/*************************************************************************\
* Copyright (c) 2002 The University of Chicago, as Operator of Argonne
* National Laboratory.
* Copyright (c) 2002 The Regents of the University of California, as
* Operator of Los Alamos National Laboratory.
* EPICS BASE Versions 3.13.7
* and higher are distributed subject to a Software License Agreement found
* in file LICENSE that is included with this distribution.
\*************************************************************************/
NAME
flex - 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 back-
tracking 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. When-
ever 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 look-
ing 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 (typi-
cally 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 tak-
ing advantages of similar transition functions for dif-
ferent states.
-CF specifies that the alternate fast scanner represen-
tation (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. Other-
wise 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 pre-
cedence.
Some notes on patterns:
- Negated character classes match newlines unless "\n"
(or an equivalent escape sequence) is one of the char-
acters explicitly present in the negated character
class (e.g., "[^A-Z\n]").
- A rule can have at most one instance of trailing con-
text (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 rede-
fined.
- 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 mid-
dle 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 ter-
minates, 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 exam-
ple). 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 encoun-
tered 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 char-
acter 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 inspira-
tion 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 con-
text"). 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 unde-
fined.)
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 hap-
pens 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 some-
what 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.
Man(1) output converted with
man2html