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FLEX(1) UNIX Programmer's Manual FLEX(1) NAME flex - fast lexical analyzer generator SYNOPSIS flex [-bcdfinpstvFILT8 -C[efmF] -Sskeleton] [_f_i_l_e_n_a_m_e ...] DESCRIPTION _f_l_e_x is a tool for generating _s_c_a_n_n_e_r_s: programs which recognized lexical patterns in text. _f_l_e_x reads the given input files, or its standard input if no file names are given, for a description of a scanner to generate. The description is in the form of pairs of regular expressions and C code, called _r_u_l_e_s. _f_l_e_x generates as output a C source file, lex.yy.c, which defines a routine yylex(). This file is compiled and linked with the -lfl library to produce an executable. When the executable is run, it analyzes its input for occurrences of the regular expressions. Whenever it finds one, it executes the corresponding C code. For full documentation, see flexdoc(1). This manual entry is intended for use as a quick reference. OPTIONS _f_l_e_x has the following options: -b Generate backtracking information to _l_e_x._b_a_c_k_t_r_a_c_k. This is a list of scanner states which require back- tracking and the input characters on which they do so. By adding rules one can remove backtracking states. If all backtracking states are eliminated and -f or -F is used, the generated scanner will run faster. -c is a do-nothing, deprecated option included for POSIX compliance. NOTE: in previous releases of _f_l_e_x -c specified table- compression options. This functionality is now given by the -C flag. To ease the the impact of this change, when _f_l_e_x encounters -c, it currently issues a warning message and assumes that -C was desired instead. In the future this "promotion" of -c to -C will go away in the name of full POSIX compliance (unless the POSIX meaning is removed first). -d makes the generated scanner run in _d_e_b_u_g mode. When- ever a pattern is recognized and the global yy_flex_debug is non-zero (which is the default), the scanner will write to _s_t_d_e_r_r 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 Printed 4/3/91 26 May 1990 1 FLEX(1) UNIX Programmer's Manual FLEX(1) 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. -f specifies (take your pick) _f_u_l_l _t_a_b_l_e or _f_a_s_t _s_c_a_n_n_e_r. No table compression is done. The result is large but fast. This option is equivalent to -Cf (see below). -i instructs _f_l_e_x to generate a _c_a_s_e-_i_n_s_e_n_s_i_t_i_v_e scanner. The case of letters given in the _f_l_e_x input patterns will be ignored, and tokens in the input will be matched regardless of case. The matched text given in _y_y_t_e_x_t will have the preserved case (i.e., it will not be folded). -n is another do-nothing, deprecated option included only for POSIX compliance. -p generates a performance report to stderr. The report consists of comments regarding features of the _f_l_e_x input file which will cause a loss of performance in the resulting scanner. -s causes the _d_e_f_a_u_l_t _r_u_l_e (that unmatched scanner input is echoed to _s_t_d_o_u_t) to be suppressed. If the scanner encounters input that does not match any of its rules, it aborts with an error. -t instructs _f_l_e_x to write the scanner it generates to standard output instead of lex.yy.c. -v specifies that _f_l_e_x should write to _s_t_d_e_r_r a summary of statistics regarding the scanner it generates. -F specifies that the _f_a_s_t scanner table representation should be used. This representation is about as fast as the full table representation (-_f), and for some sets of patterns will be considerably smaller (and for others, larger). See flexdoc(1) for details. This option is equivalent to -CF (see below). -I instructs _f_l_e_x to generate an _i_n_t_e_r_a_c_t_i_v_e 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. Note, -I cannot be used in conjunction with _f_u_l_l or _f_a_s_t _t_a_b_l_e_s, i.e., the -f, -F, -Cf, or -CF flags. Printed 4/3/91 26 May 1990 2 FLEX(1) UNIX Programmer's Manual FLEX(1) -L instructs _f_l_e_x not to generate #line directives in lex.yy.c. The default is to generate such directives so error messages in the actions will be correctly located with respect to the original _f_l_e_x input file, and not to the fairly meaningless line numbers of lex.yy.c. -T makes _f_l_e_x run in _t_r_a_c_e mode. It will generate a lot of messages to _s_t_d_o_u_t concerning the form of the input and the resultant non-deterministic and deterministic finite automata. This option is mostly for use in maintaining _f_l_e_x. -8 instructs _f_l_e_x to generate an 8-bit scanner. On some sites, this is the default. On others, the default is 7-bit characters. To see which is the case, check the verbose (-v) output for "equivalence classes created". If the denominator of the number shown is 128, then by default _f_l_e_x is generating 7-bit characters. If it is 256, then the default is 8-bit characters. -C[efmF] controls the degree of table compression. -Ce directs _f_l_e_x to construct _e_q_u_i_v_a_l_e_n_c_e _c_l_a_s_s_e_s, i.e., sets of characters which have identical lexical properties. Equivalence classes usually give dramatic reductions in the final table/object file sizes (typi- cally a factor of 2-5) and are pretty cheap performance-wise (one array look-up per character scanned). -Cf specifies that the _f_u_l_l scanner tables should be generated - _f_l_e_x should not compress the tables by tak- ing advantages of similar transition functions for dif- ferent states. -CF specifies that the alternate fast scanner represen- tation (described in flexdoc(1)) should be used. -Cm directs _f_l_e_x to construct _m_e_t_a-_e_q_u_i_v_a_l_e_n_c_e _c_l_a_s_s_e_s, 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 performance impact (one or two "if" tests and one array look-up per character scanned). A lone -C specifies that the scanner tables should be compressed but neither equivalence classes nor meta- equivalence classes should be used. The options -Cf or -CF and -Cm do not make sense together - there is no opportunity for meta-equivalence Printed 4/3/91 26 May 1990 3 FLEX(1) UNIX Programmer's Manual FLEX(1) classes if the table is not being compressed. Other- wise the options may be freely mixed. The default setting is -Cem, which specifies that _f_l_e_x 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 -Cem -Cm -Ce -C -C{f,F}e -C{f,F} fastest & largest -C options are not cumulative; whenever the flag is encountered, the previous -C settings are forgotten. -Sskeleton_file overrides the default skeleton file from which _f_l_e_x constructs its scanners. You'll never need this option unless you are doing _f_l_e_x maintenance or development. SUMMARY OF FLEX 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" Printed 4/3/91 26 May 1990 4 FLEX(1) UNIX Programmer's Manual FLEX(1) 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 pre- cedence. Some notes on patterns: - Negated character classes _m_a_t_c_h _n_e_w_l_i_n_e_s 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 '$' operator). The start condition, '^', and "<<EOF>>" patterns can only occur at the beginning of a pattern, and, as well as with '/' Printed 4/3/91 26 May 1990 5 FLEX(1) UNIX Programmer's Manual FLEX(1) and '$', cannot be grouped inside parentheses. The following are all illegal: foo/bar$ foo|(bar$) foo|^bar <sc1>foo<sc2>bar SUMMARY OF 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 _a_n_y of the scanner's actions it will slow down _a_l_l of the scanner's matching. Furthermore, REJECT cannot be used with the -_f or -_F options. Note also that unlike the other special actions, REJECT is a _b_r_a_n_c_h; code immediately following it in the action will _n_o_t be executed. - yymore() tells the scanner that the next time it matches a rule, the corresponding token should be _a_p_p_e_n_d_e_d 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". Printed 4/3/91 26 May 1990 6 FLEX(1) UNIX Programmer's Manual FLEX(1) 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 _F_I_L_E pointer and an integer _s_i_z_e. It returns a YY_BUFFER_STATE handle to a new input buffer large enough to accomodate _s_i_z_e 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. VALUES AVAILABLE TO THE USER - 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 _f_l_e_x 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 _f_l_e_x buffers its input. Once scanning terminates because an end-of-file has been seen, void yyrestart( FILE *new_file ) may be called to point _y_y_i_n 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. MACROS THE USER CAN REDEFINE - 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. Printed 4/3/91 26 May 1990 7 FLEX(1) UNIX Programmer's Manual FLEX(1) YY_INPUT's calling sequence is "YY_INPUT(buf,result,max_size)". Its action is to place up to _m_a_x__s_i_z_e characters in the character array _b_u_f and return in the integer variable _r_e_s_u_l_t 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 _y_y_i_n 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. FILES _f_l_e_x._s_k_e_l skeleton scanner. _l_e_x._y_y._c generated scanner (called _l_e_x_y_y._c on some systems). Printed 4/3/91 26 May 1990 8 FLEX(1) UNIX Programmer's Manual FLEX(1) _l_e_x._b_a_c_k_t_r_a_c_k backtracking information for -b flag (called _l_e_x._b_c_k on some systems). -lfl library with which to link the scanners. SEE ALSO flexdoc(1), lex(1), yacc(1), sed(1), awk(1). M. E. Lesk and E. Schmidt, _L_E_X - _L_e_x_i_c_a_l _A_n_a_l_y_z_e_r _G_e_n_e_r_a_t_o_r DIAGNOSTICS _r_e_j_e_c_t__u_s_e_d__b_u_t__n_o_t__d_e_t_e_c_t_e_d _u_n_d_e_f_i_n_e_d or _y_y_m_o_r_e__u_s_e_d__b_u_t__n_o_t__d_e_t_e_c_t_e_d _u_n_d_e_f_i_n_e_d - These errors can occur at compile time. They indicate that the scanner uses REJECT or yymore() but that _f_l_e_x failed to notice the fact, meaning that _f_l_e_x 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 _f_l_e_x input file. (Note that previously _f_l_e_x 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.) _f_l_e_x _s_c_a_n_n_e_r _j_a_m_m_e_d - a scanner compiled with -s has encoun- tered an input string which wasn't matched by any of its rules. _f_l_e_x _i_n_p_u_t _b_u_f_f_e_r _o_v_e_r_f_l_o_w_e_d - 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"). _s_c_a_n_n_e_r _r_e_q_u_i_r_e_s -_8 _f_l_a_g - Your scanner specification includes recognizing 8-bit characters and you did not specify the -8 flag (and your site has not installed flex with -8 as the default). _f_a_t_a_l _f_l_e_x _s_c_a_n_n_e_r _i_n_t_e_r_n_a_l _e_r_r_o_r--_e_n_d _o_f _b_u_f_f_e_r _m_i_s_s_e_d - 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 ); _t_o_o _m_a_n_y %_t _c_l_a_s_s_e_s! - You managed to put every single char- acter into its own %t class. _f_l_e_x requires that at least one of the classes share characters. Printed 4/3/91 26 May 1990 9 FLEX(1) UNIX Programmer's Manual FLEX(1) AUTHOR Vern Paxson, with the help of many ideas and much inspira- tion from Van Jacobson. Original version by Jef Poskanzer. See flexdoc(1) for additional credits and the address to send comments to. DEFICIENCIES / BUGS 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 _f_i_x_e_d trailing context being turned into the more expensive _v_a_r_i_a_b_l_e 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. _f_l_e_x does not generate correct #line directives for code internal to the scanner; thus, bugs in _f_l_e_x._s_k_e_l 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 _f_l_e_x rules and expect it to work. Call input() instead. Printed 4/3/91 26 May 1990 10 FLEX(1) UNIX Programmer's Manual FLEX(1) The total table entries listed by the -v 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 some- what greater than the number of states if it does. REJECT cannot be used with the -_f or -_F options. Some of the macros, such as yywrap(), may in the future become functions which live in the -lfl library. This will doubtless break a lot of code, but may be required for POSIX-compliance. The _f_l_e_x internal algorithms need documentation. Printed 4/3/91 26 May 1990 11