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1 FLEX Flex - a fast lexical analyzer generator Syntax: FLEX [qualifiers] filespec[,...] Flex is a tool for generating scanners: programs which recognize lexical patterns in text. Flex reads the given input files, or SYS$INPUT if no file names are given, for a description of a scanner to generate. The description is in the form of pairs of regular expressions and C code, called rules. Flex generates as output a C source file, lexyy.c, which defines a routine yylex(). This file is compiled and linked with your program, which calls yylex(). When the executable is run, it analyzes its input for occurrences of the regular expressions. Whenever it finds one, it executes the corresponding C code. For full documentation, see flex_manual:flexdoc.man. (This manual describes command line options in their unix-style form. See the "Unix" topic for the VMS equivalents.) This information is based on flex version 2.3.6 2 Parameters filespec The input file contains flex source code from which a lexer C subroutine is generated. There is no default for the filetype, you must enter both the file name and type (e.g. foo.lex). Wildcards are not allowed. 2 Qualifiers /OUTPUT=filespec /OUTPUT=filespec /OUTPUT=LEXYY.C (D) The name of the output file that flex will create and write the lexer code to. You must specify both the file name and type in "filespec"; there is no default type. If /OUTPUT is not supplied, the filename lexyy.c is used by default. /BACKTRACK_REPORT /BACKTRACK_REPORT /NOBACKTRACK_REPORT (D) Generate backtracking information to the file lex.backtrack. This is a list of scanner states which require backtracking and the input characters on which they do so. By adding rules one can remove backtracking states. If all backtracking states are eliminated and /TABLES=FAST or /TABLES=FULL is used, the generated scanner will run faster. /BACKTRACK_REPORT is equivalent to -b in the unix version. /DEBUG /DEBUG /NODEBUG (D) Makes the generated scanner run in debug mode. When- ever a pattern is recognized and the global vatiable yy_flex_debug is non-zero (which is the default), the scanner will write to SYS$ERROR a line of the form: --accepting rule at line 53 ("the matched text") The line number refers to the location of the rule in the file defining the scanner (i.e., the file that was fed to flex). Messages are also generated when the scanner backtracks, accepts the default rule, reaches the end of its input buffer (or encounters a NUL; the two look the same as far as the scanner's concerned), or reaches an end-of-file. /DEBUG is equivalent to -d in the unix version. /CASE_SENSITIVE /CASE_SENSITIVE (D) /NOCASE_SENSITIVE Instructs flex to generate a case-insensitive scanner. The case of letters given in the flex input patterns will be ignored, and tokens in the input will be matched regardless of case. The matched text given in yytext will have the preserved case (i.e., it will not be folded). /CASE_SENSITIVE is equivalent to -i in the unix version. /STATISTICS=([PERFORMANCE],[SUMMARY]) /STATISTICS=([PERFORMANCE],[SUMMARY]) /NOSTATISTICS (D) If the keyword PERFORMANCE is specified, flex generates a performance report to SYS$ERROR. The report consists of comments regarding features of the flex input file which will cause a loss of performance in the resulting scanner. The keyword SUMMARY specifies that flex should write to SYS$ERROR a summary of statistics regarding the scanner it generates. You can specify either or both keywords. /STATISTICS=PERFORMANCE is equivalent to -p in the unix version. /STATISTICS=SUMMARY is equivalent to -v in the unix version. /ECHO /ECHO (D) /NOECHO /NOECHO causes the default rule (that unmatched scanner input is echoed to SYS$OUTPUT) to be suppressed. If the scanner encounters input that does not match any of its rules, it aborts with an error. /ECHO is equivalent to -s in the unix version. /INTERACTIVE /INTERACTIVE /NOINTERACTIVE (D) Instructs flex to generate an interactive scanner, that is, a scanner which stops immediately rather than look- ing ahead if it knows that the currently scanned text cannot be part of a longer rule's match. Again, see flexdoc(1) for details. /INTERACTIVE cannot be used in conjunction with full or fast tables, i.e., /TABLES=FULL or /TABLES=FAST. /INTERACTIVE is equivalent to -I in the unix version. /LINE /LINE (D) /NOLINE Instructs flex not to generate #line directives in output file. The default is to generate such directives so error messages in the actions will be correctly located with respect to the original flex input file, and not to the fairly meaningless line numbers of lex.yy.c. /NOLINE is equivalent to -L in the unix version. /TRACE /TRACE /NOTRACE (D) Makes flex run in trace mode. It will generate a lot of messages concerning the form of the input and the resultant non-deterministic and deterministic finite automata. This option is mostly for use in maintaining flex. Because of the large amount of output that can be generated, you may want to redefine SYS$OUTPUT to a file when using this option. /TRACE is equivalent to -t in the unix version. /EIGHTBIT /EIGHTBIT /NOEIGHTBIT (D) Instructs flex to generate an 8-bit scanner. At some sites, this is the default. On others, the default is 7-bit characters. To see which is the case, check the /STATISTICS=SUMARY output for "equivalence classes created". If the denominator of the number shown is 128, then by default flex is generating 7-bit characters. If it is 256, then the default is 8-bit characters. /EIGHTBIT is equivalent to -8 in the unix version. /TABLES=[type,...] /TABLES /TABLES=(EQUIVALENCE,FAST,FULL,META_EQUIVALENCE,NORMAL) /TABLES=(EQUIVALENCE,META_EQUIVALENCE) (D) Controls the degree of table compression. /TABLES=EQUIVALENCE directs flex to construct equivalence classes, i.e., sets of characters which have identical lexical properties. Equivalence classes usually give dramatic reductions in the final table/object file sizes (typically a factor of 2-5) and are pretty cheap performance-wise (one array look-up per char- acter scanned). /TABLES=FULL specifies that the full scanner tables should be generated - flex should not compress the tables by taking advan- tages of similar transition functions for different states. /TABLES=FAST specifies that the alternate fast scanner represen- tation (described in flexdoc(1)) should be used. /TABLES=META_EQUIVALENCE directs flex to construct meta- equivalence classes, which are sets of equivalence classes (or characters, if equivalence classes are not being used) that are commonly used together. Meta-equivalence classes are often a big win when using compressed tables, but they have a moderate per- formance impact (one or two "if" tests and one array look-up per character scanned). /TABLES=NORMAL (or just /TABLES) specifies that the scanner tables should be compressed but neither equivalence classes nor meta-equivalence classes should be used. The options FULL or FAST and META_EQUIVALENCE do not make sense together - there is no opportunity for meta-equivalence classes if the table is not being compressed. Otherwise the options may be freely mixed. FAST or FULL are also incompatible with /INTERACTIVE. The default setting is /TABLES=(EQUIVALENCE,META_EQUIVALENCE), which specifies that flex should generate equivalence classes and meta-equivalence classes. This setting provides the highest degree of table compression. You can trade off faster-executing scanners at the cost of larger tables with the following generally being true: slowest smallest /TABLES=(EQUIVALENCE,META_EQUIVALENCE) (default) /TABLES=META_EQUIVALENCE /TABLES=EQUIVALENCE /TABLES=NORMAL or /TABLES /TABLES=(FULL or FAST, EQUIVALENCE) /TABLES=FULL or FAST fastest largest /TABLES is equivalent to -C in the unix version. /TABLES=EQUIV is equivalent to -Ce in the unix version. /TABLES=META is equivalent to -Cm in the unix version. /TABLES=FULL is equivalent to -Cf or -f in the unix version. /TABLES=FAST is equivalent to -CF or -F in the unix version. /SKELETON=filespec /SKELETON=filespec Overrides the default skeleton file from which flex constructs its scanners. The default is flex.skel in a site dependent publicly accessible directory. You'll never need this option unless you are doing flex maint- enance or development. /SKELETON is equivalent to -Sfile in the unix version. 2 Regular_expressions The patterns in the input are written using an extended set of regular expressions. These are: x match the character 'x' . any character except newline [xyz] a "character class"; in this case, the pattern matches either an 'x', a 'y', or a 'z' [abj-oZ] a "character class" with a range in it; matches an 'a', a 'b', any letter from 'j' through 'o', or a 'Z' [^A-Z] a "negated character class", i.e., any character but those in the class. In this case, any character EXCEPT an uppercase letter. [^A-Z\n] any character EXCEPT an uppercase letter or a newline r* zero or more r's, where r is any regular expression r+ one or more r's r? zero or one r's (that is, "an optional r") r{2,5} anywhere from two to five r's r{2,} two or more r's r{4} exactly 4 r's {name} the expansion of the "name" definition (see above) "[xyz]\"foo" the literal string: [xyz]"foo \X if X is an 'a', 'b', 'f', 'n', 'r', 't', or 'v', then the ANSI-C interpretation of \x. Otherwise, a literal 'X' (used to escape operators such as '*') \123 the character with octal value 123 \x2a the character with hexadecimal value 2a (r) match an r; parentheses are used to override precedence (see below) rs the regular expression r followed by the regular expression s; called "concatenation" r|s either an r or an s r/s an r but only if it is followed by an s. The s is not part of the matched text. This type of pattern is called as "trailing context". ^r an r, but only at the beginning of a line r$ an r, but only at the end of a line. Equivalent to "r/\n". <s>r an r, but only in start condition s (see below for discussion of start conditions) <s1,s2,s3>r same, but in any of start conditions s1, s2, or s3 <<EOF>> an end-of-file <s1,s2><<EOF>> an end-of-file when in start condition s1 or s2 The regular expressions listed above are grouped according to precedence, from highest precedence at the top to lowest at the bottom. Those grouped together have equal precedence. Some notes on patterns: - Negated character classes match newlines unless "\n" (or an equivalent escape sequence) is one of the char- acters explicitly present in the negated character class (e.g., "[^A-Z\n]"). - A rule can have at most one instance of trailing con- text (the '/' operator or the '$' operator). The start condition, '^', and "<<EOF>>" patterns can only occur at the beginning of a pattern, and, as well as with '/' and '$', cannot be grouped inside parentheses. The following are all illegal: foo/bar$ foo|(bar$) foo|^bar <sc1>foo<sc2>bar 2 Special_actions In addition to arbitrary C code, the following can appear in actions: - ECHO copies yytext to the scanner's output. - BEGIN followed by the name of a start condition places the scanner in the corresponding start condition. - REJECT directs the scanner to proceed on to the "second best" rule which matched the input (or a prefix of the input). yytext and yyleng are set up appropriately. Note that REJECT is a particularly expensive feature in terms scanner performance; if it is used in any of the scanner's actions it will slow down all of the scanner's matching. Furthermore, REJECT cannot be used with the /TABLES=FULL or /TABLES=FAST options. Note also that unlike the other special actions, REJECT is a branch; code immediately following it in the action will not be executed. - yymore() tells the scanner that the next time it matches a rule, the corresponding token should be appended onto the current value of yytext rather than replacing it. - yyless(n) returns all but the first n characters of the current token back to the input stream, where they will be rescanned when the scanner looks for the next match. yytext and yyleng are adjusted appropriately (e.g., yyleng will now be equal to n ). - unput(c) puts the character c back onto the input stream. It will be the next character scanned. - input() reads the next character from the input stream (this routine is called yyinput() if the scanner is compiled using C++). - yyterminate() can be used in lieu of a return statement in an action. It terminates the scanner and returns a 0 to the scanner's caller, indicating "all done". By default, yyterminate() is also called when an end- of-file is encountered. It is a macro and may be rede- fined. - YY_NEW_FILE is an action available only in <<EOF>> rules. It means "Okay, I've set up a new input file, continue scanning". - yy_create_buffer( file, size ) takes a FILE pointer and an integer size. It returns a YY_BUFFER_STATE handle to a new input buffer large enough to accomodate size characters and associated with the given file. When in doubt, use YY_BUF_SIZE for the size. - yy_switch_to_buffer( new_buffer ) switches the scanner's processing to scan for tokens from the given buffer, which must be a YY_BUFFER_STATE. - yy_delete_buffer( buffer ) deletes the given buffer. 2 Variables - char *yytext holds the text of the current token. It may not be modified. - int yyleng holds the length of the current token. It may not be modified. - FILE *yyin is the file which by default flex reads from. It may be redefined but doing so only makes sense before scanning begins. Changing it in the mid- dle of scanning will have unexpected results since flex buffers its input. Once scanning terminates because an end-of-file has been seen, void yyrestart( FILE *new_file ) may be called to point yyin at the new input file. - FILE *yyout is the file to which ECHO actions are done. It can be reassigned by the user. - YY_CURRENT_BUFFER returns a YY_BUFFER_STATE handle to the current buffer. 2 Macros - YY_DECL controls how the scanning routine is declared. By default, it is "int yylex()", or, if prototypes are being used, "int yylex(void)". This definition may be changed by redefining the "YY_DECL" macro. Note that if you give arguments to the scanning routine using a K&R-style/non-prototyped function declaration, you must terminate the definition with a semi-colon (;). - The nature of how the scanner gets its input can be controlled by redefining the YY_INPUT macro. YY_INPUT's calling sequence is "YY_INPUT(buf,result,max_size)". Its action is to place up to max_size characters in the character array buf and return in the integer variable result either the number of characters read or the constant YY_NULL (0 on Unix systems) to indicate EOF. The default YY_INPUT reads from the global file-pointer "yyin". A sample redefinition of YY_INPUT (in the definitions section of the input file): %{ #undef YY_INPUT #define YY_INPUT(buf,result,max_size) \ { \ int c = getchar(); \ result = (c == EOF) ? YY_NULL : (buf[0] = c, 1); \ } %} - When the scanner receives an end-of-file indication from YY_INPUT, it then checks the yywrap() function. If yywrap() returns false (zero), then it is assumed that the function has gone ahead and set up yyin to point to another input file, and scanning continues. If it returns true (non-zero), then the scanner ter- minates, returning 0 to its caller. The default yywrap() always returns 1. Presently, to redefine it you must first "#undef yywrap", as it is currently implemented as a macro. It is likely that yywrap() will soon be defined to be a function rather than a macro. - YY_USER_ACTION can be redefined to provide an action which is always executed prior to the matched rule's action. - The macro YY_USER_INIT may be redefined to provide an action which is always executed before the first scan. - In the generated scanner, the actions are all gathered in one large switch statement and separated using YY_BREAK, which may be redefined. By default, it is simply a "break", to separate each rule's action from the following rule's. 2 Diagnostics reject_used_but_not_detected undefined or yymore_used_but_not_detected undefined - These errors can occur at compile time. They indicate that the scanner uses REJECT or yymore() but that flex failed to notice the fact, meaning that flex scanned the first two sections looking for occurrences of these actions and failed to find any, but somehow you snuck some in (via a #include file, for exam- ple). Make an explicit reference to the action in your flex input file. (Note that previously flex supported a %used/%unused mechanism for dealing with this problem; this feature is still supported but now deprecated, and will go away soon unless the author hears from people who can argue compellingly that they need it.) flex scanner jammed - a scanner compiled with /NOECHO has encountered an input string which wasn't matched by any of its rules. flex input buffer overflowed - a scanner rule matched a string long enough to overflow the scanner's internal input buffer (16K bytes - controlled by YY_BUF_MAX in "flex.skel"). scanner requires -8 flag - Your scanner specification includes recognizing 8-bit characters and you did not specify the /EIGHTBIT qualifier (and your site has not installed flex with /EIGHTBIT as the default). fatal flex scanner internal error--end of buffer missed - This can occur in an scanner which is reentered after a long-jump has jumped out (or over) the scanner's activation frame. Before reentering the scanner, use: yyrestart( yyin ); too many %t classes! - You managed to put every single char- acter into its own %t class. flex requires that at least one of the classes share characters. 2 Author Vern Paxson, with the help of many ideas and much inspira- tion from Van Jacobson. Original version by Jef Poskanzer. See flexdoc.man for additional credits and the address to send comments to. 2 Problems Some trailing context patterns cannot be properly matched and generate warning messages ("Dangerous trailing con- text"). These are patterns where the ending of the first part of the rule matches the beginning of the second part, such as "zx*/xy*", where the 'x*' matches the 'x' at the beginning of the trailing context. (Note that the POSIX draft states that the text matched by such patterns is unde- fined.) For some trailing context rules, parts which are actually fixed-length are not recognized as such, leading to the abovementioned performance loss. In particular, parts using '|' or {n} (such as "foo{3}") are always considered variable-length. Combining trailing context with the special '|' action can result in fixed trailing context being turned into the more expensive variable trailing context. For example, this hap- pens in the following example: %% abc | xyz/def Use of unput() invalidates yytext and yyleng. Use of unput() to push back more text than was matched can result in the pushed-back text matching a beginning-of-line ('^') rule even though it didn't come at the beginning of the line (though this is rare!). Pattern-matching of NUL's is substantially slower than matching other characters. flex does not generate correct #line directives for code internal to the scanner; thus, bugs in flex.skel yield bogus line numbers. Due to both buffering of input and read-ahead, you cannot intermix calls to <stdio.h> routines, such as, for example, getchar(), with flex rules and expect it to work. Call input() instead. The total table entries listed by the /STATISTICS=SUMMARY flag excludes the number of table entries needed to determine what rule has been matched. The number of entries is equal to the number of DFA states if the scanner does not use REJECT, and somewhat greater than the number of states if it does. REJECT cannot be used with the /TABLES=FAST or /TABLES=FULL options. Some of the macros, such as yywrap(), may in the future become functions which live in the flex library. This will doubtless break a lot of code, but may be required for POSIX-compliance. The flex internal algorithms need documentation. 2 Unix The unix command line options and their VMS equivalents are: -b /BACKTRACK_REPORT -C[efFm] /TABLES=[([EQUIV],[FULL],[FAST],[META])] -f /TABLES=FULL -F /TABLES=FAST -i /NOCASE_SENSITIVE -I /INTERACTIVE -L /NOLINE -p /STATISTICS=PERFORMANCE -s /NOECHO -Sfile /SKELETON=file -t /OUTPUT=SYS$OUTPUT -T /TRACE -v /STATISTICS=SUMMARY -8 /EIGHTBIT