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FLEX(1) FLEX(1) NNAAMMEE flex - fast lexical analyzer generator SSYYNNOOPPSSIISS fflleexx [[--bbccddffhhiillnnppssttvvwwBBFFIILLTTVV7788++ --CC[[aaeeffFFmmrr]] --PPpprreeffiixx --SSsskkeellee-- ttoonn]] _[_f_i_l_e_n_a_m_e _._._._] DDEESSCCRRIIPPTTIIOONN _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, lleexx..yyyy..cc,, which defines a routine yyyylleexx(()).. This file is compiled and linked with the --llffll library to produce an executable. When the executable is run, it analyzes its input for occurrences of the regular expres- sions. Whenever it finds one, it executes the correspond- ing C code. For full documentation, see fflleexxddoocc((11)).. This manual entry is intended for use as a quick reference. OOPPTTIIOONNSS _f_l_e_x has the following options: --bb generate backing-up information to _l_e_x_._b_a_c_k_u_p_. This is a list of scanner states which require backing up and the input characters on which they do so. By adding rules one can remove backing-up states. If all backing-up states are eliminated and --CCff or --CCFF is used, the generated scanner will run faster. --cc is a do-nothing, deprecated option included for POSIX compliance. NNOOTTEE:: in previous releases of _f_l_e_x --cc specified table-compression options. This functionality is now given by the --CC flag. To ease the the impact of this change, when _f_l_e_x encounters --cc,, it cur- rently issues a warning message and assumes that --CC was desired instead. In the future this "promo- tion" of --cc to --CC will go away in the name of full POSIX compliance (unless the POSIX meaning is removed first). --dd makes the generated scanner run in _d_e_b_u_g mode. Whenever a pattern is recognized and the global yyyy__fflleexx__ddeebbuugg is non-zero (which is the default), the scanner will write to _s_t_d_e_r_r a line of the form: Version 2.4 November 1993 1 FLEX(1) FLEX(1) --accepting rule at line 53 ("the matched text") The line number refers to the location of the rule in the file defining the scanner (i.e., the file that was fed to flex). Messages are also generated when the scanner backs up, accepts the default rule, reaches the end of its input buffer (or encounters a NUL; the two look the same as far as the scanner's concerned), or reaches an end-of- file. --ff specifies _f_a_s_t _s_c_a_n_n_e_r_. No table compression is done and stdio is bypassed. The result is large but fast. This option is equivalent to --CCffrr (see below). --hh generates a "help" summary of _f_l_e_x_'_s options to _s_t_d_e_r_r and then exits. --ii instructs _f_l_e_x to generate a _c_a_s_e_-_i_n_s_e_n_s_i_t_i_v_e scan- ner. 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). --ll turns on maximum compatibility with the original AT&T lex implementation, at a considerable perfor- mance cost. This option is incompatible with --++,, --ff,, --FF,, --CCff,, or --CCFF.. See _f_l_e_x_d_o_c_(_1_) for details. --nn is another do-nothing, deprecated option included only for POSIX compliance. --pp 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 per- formance in the resulting scanner. If you give the flag twice, you will also get comments regarding features that lead to minor performance losses. --ss 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. --tt instructs _f_l_e_x to write the scanner it generates to standard output instead of lleexx..yyyy..cc.. --vv specifies that _f_l_e_x should write to _s_t_d_e_r_r a sum- mary of statistics regarding the scanner it gener- ates. --ww suppresses warning messages. Version 2.4 November 1993 2 FLEX(1) FLEX(1) --BB instructs _f_l_e_x to generate a _b_a_t_c_h scanner instead of an _i_n_t_e_r_a_c_t_i_v_e scanner (see --II below). See _f_l_e_x_d_o_c_(_1_) for details. Scanners using --CCff or --CCFF compression options automatically specify this option, too. --FF specifies that the _f_a_s_t scanner table representa- tion should be used (and stdio bypassed). This representation is about as fast as the full table representation ((--ff)),, and for some sets of patterns will be considerably smaller (and for others, larger). It cannot be used with the --++ option. See fflleexxddoocc((11)) for more details. This option is equivalent to --CCFFrr (see below). --II 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 looking ahead if it knows that the currently scanned text cannot be part of a longer rule's match. This is the opposite of _b_a_t_c_h scanners (see --BB above). See fflleexxddoocc((11)) for details. Note, --II cannot be used in conjunction with _f_u_l_l or _f_a_s_t _t_a_b_l_e_s_, i.e., the --ff,, --FF,, --CCff,, or --CCFF flags. For other table compression options, --II is the default. --LL instructs _f_l_e_x not to generate ##lliinnee directives in lleexx..yyyy..cc.. The default is to generate such direc- tives so error messages in the actions will be cor- rectly located with respect to the original _f_l_e_x input file, and not to the fairly meaningless line numbers of lleexx..yyyy..cc.. --TT makes _f_l_e_x run in _t_r_a_c_e mode. It will generate a lot of messages to _s_t_d_e_r_r 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_. --VV prints the version number to _s_t_d_e_r_r and exits. --77 instructs _f_l_e_x to generate a 7-bit scanner, which can save considerable table space, especially when using --CCff or --CCFF (and, at most sites, --77 is on by default for these options. To see if this is the case, use the --vv verbose flag and check the flag summary it reports). --88 instructs _f_l_e_x to generate an 8-bit scanner. This is the default except for the --CCff and --CCFF compres- sion options, for which the default is site- dependent, and can be checked by inspecting the Version 2.4 November 1993 3 FLEX(1) FLEX(1) flag summary generated by the --vv option. --++ specifies that you want flex to generate a C++ scanner class. See the section on Generating C++ Scanners in _f_l_e_x_d_o_c_(_1_) for details. --CC[[aaeeffFFmmrr]] controls the degree of table compression and scan- ner optimization. --CCaa trade off larger tables in the generated scan- ner for faster performance because the elements of the tables are better aligned for memory access and computation. This option can double the size of the tables used by your scanner. --CCee 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 lexi- cal 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). --CCff specifies that the _f_u_l_l scanner tables should be generated - _f_l_e_x should not compress the tables by taking advantages of similar transition func- tions for different states. --CCFF specifies that the alternate fast scanner rep- resentation (described in fflleexxddoocc((11)))) should be used. This option cannot be used with --++.. --CCmm 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 perfor- mance impact (one or two "if" tests and one array look-up per character scanned). --CCrr causes the generated scanner to _b_y_p_a_s_s using stdio for input. In general this option results in a minor performance gain only worthwhile if used in conjunction with --CCff or --CCFF.. It can cause surpris- ing behavior if you use stdio yourself to read from _y_y_i_n prior to calling the scanner. A lone --CC specifies that the scanner tables should be compressed but neither equivalence classes nor meta-equivalence classes should be used. The options --CCff or --CCFF and --CCmm do not make sense Version 2.4 November 1993 4 FLEX(1) FLEX(1) together - there is no opportunity for meta- equivalence classes if the table is not being com- pressed. Otherwise the options may be freely mixed. The default setting is --CCeemm,, 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} -C{f,F}a fastest & largest --CC options are cumulative. --PPpprreeffiixx changes the default _y_y prefix used by _f_l_e_x to be _p_r_e_f_i_x instead. See _f_l_e_x_d_o_c_(_1_) for a description of all the global variables and file names that this affects. --SSsskkeelleettoonn__ffiillee 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. SSUUMMMMAARRYY OOFF FFLLEEXX RREEGGUULLAARR EEXXPPRREESSSSIIOONNSS 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 Version 2.4 November 1993 5 FLEX(1) FLEX(1) 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 <*>r an r in any start condition, even an exclusive one. <<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 low- est at the bottom. Those grouped together have equal precedence. Some notes on patterns: - Negated character classes _m_a_t_c_h _n_e_w_l_i_n_e_s unless Version 2.4 November 1993 6 FLEX(1) FLEX(1) "\n" (or an equivalent escape sequence) is one of the characters explicitly present in the negated character class (e.g., "[^A-Z\n]"). - A rule can have at most one instance of trailing context (the '/' operator or the '$' operator). The start condition, '^', and "<<EOF>>" patterns can only occur at the beginning of a pattern, and, as well as with '/' and '$', cannot be grouped inside parentheses. The following are all illegal: foo/bar$ foo|(bar$) foo|^bar <sc1>foo<sc2>bar SSUUMMMMAARRYY OOFF SSPPEECCIIAALL AACCTTIIOONNSS In addition to arbitrary C code, the following can appear in actions: - EECCHHOO copies yytext to the scanner's output. - BBEEGGIINN followed by the name of a start condition places the scanner in the corresponding start con- dition. - RREEJJEECCTT directs the scanner to proceed on to the "second best" rule which matched the input (or a prefix of the input). yyyytteexxtt and yyyylleenngg are set up appropriately. Note that RREEJJEECCTT 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. Further- more, RREEJJEECCTT cannot be used with the --ff or --FF options. Note also that unlike the other special actions, RREEJJEECCTT is a _b_r_a_n_c_h_; code immediately following it in the action will _n_o_t be executed. - yyyymmoorree(()) 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 yyyytteexxtt rather than replacing it. - yyyylleessss((nn)) 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. yyyytteexxtt and yyyylleenngg are adjusted appropriately (e.g., yyyylleenngg will now be equal to _n ). - uunnppuutt((cc)) puts the character _c back onto the input Version 2.4 November 1993 7 FLEX(1) FLEX(1) stream. It will be the next character scanned. - iinnppuutt(()) reads the next character from the input stream (this routine is called yyyyiinnppuutt(()) if the scanner is compiled using CC++++)).. - yyyytteerrmmiinnaattee(()) 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, yyyytteerrmmiinnaattee(()) is also called when an end-of-file is encountered. It is a macro and may be redefined. - YYYY__NNEEWW__FFIILLEE is an action available only in <<EOF>> rules. It means "Okay, I've set up a new input file, continue scanning". It is no longer required; you can just assign _y_y_i_n to point to a new file in the <<EOF>> action. - yyyy__ccrreeaattee__bbuuffffeerr(( ffiillee,, ssiizzee )) 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 acco- modate _s_i_z_e characters and associated with the given file. When in doubt, use YYYY__BBUUFF__SSIIZZEE for the size. - yyyy__sswwiittcchh__ttoo__bbuuffffeerr(( nneeww__bbuuffffeerr )) switches the scanner's processing to scan for tokens from the given buffer, which must be a YY_BUFFER_STATE. - yyyy__ddeelleettee__bbuuffffeerr(( bbuuffffeerr )) deletes the given buffer. VVAALLUUEESS AAVVAAIILLAABBLLEE TTOO TTHHEE UUSSEERR - cchhaarr **yyyytteexxtt holds the text of the current token. It may be modified but not lengthened (you cannot append characters to the end). Modifying the last character may affect the activity of rules anchored using '^' during the next scan; see fflleexxddoocc((11)) for details. If the special directive %%aarrrraayy appears in the first section of the scanner description, then yyyytteexxtt is instead declared cchhaarr yyyytteexxtt[[YYYYLLMMAAXX]],, where YYYYLLMMAAXX is a macro definition that you can redefine in the first section if you don't like the default value (generally 8KB). Using %%aarrrraayy results in somewhat slower scanners, but the value of yyyytteexxtt becomes immune to calls to _i_n_p_u_t_(_) and _u_n_p_u_t_(_)_, which potentially destroy its value when yyyytteexxtt is a character pointer. The opposite of %%aarrrraayy is %%ppooiinntteerr,, which is the default. Version 2.4 November 1993 8 FLEX(1) FLEX(1) You cannot use %%aarrrraayy when generating C++ scanner classes (the --++ flag). - iinntt yyyylleenngg holds the length of the current token. - FFIILLEE **yyyyiinn 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 or after an EOF has been encountered. Changing it in the midst of scanning will have unexpected results since _f_l_e_x buffers its input; use yyyyrreessttaarrtt(()) instead. Once scanning terminates because an end-of-file has been seen, yyoouu ccaann aassssiiggnn _y_y_i_n at the new input file and then call the scanner again to continue scanning. - vvooiidd yyyyrreessttaarrtt(( FFIILLEE **nneeww__ffiillee )) may be called to point _y_y_i_n at the new input file. The switch-over to the new file is immediate (any previously buffered-up input is lost). Note that calling yyyyrreessttaarrtt(()) with _y_y_i_n as an argument thus throws away the current input buffer and continues scan- ning the same input file. - FFIILLEE **yyyyoouutt is the file to which EECCHHOO actions are done. It can be reassigned by the user. - YYYY__CCUURRRREENNTT__BBUUFFFFEERR returns a YYYY__BBUUFFFFEERR__SSTTAATTEE handle to the current buffer. - YYYY__SSTTAARRTT returns an integer value corresponding to the current start condition. You can subsequently use this value with BBEEGGIINN to return to that start condition. MMAACCRROOSS AANNDD FFUUNNCCTTIIOONNSS YYOOUU CCAANN RREEDDEEFFIINNEE - YYYY__DDEECCLL 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 YYYY__IINNPPUUTT macro. 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 con- stant 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 Version 2.4 November 1993 9 FLEX(1) FLEX(1) (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 function yyyywwrraapp(()) function. If yyyywwrraapp(()) 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 terminates, returning 0 to its caller. The default yyyywwrraapp(()) always returns 1. - YY_USER_ACTION can be redefined to provide an action which is always executed prior to the matched rule's action. - The macro YYYY__UUSSEERR__IINNIITT may be redefined to provide an action which is always executed before the first scan. - In the generated scanner, the actions are all gath- ered in one large switch statement and separated using YYYY__BBRREEAAKK,, which may be redefined. By default, it is simply a "break", to separate each rule's action from the following rule's. FFIILLEESS --llffll library with which to link scanners to obtain the default versions of _y_y_w_r_a_p_(_) and/or _m_a_i_n_(_)_. _l_e_x_._y_y_._c generated scanner (called _l_e_x_y_y_._c on some systems). _l_e_x_._y_y_._c_c generated C++ scanner class, when using --++.. _<_F_l_e_x_L_e_x_e_r_._h_> header file defining the C++ scanner base class, FFlleexxLLeexxeerr,, and its derived class, yyyyFFlleexxLLeexxeerr.. _f_l_e_x_._s_k_l skeleton scanner. This file is only used when building flex, not when flex executes. Version 2.4 November 1993 10 FLEX(1) FLEX(1) _l_e_x_._b_a_c_k_u_p backing-up information for --bb flag (called _l_e_x_._b_c_k on some systems). SSEEEE AALLSSOO 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 DDIIAAGGNNOOSSTTIICCSS _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 RREEJJEECCTT or yyyymmoorree(()) 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 example). Make an explicit reference to the action in your _f_l_e_x input file. (Note that previously _f_l_e_x supported a %%uusseedd//%%uunnuusseedd mechanism for dealing with this problem; this feature is still supported but now dep- recated, 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 --ss has encountered an input string which wasn't matched by any of its rules. _w_a_r_n_i_n_g_, _r_u_l_e _c_a_n_n_o_t _b_e _m_a_t_c_h_e_d indicates that the given rule cannot be matched because it follows other rules that will always match the same text as it. See _f_l_e_x_d_o_c_(_1_) for an example. _w_a_r_n_i_n_g_, --ss _o_p_t_i_o_n _g_i_v_e_n _b_u_t _d_e_f_a_u_l_t _r_u_l_e _c_a_n _b_e _m_a_t_c_h_e_d means that it is possible (perhaps only in a particular start condition) that the default rule (match any single character) is the only one that will match a particular input. Since _s_c_a_n_n_e_r _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 more text than the available dynamic memory. _t_o_k_e_n _t_o_o _l_a_r_g_e_, _e_x_c_e_e_d_s _Y_Y_L_M_A_X _- your scanner uses %%aarrrraayy and one of its rules matched a string longer than the YYYYLL-- MMAAXX constant (8K bytes by default). You can increase the value by #define'ing YYYYLLMMAAXX in the definitions section of your _f_l_e_x input. _s_c_a_n_n_e_r _r_e_q_u_i_r_e_s _-_8 _f_l_a_g _t_o _u_s_e _t_h_e _c_h_a_r_a_c_t_e_r _'_x_' _- Your scanner specification includes recognizing the 8-bit char- acter _'_x_' and you did not specify the -8 flag, and your scanner defaulted to 7-bit because you used the --CCff or --CCFF Version 2.4 November 1993 11 FLEX(1) FLEX(1) table compression options. _f_l_e_x _s_c_a_n_n_e_r _p_u_s_h_-_b_a_c_k _o_v_e_r_f_l_o_w _- you used uunnppuutt(()) to push back so much text that the scanner's buffer could not hold both the pushed-back text and the current token in yyyytteexxtt.. Ideally the scanner should dynamically resize the buffer in this case, but at present it does not. _i_n_p_u_t _b_u_f_f_e_r _o_v_e_r_f_l_o_w_, _c_a_n_'_t _e_n_l_a_r_g_e _b_u_f_f_e_r _b_e_c_a_u_s_e _s_c_a_n_- _n_e_r _u_s_e_s _R_E_J_E_C_T _- the scanner was working on matching an extremely large token and needed to expand the input buffer. This doesn't work with scanners that use RREEJJEECCTT.. _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 activa- tion frame. Before reentering the scanner, use: yyrestart( yyin ); or use C++ scanner classes (the --++ option), which are fully reentrant. AAUUTTHHOORR 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. DDEEFFIICCIIEENNCCIIEESS // BBUUGGSS 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 undefined.) 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, in the following: %% abc | Version 2.4 November 1993 12 FLEX(1) FLEX(1) xyz/def Use of uunnppuutt(()) or iinnppuutt(()) invalidates yytext and yyleng, unless the %%aarrrraayy directive or the --ll option has been used. 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 begin- ning of the line (though this is rare!). Pattern-matching of NUL's is substantially slower than matching other characters. Dynamic resizing of the input buffer is slow, as it entails rescanning all the text matched so far by the cur- rent (generally huge) token. _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_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 exam- ple, ggeettcchhaarr(()),, with _f_l_e_x rules and expect it to work. Call iinnppuutt(()) instead. The total table entries listed by the --vv flag excludes the number of table entries needed to determine what rule has been matched. The number of entries is equal to the num- ber of DFA states if the scanner does not use RREEJJEECCTT,, and somewhat greater than the number of states if it does. RREEJJEECCTT cannot be used with the --ff or --FF options. The _f_l_e_x internal algorithms need documentation. Version 2.4 November 1993 13