FLEX

NAME

SYNOPSIS

DESCRIPTION

OPTIONS

-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.

-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).

-r

specifies that the scanner uses the REJECT action.

-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.

-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 possible cost of some scanning overhead (it's not clear that more overhead is involved), 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 standard out 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

-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.

-

yymore() is not supported.

-

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

-

If your input uses REJECT, you must run flex with the -r flag. If you leave out the flag, the scanner will abort at run-time with a message that the scanner was compiled without the flag being specified.

-

The input() routine is not redefinable, though may be called to read characters following whatever has been matched by a rule. If input() encounters and 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).

-

Trailing context is restricted to patterns which have either a fixed-sized leading part or a fixed-sized trailing part. For example, "a*/b" and "a/b*" are okay, but not "a*/b*". This restriction is due to a bug in the trailing context algorithm given in Principles of Compiler Design (and Compilers - Principles, Techniques, and Tools) which can result in mismatches. Try the following lex program

    %%
    x+/xy           printf( "I found \"%s\"\n", yytext );

on the input "xxy". (If anyone knows of a fast algorithm for finding the beginning of trailing context for an arbitrary pair of regular expressions, please let me know!) If you must have arbitrary trailing context, you can use yyless() to effect it.

-

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

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).

-

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.

-

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.

FILES

flex.skel

skeleton scanner

flex.fastskel

skeleton scanner for -f and -F

flexskelcom.h

common definitions for skeleton files

flexskeldef.h

definitions for compressed skeleton file

fastskeldef.h

definitions for -f, -F skeleton file

SEE ALSO

lex(1)

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

AUTHOR

Thanks to the many flex beta-testers, especially Casey Leedom, Nick Christopher, Chris Faylor, Eric Goldman, Craig Leres, Mohamed el Lozy, Esmond Pitt, Jef Poskanzer, and Dave Tallman. Thanks to 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@lbl-{csam,rtsg}.arpa
     ucbvax!lbl-csam.arpa!vern

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 (as large as BUFSIZ in "/usr/include/stdio.h"). You can edit flexskelcom.h and increase YY_BUF_SIZE and YY_MAX_LINE to increase this limit.

REJECT used and scanner was not generated using -r - just like it sounds. Your scanner uses REJECT. You must run flex on the scanner description using the -r flag.

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

BUGS

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.

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

Do not mix trailing context with the '|' operator used to specify that multiple rules use the same action. That is, avoid constructs like:

        foo/bar      |
        bletch       |
        bugprone     { ... }

The pattern:

        x{3}

Due to both buffering of input and read-ahead, you cannot intermix calls to, 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 was not compiled with -r, and greater than the number of states if it was.

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

Index

NAME

SYNOPSIS

DESCRIPTION

OPTIONS

INCOMPATIBILITIES WITH LEX

ENHANCEMENTS

FILES

SEE ALSO

AUTHOR

DIAGNOSTICS

BUGS