 %{
3047 int mylineno = 0;
3048 %}
3050 string \"[^\n"]+\"
3052 ws [ \t]+
3054 alpha [A-Za-z]
3055 dig [0-9]
3056 name ({alpha}|{dig}|\$)({alpha}|{dig}|[_.\-/$])*
3057 num1 [-+]?{dig}+\.?([eE][-+]?{dig}+)?
3058 num2 [-+]?{dig}*\.{dig}+([eE][-+]?{dig}+)?
3059 number {num1}|{num2}
3061 %%
3063 {ws} /* skip blanks and tabs */
3065 "/*" {
3066 int c;
3068 while((c = yyinput()) != 0)
3069 {
3070 if(c == '\n')
3071 ++mylineno;
3073 else if(c == '*')
3074 {
3075 if((c = yyinput()) == '/')
3076 break;
3077 else
3078 unput(c);
3079 }
3080 }
3081 }
3083 {number} cout << "number " << YYText() << '\n';
3085 \n mylineno++;
3087 {name} cout << "name " << YYText() << '\n';
3089 {string} cout << "string " << YYText() << '\n';
3091 %%
3093 int main( int /* argc */, char** /* argv */ )
3094 {
3095 FlexLexer* lexer = new yyFlexLexer;
3099 Version 2.5 Last change: April 1995 47
3106 FLEX(1) USER COMMANDS FLEX(1)
3110 while(lexer->yylex() != 0)
3111 ;
3112 return 0;
3113 }
3114 If you want to create multiple (different) lexer classes,
3115 you use the -P flag (or the prefix= option) to rename each
3116 yyFlexLexer to some other xxFlexLexer. You then can include
3117 <FlexLexer.h> in your other sources once per lexer class,
3118 first renaming yyFlexLexer as follows:
3120 #undef yyFlexLexer
3121 #define yyFlexLexer xxFlexLexer
3122 #include <FlexLexer.h>
3124 #undef yyFlexLexer
3125 #define yyFlexLexer zzFlexLexer
3126 #include <FlexLexer.h>
3128 if, for example, you used %option prefix="xx" for one of
3129 your scanners and %option prefix="zz" for the other.
3131 IMPORTANT: the present form of the scanning class is experi-
3132 mental and may change considerably between major releases.
3134 INCOMPATIBILITIES WITH LEX AND POSIX
3135 flex is a rewrite of the AT&T Unix lex tool (the two imple-
3136 mentations do not share any code, though), with some exten-
3137 sions and incompatibilities, both of which are of concern to
3138 those who wish to write scanners acceptable to either imple-
3139 mentation. Flex is fully compliant with the POSIX lex
3140 specification, except that when using %pointer (the
3141 default), a call to unput() destroys the contents of yytext,
3142 which is counter to the POSIX specification.
3144 In this section we discuss all of the known areas of incom-
3145 patibility between flex, AT&T lex, and the POSIX specifica-
3146 tion.
3148 flex's -l option turns on maximum compatibility with the
3149 original AT&T lex implementation, at the cost of a major
3150 loss in the generated scanner's performance. We note below
3151 which incompatibilities can be overcome using the -l option.
3153 flex is fully compatible with lex with the following excep-
3154 tions:
3156 - The undocumented lex scanner internal variable yylineno
3157 is not supported unless -l or %option yylineno is used.
3159 yylineno should be maintained on a per-buffer basis,
3160 rather than a per-scanner (single global variable)
3161 basis.
3165 Version 2.5 Last change: April 1995 48
3172 FLEX(1) USER COMMANDS FLEX(1)
3176 yylineno is not part of the POSIX specification.
3178 - The input() routine is not redefinable, though it may
3179 be called to read characters following whatever has
3180 been matched by a rule. If input() encounters an end-
3181 of-file the normal yywrap() processing is done. A
3182 ``real'' end-of-file is returned by input() as EOF.
3184 Input is instead controlled by defining the YY_INPUT
3185 macro.
3187 The flex restriction that input() cannot be redefined
3188 is in accordance with the POSIX specification, which
3189 simply does not specify any way of controlling the
3190 scanner's input other than by making an initial assign-
3191 ment to yyin.
3193 - The unput() routine is not redefinable. This restric-
3194 tion is in accordance with POSIX.
3196 - flex scanners are not as reentrant as lex scanners. In
3197 particular, if you have an interactive scanner and an
3198 interrupt handler which long-jumps out of the scanner,
3199 and the scanner is subsequently called again, you may
3200 get the following message:
3202 fatal flex scanner internal error--end of buffer missed
3204 To reenter the scanner, first use
3206 yyrestart( yyin );
3208 Note that this call will throw away any buffered input;
3209 usually this isn't a problem with an interactive
3210 scanner.
3212 Also note that flex C++ scanner classes are reentrant,
3213 so if using C++ is an option for you, you should use
3214 them instead. See "Generating C++ Scanners" above for
3215 details.
3217 - output() is not supported. Output from the ECHO macro
3218 is done to the file-pointer yyout (default stdout).
3220 output() is not part of the POSIX specification.
3222 - lex does not support exclusive start conditions (%x),
3223 though they are in the POSIX specification.
3225 - When definitions are expanded, flex encloses them in
3226 parentheses. With lex, the following:
3231 Version 2.5 Last change: April 1995 49
3238 FLEX(1) USER COMMANDS FLEX(1)
3242 NAME [A-Z][A-Z0-9]*
3243 %%
3244 foo{NAME}? printf( "Found it\n" );
3245 %%
3247 will not match the string "foo" because when the macro
3248 is expanded the rule is equivalent to "foo[A-Z][A-Z0-
3249 9]*?" and the precedence is such that the '?' is asso-
3250 ciated with "[A-Z0-9]*". With flex, the rule will be
3251 expanded to "foo([A-Z][A-Z0-9]*)?" and so the string
3252 "foo" will match.
3254 Note that if the definition begins with ^ or ends with
3255 $ then it is not expanded with parentheses, to allow
3256 these operators to appear in definitions without losing
3257 their special meanings. But the <s>, /, and <<EOF>>
3258 operators cannot be used in a flex definition.
3260 Using -l results in the lex behavior of no parentheses
3261 around the definition.
3263 The POSIX specification is that the definition be
3264 enclosed in parentheses.
3266 - Some implementations of lex allow a rule's action to
3267 begin on a separate line, if the rule's pattern has
3268 trailing whitespace:
3270 %%
3271 foo|bar<space here>
3272 { foobar_action(); }
3274 flex does not support this feature.
3276 - The lex %r (generate a Ratfor scanner) option is not
3277 supported. It is not part of the POSIX specification.
3279 - After a call to unput(), yytext is undefined until the
3280 next token is matched, unless the scanner was built
3281 using %array. This is not the case with lex or the
3282 POSIX specification. The -l option does away with this
3283 incompatibility.
3285 - The precedence of the {} (numeric range) operator is
3286 different. lex interprets "abc{1,3}" as "match one,
3287 two, or three occurrences of 'abc'", whereas flex
3288 interprets it as "match 'ab' followed by one, two, or
3289 three occurrences of 'c'". The latter is in agreement
3290 with the POSIX specification.
3292 - The precedence of the ^ operator is different. lex
3293 interprets "^foo|bar" as "match either 'foo' at the
3297 Version 2.5 Last change: April 1995 50
3304 FLEX(1) USER COMMANDS FLEX(1)
3308 beginning of a line, or 'bar' anywhere", whereas flex
3309 interprets it as "match either 'foo' or 'bar' if they
3310 come at the beginning of a line". The latter is in
3311 agreement with the POSIX specification.
3313 - The special table-size declarations such as %a sup-
3314 ported by lex are not required by flex scanners; flex
3315 ignores them.
3317 - The name FLEX_SCANNER is #define'd so scanners may be
3318 written for use with either flex or lex. Scanners also
3319 include YY_FLEX_MAJOR_VERSION and YY_FLEX_MINOR_VERSION
3320 indicating which version of flex generated the scanner
3321 (for example, for the 2.5 release, these defines would
3322 be 2 and 5 respectively).
3324 The following flex features are not included in lex or the
3325 POSIX specification:
3327 C++ scanners
3328 %option
3329 start condition scopes
3330 start condition stacks
3331 interactive/non-interactive scanners
3332 yy_scan_string() and friends
3333 yyterminate()
3334 yy_set_interactive()
3335 yy_set_bol()
3336 YY_AT_BOL()
3337 <<EOF>>
3338 <*>
3339 YY_DECL
3340 YY_START
3341 YY_USER_ACTION
3342 YY_USER_INIT
3343 #line directives
3344 %{}'s around actions
3345 multiple actions on a line
3347 plus almost all of the flex flags. The last feature in the
3348 list refers to the fact that with flex you can put multiple
3349 actions on the same line, separated with semi-colons, while
3350 with lex, the following
3352 foo handle_foo(); ++num_foos_seen;
3354 is (rather surprisingly) truncated to
3356 foo handle_foo();
3358 flex does not truncate the action. Actions that are not
3359 enclosed in braces are simply terminated at the end of the
3363 Version 2.5 Last change: April 1995 51
3370 FLEX(1) USER COMMANDS FLEX(1)
3374 line.
3376 DIAGNOSTICS
3377 warning, rule cannot be matched indicates that the given
3378 rule cannot be matched because it follows other rules that
3379 will always match the same text as it. For example, in the
3380 following "foo" cannot be matched because it comes after an
3381 identifier "catch-all" rule:
3383 [a-z]+ got_identifier();
3384 foo got_foo();
3386 Using REJECT in a scanner suppresses this warning.
3388 warning, -s option given but default rule can be matched
3389 means that it is possible (perhaps only in a particular
3390 start condition) that the default rule (match any single
3391 character) is the only one that will match a particular
3392 input. Since -s was given, presumably this is not intended.
3394 reject_used_but_not_detected undefined or
3395 yymore_used_but_not_detected undefined - These errors can
3396 occur at compile time. They indicate that the scanner uses
3397 REJECT or yymore() but that flex failed to notice the fact,
3398 meaning that flex scanned the first two sections looking for
3399 occurrences of these actions and failed to find any, but
3400 somehow you snuck some in (via a #include file, for exam-
3401 ple). Use %option reject or %option yymore to indicate to
3402 flex that you really do use these features.
3404 flex scanner jammed - a scanner compiled with -s has encoun-
3405 tered an input string which wasn't matched by any of its
3406 rules. This error can also occur due to internal problems.
3408 token too large, exceeds YYLMAX - your scanner uses %array
3409 and one of its rules matched a string longer than the YYLMAX
3410 constant (8K bytes by default). You can increase the value
3411 by #define'ing YYLMAX in the definitions section of your
3412 flex input.
3414 scanner requires -8 flag to use the character 'x' - Your
3415 scanner specification includes recognizing the 8-bit charac-
3416 ter 'x' and you did not specify the -8 flag, and your
3417 scanner defaulted to 7-bit because you used the -Cf or -CF
3418 table compression options. See the discussion of the -7
3419 flag for details.
3421 flex scanner push-back overflow - you used unput() to push
3422 back so much text that the scanner's buffer could not hold
3423 both the pushed-back text and the current token in yytext.
3424 Ideally the scanner should dynamically resize the buffer in
3425 this case, but at present it does not.
3429 Version 2.5 Last change: April 1995 52
3436 FLEX(1) USER COMMANDS FLEX(1)
3440 input buffer overflow, can't enlarge buffer because scanner
3441 uses REJECT - the scanner was working on matching an
3442 extremely large token and needed to expand the input buffer.
3443 This doesn't work with scanners that use REJECT.
3445 fatal flex scanner internal error--end of buffer missed -
3446 This can occur in an scanner which is reentered after a
3447 long-jump has jumped out (or over) the scanner's activation
3448 frame. Before reentering the scanner, use:
3450 yyrestart( yyin );
3452 or, as noted above, switch to using the C++ scanner class.
3454 too many start conditions in <> you listed more start condi-
3455 tions in a <> construct than exist (so you must have listed
3456 at least one of them twice).
3458 FILES
3459 -lfl library with which scanners must be linked.
3461 lex.yy.c
3462 generated scanner (called lexyy.c on some systems).
3464 lex.yy.cc
3465 generated C++ scanner class, when using -+.
3467 <FlexLexer.h>
3468 header file defining the C++ scanner base class, Flex-
3469 Lexer, and its derived class, yyFlexLexer.
3471 flex.skl
3472 skeleton scanner. This file is only used when building
3473 flex, not when flex executes.
3475 lex.backup
3476 backing-up information for -b flag (called lex.bck on
3477 some systems).
3479 DEFICIENCIES / BUGS
3480 Some trailing context patterns cannot be properly matched
3481 and generate warning messages ("dangerous trailing con-
3482 text"). These are patterns where the ending of the first
3483 part of the rule matches the beginning of the second part,
3484 such as "zx*/xy*", where the 'x*' matches the 'x' at the
3485 beginning of the trailing context. (Note that the POSIX
3486 draft states that the text matched by such patterns is unde-
3487 fined.)
3489 For some trailing context rules, parts which are actually
3490 fixed-length are not recognized as such, leading to the
3491 abovementioned performance loss. In particular, parts using
3495 Version 2.5 Last change: April 1995 53
3502 FLEX(1) USER COMMANDS FLEX(1)
3506 '|' or {n} (such as "foo{3}") are always considered
3507 variable-length.
3509 Combining trailing context with the special '|' action can
3510 result in fixed trailing context being turned into the more
3511 expensive variable trailing context. For example, in the
3512 following:
3514 %%
3515 abc |
3516 xyz/def
3519 Use of unput() invalidates yytext and yyleng, unless the
3520 %array directive or the -l option has been used.
3522 Pattern-matching of NUL's is substantially slower than
3523 matching other characters.
3525 Dynamic resizing of the input buffer is slow, as it entails
3526 rescanning all the text matched so far by the current (gen-
3527 erally huge) token.
3529 Due to both buffering of input and read-ahead, you cannot
3530 intermix calls to <stdio.h> routines, such as, for example,
3531 getchar(), with flex rules and expect it to work. Call
3532 input() instead.
3534 The total table entries listed by the -v flag excludes the
3535 number of table entries needed to determine what rule has
3536 been matched. The number of entries is equal to the number
3537 of DFA states if the scanner does not use REJECT, and some-
3538 what greater than the number of states if it does.
3540 REJECT cannot be used with the -f or -F options.
3542 The flex internal algorithms need documentation.
3544 SEE ALSO
3545 lex(1), yacc(1), sed(1), awk(1).
3547 John Levine, Tony Mason, and Doug Brown, Lex & Yacc,
3548 O'Reilly and Associates. Be sure to get the 2nd edition.
3550 M. E. Lesk and E. Schmidt, LEX - Lexical Analyzer Generator
3552 Alfred Aho, Ravi Sethi and Jeffrey Ullman, Compilers: Prin-
3553 ciples, Techniques and Tools, Addison-Wesley (1986).
3554 Describes the pattern-matching techniques used by flex
3555 (deterministic finite automata).
3561 Version 2.5 Last change: April 1995 54
3568 FLEX(1) USER COMMANDS FLEX(1)
3572 AUTHOR
3573 Vern Paxson, with the help of many ideas and much inspira-
3574 tion from Van Jacobson. Original version by Jef Poskanzer.
3575 The fast table representation is a partial implementation of
3576 a design done by Van Jacobson. The implementation was done
3577 by Kevin Gong and Vern Paxson.
3579 Thanks to the many flex beta-testers, feedbackers, and con-
3580 tributors, especially Francois Pinard, Casey Leedom, Robert
3581 Abramovitz, Stan Adermann, Terry Allen, David Barker-
3582 Plummer, John Basrai, Neal Becker, Nelson H.F. Beebe,
3583 benson@odi.com, Karl Berry, Peter A. Bigot, Simon Blanchard,
3584 Keith Bostic, Frederic Brehm, Ian Brockbank, Kin Cho, Nick
3585 Christopher, Brian Clapper, J.T. Conklin, Jason Coughlin,
3586 Bill Cox, Nick Cropper, Dave Curtis, Scott David Daniels,
3587 Chris G. Demetriou, Theo Deraadt, Mike Donahue, Chuck
3588 Doucette, Tom Epperly, Leo Eskin, Chris Faylor, Chris
3589 Flatters, Jon Forrest, Jeffrey Friedl, Joe Gayda, Kaveh R.
3590 Ghazi, Wolfgang Glunz, Eric Goldman, Christopher M. Gould,
3591 Ulrich Grepel, Peer Griebel, Jan Hajic, Charles Hemphill,
3592 NORO Hideo, Jarkko Hietaniemi, Scott Hofmann, Jeff Honig,
3593 Dana Hudes, Eric Hughes, John Interrante, Ceriel Jacobs,
3594 Michal Jaegermann, Sakari Jalovaara, Jeffrey R. Jones, Henry
3595 Juengst, Klaus Kaempf, Jonathan I. Kamens, Terrence O Kane,
3596 Amir Katz, ken@ken.hilco.com, Kevin B. Kenny, Steve Kirsch,
3597 Winfried Koenig, Marq Kole, Ronald Lamprecht, Greg Lee,
3598 Rohan Lenard, Craig Leres, John Levine, Steve Liddle, David
3599 Loffredo, Mike Long, Mohamed el Lozy, Brian Madsen, Malte,
3600 Joe Marshall, Bengt Martensson, Chris Metcalf, Luke Mewburn,
3601 Jim Meyering, R. Alexander Milowski, Erik Naggum, G.T.
3602 Nicol, Landon Noll, James Nordby, Marc Nozell, Richard
3603 Ohnemus, Karsten Pahnke, Sven Panne, Roland Pesch, Walter
3604 Pelissero, Gaumond Pierre, Esmond Pitt, Jef Poskanzer, Joe
3605 Rahmeh, Jarmo Raiha, Frederic Raimbault, Pat Rankin, Rick
3606 Richardson, Kevin Rodgers, Kai Uwe Rommel, Jim Roskind,
3607 Alberto Santini, Andreas Scherer, Darrell Schiebel, Raf
3608 Schietekat, Doug Schmidt, Philippe Schnoebelen, Andreas
3609 Schwab, Larry Schwimmer, Alex Siegel, Eckehard Stolz, Jan-
3610 Erik Strvmquist, Mike Stump, Paul Stuart, Dave Tallman, Ian
3611 Lance Taylor, Chris Thewalt, Richard M. Timoney, Jodi Tsai,
3612 Paul Tuinenga, Gary Weik, Frank Whaley, Gerhard Wilhelms,
3613 Kent Williams, Ken Yap, Ron Zellar, Nathan Zelle, David
3614 Zuhn, and those whose names have slipped my marginal mail-
3615 archiving skills but whose contributions are appreciated all
3616 the same.
3618 Thanks to Keith Bostic, Jon Forrest, Noah Friedman, John
3619 Gilmore, Craig Leres, John Levine, Bob Mulcahy, G.T. Nicol,
3620 Francois Pinard, Rich Salz, and Richard Stallman for help
3621 with various distribution headaches.
3627 Version 2.5 Last change: April 1995 55
3634 FLEX(1) USER COMMANDS FLEX(1)
3638 Thanks to Esmond Pitt and Earle Horton for 8-bit character
3639 support; to Benson Margulies and Fred Burke for C++ support;
3640 to Kent Williams and Tom Epperly for C++ class support; to
3641 Ove Ewerlid for support of NUL's; and to Eric Hughes for
3642 support of multiple buffers.
3644 This work was primarily done when I was with the Real Time
3645 Systems Group at the Lawrence Berkeley Laboratory in Berke-
3646 ley, CA. Many thanks to all there for the support I
3647 received.
3649 Send comments to vern@ee.lbl.gov.
3693 Version 2.5 Last change: April 1995 56