FLEX

NAME

SYNOPSIS

DESCRIPTION

OPTIONS

-b

Generate backtracking information to 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 -f or -F is used, the generated scanner will run faster (see the -p flag). Only users who wish to squeeze every last cycle out of their scanners need worry about this option.

-d

makes the generated scanner run in debug mode. Whenever a pattern is recognized the scanner will write to stderr a line of the form:

    --accepting rule #n

Rules are numbered sequentially with the first one being 1. Rule #0 is executed when the scanner backtracks; Rule #(n+1) (where n is the number of rules) indicates the default action; Rule #(n+2) indicates that the input buffer is empty and needs to be refilled and then the scan restarted. Rules beyond (n+2) are end-of-file actions.

-f

has the same effect as lex's -f flag (do not compress the scanner tables); the mnemonic changes from fast compilation to (take your pick) full table or fast scanner. The actual compilation takes longer, since flex is I/O bound writing out the big table.

This option is equivalent to -cf (see below).

-i

instructs flex to generate a case-insensitive scanner. The case of letters given in the flex input patterns will be ignored, and the rules will be matched regardless of case. The matched text given in yytext will have the preserved case (i.e., it will not be folded).

-p

generates a performance report to stderr. The report consists of comments regarding features of the flex input file which will cause a loss of performance in the resulting scanner. Note that the use of REJECT and variable trailing context (see BUGS) entails a substantial performance penalty; use of yymore(), the ^ operator, and the -I flag entail minor performance penalties.

-s

causes the default rule (that unmatched scanner input is echoed to stdout) to be suppressed. If the scanner encounters input that does not match any of its rules, it aborts with an error. This option is useful for finding holes in a scanner's rule set.

-v

has the same meaning as for lex (print to stderr a summary of statistics of the generated scanner). Many more statistics are printed, though, and the summary spans several lines. Most of the statistics are meaningless to the casual flex user, but the first line identifies the version of flex, which is useful for figuring out where you stand with respect to patches and new releases.

-F

specifies that the fast 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). In general, if the pattern set contains both "keywords" and a catch-all, "identifier" rule, such as in the set:

        "case"    return ( TOK_CASE );
        "switch"  return ( TOK_SWITCH );
        ...
        "default" return ( TOK_DEFAULT );
        [a-z]+    return ( TOK_ID );

then you're better off using the full table representation. If only the "identifier" rule is present and you then use a hash table or some such to detect the keywords, you're better off using -F.

This option is equivalent to -cF (see below).

-I

instructs flex to generate an interactive scanner. Normally, scanners generated by flex always look ahead one character before deciding that a rule has been matched. At the cost of some scanning overhead, flex will generate a scanner which only looks ahead when needed. Such scanners are called interactive because if you want to write a scanner for an interactive system such as a command shell, you will probably want the user's input to be terminated with a newline, and without -I the user will have to type a character in addition to the newline in order to have the newline recognized. This leads to dreadful interactive performance.

If all this seems to confusing, here's the general rule: if a human will be typing in input to your scanner, use -I, otherwise don't; if you don't care about how fast your scanners run and don't want to make any assumptions about the input to your scanner, always use -I.

Note, -I cannot be used in conjunction with full or fast tables, i.e., the -f, -F, -cf, or -cF flags.

-L

instructs flex to not generate #line directives (see below).

-T

makes flex run in trace mode. It will generate a lot of messages to stdout concerning the form of the input and the resultant non-deterministic and deterministic finite automatons. This option is mostly for use in maintaining flex.

-c[efmF]

controls the degree of table compression. -ce directs flex to construct equivalence classes, i.e., sets of characters which have identical lexical properties (for example, if the only appearance of digits in the flex input is in the character class "[0-9]" then the digits '0', '1', ..., '9' will all be put in the same equivalence class). -cf specifies that the full scanner tables should be generated - flex should not compress the tables by taking advantages of similar transition functions for different states. -cF specifies that the alternate fast scanner representation (described above under the -F flag) should be used. -cm 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. 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 classes if the table is not being compressed. Otherwise the options may be freely mixed.

The default setting is -cem 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
               -cem
               -ce
               -cm
               -c
               -c{f,F}e
               -c{f,F}
    fastest            largest

Note that scanners with the smallest tables compile the quickest, so during development you will usually want to use the default, maximal compression.

-Sskeleton_file

overrides the default skeleton file from which flex constructs its scanners. You'll never need this option unless you are doing flex maintenance or development.

INCOMPATIBILITIES WITH LEX

-

There is no run-time library to link with. You needn't specify -ll when linking, and you must supply a main program. (Hacker's note: since the lex library contains a main() which simply calls yylex(), you actually can be lazy and not supply your own main program and link with -ll.)

-

lex's %r (Ratfor scanners) and %t (translation table) options are not supported.

-

The do-nothing -n flag is not supported.

-

When definitions are expanded, flex encloses them in parentheses. With lex, the following

    NAME    [A-Z][A-Z0-9]*
    %%
    foo{NAME}?      printf( "Found it\n" );
    %%

will not match the string "foo" because when the macro is expanded the rule is equivalent to "foo[A-Z][A-Z0-9]*?" and the precedence is such that the '?' is associated with "[A-Z0-9]*". With flex, the rule will be expanded to "foo([A-z][A-Z0-9]*)?" and so the string "foo" will match. Note that because of this, the ^, $, <s>, and / operators cannot be used in a definition.

-

The undocumented lex-scanner internal variable yylineno is not supported.

-

The input() routine is not redefinable, though may be called to read characters following whatever has been matched by a rule. If input() encounters an end-of-file the normal yywrap() processing is done. A ``real'' end-of-file is returned as EOF.

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 buffer "buf" and return in the integer variable "result" either the number of characters read or the constant YY_NULL (0 on Unix systems) systems) to indicate EOF. The default YY_INPUT reads from the file-pointer "yyin" (which is by default stdin), so if you just want to change the input file, you needn't redefine YY_INPUT - just point yyin at the input file.

A sample redefinition of YY_INPUT (in the first section of the input file):

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

You also can add in things like counting keeping track of the input line number this way; but don't expect your scanner to go very fast.

-

output() is not supported. Output from the ECHO macro is done to the file-pointer "yyout" (default stdout).

-

If you are providing your own yywrap() routine, you must "#undef yywrap" first.

-

To refer to yytext outside of your scanner source file, use "extern char *yytext;" rather than "extern char yytext[];".

-

yyleng is a macro and not a variable, and hence cannot be accessed outside of the scanner source file.

-

flex reads only one input file, while lex's input is made up of the concatenation of its input files.

-

The name FLEX_SCANNER is #define'd so scanners may be written for use with either flex or lex.

-

The macro YY_USER_ACTION can be redefined to provide an action which is always executed prior to the matched rule's action. For example, it could be #define'd to call a routine to convert yytext to lower-case, or to copy yyleng to a global variable to make it accessible outside of the scanner source file.

-

In the generated scanner, rules are separated using YY_BREAK instead of simple "break"'s. This allows, for example, C++ users to #define YY_BREAK to do nothing (while being very careful that every rule ends with a "break" or a "return"!) to avoid suffering from unreachable statement warnings where a rule's action ends with "return".

ENHANCEMENTS

-

Exclusive start-conditions can be declared by using %x instead of %s. These start-conditions have the property that when they are active, no other rules are active. Thus a set of rules governed by the same exclusive start condition describe a scanner which is independent of any of the other rules in the flex input. This feature makes it easy to specify "mini-scanners" which scan portions of the input that are syntactically different from the rest (e.g., comments).

-

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".

-

End-of-file rules. The special rule "<<EOF>>" indicates actions which are to be taken when an end-of-file is encountered and yywrap() returns non-zero (i.e., indicates no further files to process). The action can either point yyin at a new file to process, in which case the action should finish with YY_NEW_FILE (this is a branch, so subsequent code in the action won't be executed), or it should finish with a return statement. <<EOF>> rules may not be used with other patterns; they may only be qualified with a list of start conditions. If an unqualified <<EOF>> rule is given, it applies only to the INITIAL start condition, and not to %s start conditions. These rules are useful for catching things like unclosed comments. An example:

    %x quote
    %%
    ...
    <quote><<EOF>>   {
             error( "unterminated quote" );
             yyterminate();
             }
    <<EOF>>          {
             yyin = fopen( next_file, "r" );
             YY_NEW_FILE;
             }

-

flex dynamically resizes its internal tables, so directives like "%a 3000" are not needed when specifying large scanners.

-

The scanning routine generated by flex is declared using the macro YY_DECL. By redefining this macro you can change the routine's name and its calling sequence. For example, you could use:

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

to give it the name lexscan, returning a float, and taking two floats as arguments. Note that if you give arguments to the scanning routine, you must terminate the definition with a semi-colon (;).

-

flex generates #line directives mapping lines in the output to their origin in the input file.

-

You can put multiple actions on the same line, separated with semi-colons. With lex, the following

    foo    handle_foo(); return 1;

is truncated to

    foo    handle_foo();

flex does not truncate the action. Actions that are not enclosed in braces are terminated at the end of the line.

-

Actions can be begun with %{ and terminated with %}. In this case, flex does not count braces to figure out where the action ends - actions are terminated by the closing %}. This feature is useful when the enclosed action has extraneous braces in it (usually in comments or inside inactive #ifdef's) that throw off the brace-count.

-

All of the scanner actions (e.g., ECHO, yywrap ...) except the unput() and input() routines, are written as macros, so they can be redefined if necessary without requiring a separate library to link to.

-

When yywrap() indicates that the scanner is done processing (it does this by returning non-zero), on subsequent calls the scanner will always immediately return a value of 0. To restart it on a new input file, the action yyrestart() is used. It takes one argument, the new input file. It closes the previous yyin (unless stdin) and sets up the scanners internal variables so that the next call to yylex() will start scanning the new file. This functionality is useful for, e.g., programs which will process a file, do some work, and then get a message to parse another file.

-

Flex scans the code in section 1 (inside %{}'s) and the actions for occurrences of REJECT and yymore(). If it doesn't see any, it assumes the features are not used and generates higher-performance scanners. Flex tries to be correct in identifying uses but can be fooled (for example, if a reference is made in a macro from a #include file). If this happens (a feature is used and flex didn't realize it) you will get a compile-time error of the form

    reject_used_but_not_detected undefined

You can tell flex that a feature is used even if it doesn't think so with %used followed by the name of the feature (for example, "%used REJECT"); similarly, you can specify that a feature is not used even though it thinks it is with %unused.

-

Comments may be put in the first section of the input by preceding them with '#'.

FILES

flex.skel

skeleton scanner

lex.yy.c

generated scanner (called lexyy.c on some systems).

lex.backtrack

backtracking information for -b flag (called lex.bck on some systems).

SEE ALSO

lex(1)

M. E. Lesk and E. Schmidt, LEX - Lexical Analyzer Generator  

AUTHOR

Thanks to the many flex beta-testers and feedbackers, especially Casey Leedom, Frederic Brehm, Nick Christopher, Chris Faylor, Eric Goldman, Eric Hughes, Greg Lee, Craig Leres, Mohamed el Lozy, Jim Meyering, Esmond Pitt, Jef Poskanzer, and Dave Tallman. Thanks to Keith Bostic, John Gilmore, Bob Mulcahy, Rich Salz, and Richard Stallman for help with various distribution headaches.

Send comments to:

     Vern Paxson
     Real Time Systems
     Bldg. 46A
     Lawrence Berkeley Laboratory
     1 Cyclotron Rd.
     Berkeley, CA 94720

     (415) 486-6411

     vern@csam.lbl.gov
     vern@rtsg.ee.lbl.gov
     ucbvax!csam.lbl.gov!vern

     vern@cs.cornell.edu

     Vern Paxson
     CS Department
     Grad Office
     4126 Upson
     Cornell University
     Ithaca, NY 14853-7501

     <no phone number yet>

DIAGNOSTICS

flex scanner jammed - a scanner compiled with -s 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").

old-style lex command ignored - the flex input contains a lex command (e.g., "%n 1000") which is being ignored.  

BUGS

Some trailing context patterns cannot be properly matched and generate warning messages ("Dangerous trailing context"). 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. (Lex doesn't get these patterns right either.) If desperate, you can use yyless() to effect arbitrary trailing context.

variable trailing context (where both the leading and trailing parts do not have a fixed length) entails the same performance loss as REJECT (i.e., substantial).

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} are always considered variable-length.

Use of unput() or input() trashes the current 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.

yytext and yyleng cannot be modified within a flex action.

Nulls are not allowed in flex inputs or in the inputs to scanners generated by flex. Their presence generates fatal errors.

Flex does not generate correct #line directives for code internal to the scanner; thus, bugs in flex.skel yield bogus line numbers.

Pushing back definitions enclosed in ()'s can result in nasty, difficult-to-understand problems like:

        {DIG}  [0-9] /* a digit */

Due to both buffering of input and read-ahead, you cannot intermix calls to stdio routines, such as, for example, getchar() with flex rules and expect it to work. Call input() instead.

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 somewhat greater than the number of states if it does.

To be consistent with ANSI C, the escape sequence \xhh should be recognized for hexadecimal escape sequences, such as '\x41' for 'A'.

It would be useful if flex wrote to lex.yy.c a summary of the flags used in its generation (such as which table compression options).

The scanner run-time speeds still have not been optimized as much as they deserve. Van Jacobson's work shows that the can go faster still.

The utility needs more complete documentation.

Index

NAME

SYNOPSIS

DESCRIPTION

OPTIONS

INCOMPATIBILITIES WITH LEX

ENHANCEMENTS

FILES

SEE ALSO

AUTHOR

DIAGNOSTICS

BUGS