LEX

NAME

SYNOPSIS

DESCRIPTION

For full documentation, see Lexdoc This manual entry is intended for use as a quick reference.  

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.

-c

is a do-nothing, deprecated option included for POSIX compliance.

NOTE in previous releases of Lex [-c ] specified table-compression options. This functionality is now given by the -C flag. To ease the the impact of this change, when lex 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 debug mode. Whenever a pattern is recognized and the global yy_Lex_debug is non-zero (which is the default), the scanner will write to stderr 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 lex). 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) full table or fast scanner No table compression is done. The result is large but fast. This option is equivalent to -Cf (see below).

-i

Instructs lex to generate a case-insensitive scanner. The case of letters given in the lex 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).

-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 lex input file which will cause a loss of performance in the resulting scanner.

-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.

-t

Instructs lex to write the scanner it generates to standard output instead of lex.yy.c

-v

Specifies that lex should write to stderr a summary of statistics regarding the scanner it generates.

-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). See Lexdoc for details.

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

-I

Instructs lex to generate an interactive 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. Again, see Lexdoc for details.

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

-L

Instructs lex 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 lex input file, and not to the fairly meaningless line numbers of lex.yy.c

-T

Makes lex 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 automata. This option is mostly for use in maintaining lex

-8

Instructs lex 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 lex is generating 7-bit characters. If it is 256, then the default is 8-bit characters.

-C [efmF ]

Controls the degree of table compression. The default setting is -Cem

-C

A lone -C specifies that the scanner tables should be compressed but neither equivalence classes nor meta-equivalence classes should be used.

-Ce Directs

lex 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 character scanned).

-Cf Specifies that the

full scanner tables should be generated - lex should not compress the tables by taking advantages of similar transition functions for different states.

-CF

Specifies that the alternate fast scanner representation (described in Lexdoc should be used.

-Directs

lex 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 performance impact (one or two "if" tests and one array look-up per character scanned).

-Cem

(Default) Generate both equivalence classes and meta-equivalence classes. This setting provides the highest degree of table compression.

Faster-executing scanners can be traded off 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.

The options -Cf or -CF and -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.

-S skeleton_file

Overrides the default skeleton file from which lex constructs its scanners. Useful for lex maintenance or development.

SUMMARY OF LEX REGULAR EXPRESSIONS

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".

rs

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 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

SUMMARY OF SPECIAL 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 -f or -F options.

Note also that unlike the other special actions, REJECT is a branch code immediately following it in the action will not be executed.

Fn 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.

Fn 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 )

Fn unput c

puts the character c back onto the input stream. It will be the next character scanned.

Fn input

reads the next character from the input stream (this routine is called Fn yyinput if the scanner is compiled using C ++ )

Fn 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, Fn yyterminate is also called when an end-of-file is encountered. It is a macro and may be redefined.

YY_NEW_FILE

is an action available only in <<EOF>> rules. It means "Okay, I've set up a new input file, continue scanning".

Fn 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.

Fn 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.

Fn 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 lex reads from. It may be redefined but doing so only makes sense before scanning begins. Changing it in the middle of scanning will have unexpected results since lex buffers its input. Once scanning terminates because an end-of-file has been seen, Fn 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.

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 (;).

YY_INPUT

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) \
    result = ((buf[0] = getchar()) == EOF) ? YY_NULL : 1;
%}

YY_INPUT

When the scanner receives an end-of-file indication from YY_INPUT, it then checks the Fn yywrap function. If Fn 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 terminates, returning 0 to its caller.

yywrap

The default Fn 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 Fn 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.

YY_USER_INIT

The macro YY _USER_INIT may be redefined to provide an action which is always executed before the first scan.

YY_BREAK

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

lex.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

"lexdoc"

M. E. Lesk E. Schmidt "LEX - Lexical Analyzer Generator"

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 Fn yymore but that lex failed to notice the fact, meaning that lex 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 lex input file. Note that previously lex 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.

lex scanner jammed

a scanner compiled with -s has encountered an input string which wasn't matched by any of its rules.

lex 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 lex.skel

scanner requires -8 flag

Your scanner specification includes recognizing 8-bit characters and you did not specify the -8 flag and your site has not installed lex with -8 as the default .

too many %t classes!

You managed to put every single character into its own %t class. Lex requires that at least one of the classes share characters.

HISTORY

AUTHOR

See "Lexdoc" for additional credits and the address to send comments to.  

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. (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 fixed trailing context being turned into the more expensive variable trailing context. This happens in the following example:

%%
abc  |
xyz/def

Use of Fn unput invalidates yytext and yyleng.

Use of Fn 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.

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

Due to both buffering of input and read-ahead, you cannot intermix calls to Aq Pa stdio.h routines, such as, for example, Fn getchar , with lex rules and expect it to work. Call Fn 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.

REJECT cannot be used with the -f or -F options.

Some of the macros, such as Fn 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 lex internal algorithms need documentation.

Index

NAME

SYNOPSIS

DESCRIPTION

OPTIONS

SUMMARY OF LEX REGULAR EXPRESSIONS

SUMMARY OF SPECIAL ACTIONS

VALUES AVAILABLE TO THE USER

MACROS THE USER CAN REDEFINE

FILES

SEE ALSO

DIAGNOSTICS

HISTORY

AUTHOR

BUGS