flex(1)
NAME
flex, lex - fast lexical analyzer generator
SYNOPSIS
flex [-bcdfinpstvFILT8 -C[efmF] -Sskeleton] [filename ...]
DESCRIPTION
flex is a tool for generating scanners: programs which recognized lexical
patterns in text. flex 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. flex 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 flexdoc(1). This manual entry is intended for
use as a quick reference.
OPTIONS
flex 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 flex -c specified table-compression
options. This functionality is now given by the -C flag. To ease
the the impact of this change, when flex 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_flex_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 flex).
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 flex to generate a case-insensitive scanner. The case of
letters given in the flex 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 flex 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 flex to write the scanner it generates to standard output
instead of lex.yy.c.
-v specifies that flex 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 flexdoc(1) for
details.
This option is equivalent to -CF (see below).
-I instructs flex 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 flexdoc(1) 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 flex 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 flex
input file, and not to the fairly meaningless line numbers of
lex.yy.c.
-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 automata. This option is
mostly for use in maintaining flex.
-8 instructs flex 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 flex is generating 7-bit characters. If it is 256,
then the default is 8-bit characters.
-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. 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 -
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 in flexdoc(1)) 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. 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).
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
-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.
-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.
SUMMARY OF FLEX 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"
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
<<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.
- YY_NEW_FILE is an action available only in <<EOF>> rules. It means
"Okay, I've set up a new input file, continue scanning".
- 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.
- 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.
- 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 flex 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 flex 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 (;).
- 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) \
{ \
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 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.
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.
- The macro YY_USER_INIT may be redefined to provide an action which
is always executed before the first scan.
- 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
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).
-lfl library with which to link the scanners.
SEE ALSO
flexdoc(1), lex(1), yacc(1), sed(1), awk(1).
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 flex failed to notice the fact, meaning that flex 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 flex input file. (Note
that previously flex 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.)
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").
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 flex with -8 as the default).
fatal flex scanner internal error--end of buffer missed - This can occur
in an scanner which is reentered after a long-jump has jumped out (or
over) the scanner's activation frame. Before reentering the scanner,
use:
yyrestart( yyin );
too many %t classes! - You managed to put every single character into its
own %t class. flex requires that at least one of the classes share
characters.
AUTHOR
Vern Paxson, with the help of many ideas and much inspiration from Van
Jacobson. Original version by Jef Poskanzer.
See flexdoc(1) for additional credits and the address to send comments
to.
DEFICIENCIES / 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. For example, 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.
flex does not generate correct #line directives for code internal to the
scanner; thus, bugs in flex.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 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.
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 flex internal algorithms need documentation.