LEX(1) 386BSD Reference Manual LEX(1)
NAME
lex - fast lexical analyzer generator
SYNOPSIS
lex [[-bcdfinpstvFILT8] -C[efmF] -Sskeleton] [file ...]
DESCRIPTION
Lex is a tool for generating scanners: programs which recognized lexical
patterns in text. Lex 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 rules. Lex 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 Lexdoc. This manual entry is intended for use
as a quick reference.
OPTIONS
Lex has the following 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.
-Cm 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 -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.
-Sskeleton_file
Overrides the default skeleton file from which lex constructs its
scanners. Useful for lex maintenance or development.
SUMMARY OF LEX 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 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
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 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.
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.
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".
By default, yyterminate() is also called when an end-of-file
is encountered. It is a macro and may be redefined.
YYNEWFILE
is an action available only in <<EOF>> rules. It means
"Okay, I've set up a new input file, continue scanning".
yycreatebuffer(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.
yyswitchtobuffer(new_buffer)
switches the scanner's processing to scan for tokens from the
given buffer, which must be a YY_BUFFER_STATE.
yydeletebuffer(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,
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 yywrap() function. If 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 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.
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
lex(1), yacc(1), sed(1), awk(1).
lexdoc.
M. E. Lesk, and 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 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
A lex appeared in Version 6 AT&T UNIX. The version this man page
describes is derived from code contributed by Vern Paxson.
AUTHOR
Vern Paxson, with the help of many ideas and much inspiration from Van
Jacobson. Original version by Jef Poskanzer.
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 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.
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 <stdio.h> routines, such as, for example, getchar(), with lex 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.
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 lex internal algorithms need documentation.
BSD Experimental July 24, 1991 8