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FLEX(1) FLEX(1)
NAME
flex - fast lexical analyzer generator
SYNOPSIS
flex [-bcdfhilnpstvwBFILTV78+? -C[aefFmr] -ooutput
-Pprefix -Sskeleton] [--help --version] [filename ...]
OVERVIEW
This manual describes flex, a tool for generating pro-
grams that perform pattern-matching on text. The manual
includes both tutorial and reference sections:
Description
a brief overview of the tool
Some Simple Examples
Format Of The Input File
Patterns
the extended regular expressions used by flex
How The Input Is Matched
the rules for determining what has been matched
Actions
how to specify what to do when a pattern is matched
The Generated Scanner
details regarding the scanner that flex produces;
how to control the input source
Start Conditions
introducing context into your scanners, and
managing "mini-scanners"
Multiple Input Buffers
how to manipulate multiple input sources; how to
scan from strings instead of files
End-of-file Rules
special rules for matching the end of the input
Miscellaneous Macros
a summary of macros available to the actions
Values Available To The User
a summary of values available to the actions
Interfacing With Yacc
connecting flex scanners together with yacc parsers
Options
flex command-line options, and the "%option"
directive
Performance Considerations
how to make your scanner go as fast as possible
Generating C++ Scanners
the (experimental) facility for generating C++
scanner classes
Incompatibilities With Lex And POSIX
how flex differs from AT&T lex and the POSIX lex
standard
Diagnostics
those error messages produced by flex (or scanners
it generates) whose meanings might not be apparent
Files
files used by flex
Deficiencies / Bugs
known problems with flex
See Also
other documentation, related tools
Author
includes contact information
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 gen-
erate. The description is in the form of pairs of regu-
lar 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.
SOME SIMPLE EXAMPLES
First some simple examples to get the flavor of how one
uses flex. The following flex input specifies a scanner
which whenever it encounters the string "username" will
replace it with the user's login name:
%%
username printf( "%s", getlogin() );
By default, any text not matched by a flex scanner is
copied to the output, so the net effect of this scanner
is to copy its input file to its output with each occur-
rence of "username" expanded. In this input, there is
just one rule. "username" is the pattern and the
"printf" is the action. The "%%" marks the beginning of
the rules.
Here's another simple example:
int num_lines = 0, num_chars = 0;
%%
\n ++num_lines; ++num_chars;
. ++num_chars;
%%
main()
{
yylex();
printf( "# of lines = %d, # of chars = %d\n",
num_lines, num_chars );
}
This scanner counts the number of characters and the
number of lines in its input (it produces no output
other than the final report on the counts). The first
line declares two globals, "num_lines" and "num_chars",
which are accessible both inside yylex() and in the
main() routine declared after the second "%%". There
are two rules, one which matches a newline ("\n") and
increments both the line count and the character count,
and one which matches any character other than a newline
(indicated by the "." regular expression).
A somewhat more complicated example:
/* scanner for a toy Pascal-like language */
%{
/* need this for the call to atof() below */
#include <math.h>
%}
DIGIT [0-9]
ID [a-z][a-z0-9]*
%%
{DIGIT}+ {
printf( "An integer: %s (%d)\n", yytext,
atoi( yytext ) );
}
{DIGIT}+"."{DIGIT}* {
printf( "A float: %s (%g)\n", yytext,
atof( yytext ) );
}
if|then|begin|end|procedure|function {
printf( "A keyword: %s\n", yytext );
}
{ID} printf( "An identifier: %s\n", yytext );
"+"|"-"|"*"|"/" printf( "An operator: %s\n", yytext );
"{"[^}\n]*"}" /* eat up one-line comments */
[ \t\n]+ /* eat up whitespace */
. printf( "Unrecognized character: %s\n", yytext );
%%
main( argc, argv )
int argc;
char **argv;
{
++argv, --argc; /* skip over program name */
if ( argc > 0 )
yyin = fopen( argv[0], "r" );
else
yyin = stdin;
yylex();
}
This is the beginnings of a simple scanner for a lan-
guage like Pascal. It identifies different types of
tokens and reports on what it has seen.
The details of this example will be explained in the
following sections.
FORMAT OF THE INPUT FILE
The flex input file consists of three sections,
separated by a line with just %% in it:
definitions
%%
rules
%%
user code
The definitions section contains declarations of simple
name definitions to simplify the scanner specification,
and declarations of start conditions, which are
explained in a later section.
Name definitions have the form:
name definition
The "name" is a word beginning with a letter or an
underscore ('_') followed by zero or more letters, dig-
its, '_', or '-' (dash). The definition is taken to
begin at the first non-white-space character following
the name and continuing to the end of the line. The
definition can subsequently be referred to using
"{name}", which will expand to "(definition)". For
example,
DIGIT [0-9]
ID [a-z][a-z0-9]*
defines "DIGIT" to be a regular expression which matches
a single digit, and "ID" to be a regular expression
which matches a letter followed by zero-or-more letters-
or-digits. A subsequent reference to
{DIGIT}+"."{DIGIT}*
is identical to
([0-9])+"."([0-9])*
and matches one-or-more digits followed by a '.' fol-
lowed by zero-or-more digits.
The rules section of the flex input contains a series of
rules of the form:
pattern action
where the pattern must be unindented and the action must
begin on the same line.
See below for a further description of patterns and
actions.
Finally, the user code section is simply copied to
lex.yy.c verbatim. It is used for companion routines
which call or are called by the scanner. The presence
of this section is optional; if it is missing, the sec-
ond %% in the input file may be skipped, too.
In the definitions and rules sections, any indented text
or text enclosed in %{ and %} is copied verbatim to the
output (with the %{}'s removed). The %{}'s must appear
unindented on lines by themselves.
In the rules section, any indented or %{} text appearing
before the first rule may be used to declare variables
which are local to the scanning routine and (after the
declarations) code which is to be executed whenever the
scanning routine is entered. Other indented or %{} text
in the rule section is still copied to the output, but
its meaning is not well-defined and it may well cause
compile-time errors (this feature is present for POSIX
compliance; see below for other such features).
In the definitions section (but not in the rules sec-
tion), an unindented comment (i.e., a line beginning
with "/*") is also copied verbatim to the output up to
the next "*/".
PATTERNS
The patterns in the input are written using an extended
set of regular expressions. These are:
x match the character 'x'
. any character (byte) 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 '*')
\0 a NUL character (ASCII code 0)
\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
text matched by s is included when determining
whether this rule is the "longest match",
but is then returned to the input before
the action is executed. So the action only
sees the text matched by r. This type
of pattern is called trailing context".
(There are some combinations of r/s that flex
cannot match correctly; see notes in the
Deficiencies / Bugs section below regarding
"dangerous trailing context".)
^r an r, but only at the beginning of a line (i.e.,
which just starting to scan, or right after a
newline has been scanned).
r$ an r, but only at the end of a line (i.e., just
before a newline). Equivalent to "r/\n".
Note that flex's notion of "newline" is exactly
whatever the C compiler used to compile flex
interprets '\n' as; in particular, on some DOS
systems you must either filter out \r's in the
input yourself, or explicitly use r/\r\n for "r$".
<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
<*>r an r in any start condition, even an exclusive one.
<<EOF>> an end-of-file
<s1,s2><<EOF>>
an end-of-file when in start condition s1 or s2
Note that inside of a character class, all regular
expression operators lose their special meaning except
escape ('\') and the character class operators, '-',
']', and, at the beginning of the class, '^'.
The regular expressions listed above are grouped accord-
ing to precedence, from highest precedence at the top to
lowest at the bottom. Those grouped together have equal
precedence. For example,
foo|bar*
is the same as
(foo)|(ba(r*))
since the '*' operator has higher precedence than con-
catenation, and concatenation higher than alternation
('|'). This pattern therefore matches either the string
"foo" or the string "ba" followed by zero-or-more r's.
To match "foo" or zero-or-more "bar"'s, use:
foo|(bar)*
and to match zero-or-more "foo"'s-or-"bar"'s:
(foo|bar)*
In addition to characters and ranges of characters,
character classes can also contain character class
expressions. These are expressions enclosed inside [:
and :] delimiters (which themselves must appear between
the '[' and ']' of the character class; other elements
may occur inside the character class, too). The valid
expressions are:
[:alnum:] [:alpha:] [:blank:]
[:cntrl:] [:digit:] [:graph:]
[:lower:] [:print:] [:punct:]
[:space:] [:upper:] [:xdigit:]
These expressions all designate a set of characters
equivalent to the corresponding standard C isXXX func-
tion. For example, [:alnum:] designates those charac-
ters for which isalnum() returns true - i.e., any alpha-
betic or numeric. Some systems don't provide isblank(),
so flex defines [:blank:] as a blank or a tab.
For example, the following character classes are all
equivalent:
[[:alnum:]]
[[:alpha:][:digit:]
[[:alpha:]0-9]
[a-zA-Z0-9]
If your scanner is case-insensitive (the -i flag), then
[:upper:] and [:lower:] are equivalent to [:alpha:].
Some notes on patterns:
- A negated character class such as the example
"[^A-Z]" above will match a newline unless "\n"
(or an equivalent escape sequence) is one of the
characters explicitly present in the negated
character class (e.g., "[^A-Z\n]"). This is
unlike how many other regular expression tools
treat negated character classes, but unfortu-
nately the inconsistency is historically
entrenched. Matching newlines means that a pat-
tern like [^"]* can match the entire input unless
there's another quote in the input.
- 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 '$', cannot be
grouped inside parentheses. A '^' which does not
occur at the beginning of a rule or a '$' which
does not occur at the end of a rule loses its
special properties and is treated as a normal
character.
The following are illegal:
foo/bar$
<sc1>foo<sc2>bar
Note that the first of these, can be written
"foo/bar\n".
The following will result in '$' or '^' being
treated as a normal character:
foo|(bar$)
foo|^bar
If what's wanted is a "foo" or a bar-followed-by-
a-newline, the following could be used (the spe-
cial '|' action is explained below):
foo |
bar$ /* action goes here */
A similar trick will work for matching a foo or a
bar-at-the-beginning-of-a-line.
HOW THE INPUT IS MATCHED
When the generated scanner is run, it analyzes its input
looking for strings which match any of its patterns. If
it finds more than one match, it takes the one matching
the most text (for trailing context rules, this includes
the length of the trailing part, even though it will
then be returned to the input). If it finds two or more
matches of the same length, the rule listed first in the
flex input file is chosen.
Once the match is determined, the text corresponding to
the match (called the token) is made available in the
global character pointer yytext, and its length in the
global integer yyleng. The action corresponding to the
matched pattern is then executed (a more detailed
description of actions follows), and then the remaining
input is scanned for another match.
If no match is found, then the default rule is executed:
the next character in the input is considered matched
and copied to the standard output. Thus, the simplest
legal flex input is:
%%
which generates a scanner that simply copies its input
(one character at a time) to its output.
Note that yytext can be defined in two different ways:
either as a character pointer or as a character array.
You can control which definition flex uses by including
one of the special directives %pointer or %array in the
first (definitions) section of your flex input. The
default is %pointer, unless you use the -l lex compati-
bility option, in which case yytext will be an array.
The advantage of using %pointer is substantially faster
scanning and no buffer overflow when matching very large
tokens (unless you run out of dynamic memory). The dis-
advantage is that you are restricted in how your actions
can modify yytext (see the next section), and calls to
the unput() function destroys the present contents of
yytext, which can be a considerable porting headache
when moving between different lex versions.
The advantage of %array is that you can then modify
yytext to your heart's content, and calls to unput() do
not destroy yytext (see below). Furthermore, existing
lex programs sometimes access yytext externally using
declarations of the form:
extern char yytext[];
This definition is erroneous when used with %pointer,
but correct for %array.
%array defines yytext to be an array of YYLMAX charac-
ters, which defaults to a fairly large value. You can
change the size by simply #define'ing YYLMAX to a dif-
ferent value in the first section of your flex input.
As mentioned above, with %pointer yytext grows dynami-
cally to accommodate large tokens. While this means
your %pointer scanner can accommodate very large tokens
(such as matching entire blocks of comments), bear in
mind that each time the scanner must resize yytext it
also must rescan the entire token from the beginning, so
matching such tokens can prove slow. yytext presently
does not dynamically grow if a call to unput() results
in too much text being pushed back; instead, a run-time
error results.
Also note that you cannot use %array with C++ scanner
classes (the c++ option; see below).
ACTIONS
Each pattern in a rule has a corresponding action, which
can be any arbitrary C statement. The pattern ends at
the first non-escaped whitespace character; the remain-
der of the line is its action. If the action is empty,
then when the pattern is matched the input token is sim-
ply discarded. For example, here is the specification
for a program which deletes all occurrences of "zap me"
from its input:
%%
"zap me"
(It will copy all other characters in the input to the
output since they will be matched by the default rule.)
Here is a program which compresses multiple blanks and
tabs down to a single blank, and throws away whitespace
found at the end of a line:
%%
[ \t]+ putchar( ' ' );
[ \t]+$ /* ignore this token */
If the action contains a '{', then the action spans till
the balancing '}' is found, and the action may cross
multiple lines. flex knows about C strings and comments
and won't be fooled by braces found within them, but
also allows actions to begin with %{ and will consider
the action to be all the text up to the next %} (regard-
less of ordinary braces inside the action).
An action consisting solely of a vertical bar ('|')
means "same as the action for the next rule." See below
for an illustration.
Actions can include arbitrary C code, including return
statements to return a value to whatever routine called
yylex(). Each time yylex() is called it continues pro-
cessing tokens from where it last left off until it
either reaches the end of the file or executes a return.
Actions are free to modify yytext except for lengthening
it (adding characters to its end--these will overwrite
later characters in the input stream). This however
does not apply when using %array (see above); in that
case, yytext may be freely modified in any way.
Actions are free to modify yyleng except they should not
do so if the action also includes use of yymore() (see
below).
There are a number of special directives which can be
included within an action:
- 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 (see below).
- REJECT directs the scanner to proceed on to the
"second best" rule which matched the input (or a
prefix of the input). The rule is chosen as
described above in "How the Input is Matched",
and yytext and yyleng set up appropriately. It
may either be one which matched as much text as
the originally chosen rule but came later in the
flex input file, or one which matched less text.
For example, the following will both count the
words in the input and call the routine special()
whenever "frob" is seen:
int word_count = 0;
%%
frob special(); REJECT;
[^ \t\n]+ ++word_count;
Without the REJECT, any "frob"'s in the input
would not be counted as words, since the scanner
normally executes only one action per token.
Multiple REJECT's are allowed, each one finding
the next best choice to the currently active
rule. For example, when the following scanner
scans the token "abcd", it will write "abcdab-
caba" to the output:
%%
a |
ab |
abc |
abcd ECHO; REJECT;
.|\n /* eat up any unmatched character */
(The first three rules share the fourth's action
since they use the special '|' action.) REJECT
is a particularly expensive feature in terms of
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 -Cf or -CF options (see below).
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. For example, given the input
"mega-kludge" the following will write "mega-
mega-kludge" to the output:
%%
mega- ECHO; yymore();
kludge ECHO;
First "mega-" is matched and echoed to the out-
put. Then "kludge" is matched, but the previous
"mega-" is still hanging around at the beginning
of yytext so the ECHO for the "kludge" rule will
actually write "mega-kludge".
Two notes regarding use of yymore(). First, yymore()
depends on the value of yyleng correctly reflecting the
size of the current token, so you must not modify yyleng
if you are using yymore(). Second, the presence of
yymore() in the scanner's action entails a minor perfor-
mance penalty in the scanner's matching speed.
- 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 ). For example, on the input "foobar"
the following will write out "foobarbar":
%%
foobar ECHO; yyless(3);
[a-z]+ ECHO;
An argument of 0 to yyless will cause the entire
current input string to be scanned again. Unless
you've changed how the scanner will subsequently
process its input (using BEGIN, for example),
this will result in an endless loop.
Note that yyless is a macro and can only be used in the
flex input file, not from other source files.
- unput(c) puts the character c back onto the input
stream. It will be the next character scanned.
The following action will take the current token
and cause it to be rescanned enclosed in paren-
theses.
{
int i;
/* Copy yytext because unput() trashes yytext */
char *yycopy = strdup( yytext );
unput( ')' );
for ( i = yyleng - 1; i >= 0; --i )
unput( yycopy[i] );
unput( '(' );
free( yycopy );
}
Note that since each unput() puts the given char-
acter back at the beginning of the input stream,
pushing back strings must be done back-to-front.
An important potential problem when using unput() is
that if you are using %pointer (the default), a call to
unput() destroys the contents of yytext, starting with
its rightmost character and devouring one character to
the left with each call. If you need the value of
yytext preserved after a call to unput() (as in the
above example), you must either first copy it elsewhere,
or build your scanner using %array instead (see How The
Input Is Matched).
Finally, note that you cannot put back EOF to attempt to
mark the input stream with an end-of-file.
- input() reads the next character from the input
stream. For example, the following is one way to
eat up C comments:
%%
"/*" {
register int c;
for ( ; ; )
{
while ( (c = input()) != '*' &&
c != EOF )
; /* eat up text of comment */
if ( c == '*' )
{
while ( (c = input()) == '*' )
;
if ( c == '/' )
break; /* found the end */
}
if ( c == EOF )
{
error( "EOF in comment" );
break;
}
}
}
(Note that if the scanner is compiled using C++,
then input() is instead referred to as yyinput(),
in order to avoid a name clash with the C++
stream by the name of input.)
- YY_FLUSH_BUFFER flushes the scanner's internal
buffer so that the next time the scanner attempts
to match a token, it will first refill the buffer
using YY_INPUT (see The Generated Scanner,
below). This action is a special case of the
more general yy_flush_buffer() function,
described below in the section Multiple Input
Buffers.
- yyterminate() can be used in lieu of a return
statement in an action. It terminates the scan-
ner and returns a 0 to the scanner's caller,
indicating "all done". By default, yyterminate()
is also called when an end-of-file is encoun-
tered. It is a macro and may be redefined.
THE GENERATED SCANNER
The output of flex is the file lex.yy.c, which contains
the scanning routine yylex(), a number of tables used by
it for matching tokens, and a number of auxiliary rou-
tines and macros. By default, yylex() is declared as
follows:
int yylex()
{
... various definitions and the actions in here ...
}
(If your environment supports function prototypes, then
it will be "int yylex( void )".) This definition may be
changed by defining the "YY_DECL" macro. For example,
you could use:
#define YY_DECL float lexscan( a, b ) float a, b;
to give the scanning routine the name lexscan, returning
a float, and taking two floats as arguments. 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 (;).
Whenever yylex() is called, it scans tokens from the
global input file yyin (which defaults to stdin). It
continues until it either reaches an end-of-file (at
which point it returns the value 0) or one of its
actions executes a return statement.
If the scanner reaches an end-of-file, subsequent calls
are undefined unless either yyin is pointed at a new
input file (in which case scanning continues from that
file), or yyrestart() is called. yyrestart() takes one
argument, a FILE * pointer (which can be nil, if you've
set up YY_INPUT to scan from a source other than yyin),
and initializes yyin for scanning from that file.
Essentially there is no difference between just assign-
ing yyin to a new input file or using yyrestart() to do
so; the latter is available for compatibility with pre-
vious versions of flex, and because it can be used to
switch input files in the middle of scanning. It can
also be used to throw away the current input buffer, by
calling it with an argument of yyin; but better is to
use YY_FLUSH_BUFFER (see above). Note that yyrestart()
does not reset the start condition to INITIAL (see Start
Conditions, below).
If yylex() stops scanning due to executing a return
statement in one of the actions, the scanner may then be
called again and it will resume scanning where it left
off.
By default (and for purposes of efficiency), the scanner
uses block-reads rather than simple getc() calls to read
characters from yyin. The nature of how it gets its
input can be controlled by defining 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 definition of YY_INPUT (in the definitions sec-
tion of the input file):
%{
#define YY_INPUT(buf,result,max_size) \
{ \
int c = getchar(); \
result = (c == EOF) ? YY_NULL : (buf[0] = c, 1); \
}
%}
This definition will change the input processing to
occur one character at a time.
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. Note that in either case,
the start condition remains unchanged; it does not
revert to INITIAL.
If you do not supply your own version of yywrap(), then
you must either use %option noyywrap (in which case the
scanner behaves as though yywrap() returned 1), or you
must link with -lfl to obtain the default version of the
routine, which always returns 1.
Three routines are available for scanning from in-memory
buffers rather than files: yy_scan_string(),
yy_scan_bytes(), and yy_scan_buffer(). See the discus-
sion of them below in the section Multiple Input
Buffers.
The scanner writes its ECHO output to the yyout global
(default, stdout), which may be redefined by the user
simply by assigning it to some other FILE pointer.
START CONDITIONS
flex provides a mechanism for conditionally activating
rules. Any rule whose pattern is prefixed with "<sc>"
will only be active when the scanner is in the start
condition named "sc". For example,
<STRING>[^"]* { /* eat up the string body ... */
...
}
will be active only when the scanner is in the "STRING"
start condition, and
<INITIAL,STRING,QUOTE>\. { /* handle an escape ... */
...
}
will be active only when the current start condition is
either "INITIAL", "STRING", or "QUOTE".
Start conditions are declared in the definitions (first)
section of the input using unindented lines beginning
with either %s or %x followed by a list of names. The
former declares inclusive start conditions, the latter
exclusive start conditions. A start condition is acti-
vated using the BEGIN action. Until the next BEGIN
action is executed, rules with the given start condition
will be active and rules with other start conditions
will be inactive. If the start condition is inclusive,
then rules with no start conditions at all will also be
active. If it is exclusive, then only rules qualified
with the start condition will be active. A set of rules
contingent on the same exclusive start condition
describe a scanner which is independent of any of the
other rules in the flex input. Because of this, exclu-
sive start conditions make it easy to specify "mini-
scanners" which scan portions of the input that are syn-
tactically different from the rest (e.g., comments).
If the distinction between inclusive and exclusive start
conditions is still a little vague, here's a simple
example illustrating the connection between the two.
The set of rules:
%s example
%%
<example>foo do_something();
bar something_else();
is equivalent to
%x example
%%
<example>foo do_something();
<INITIAL,example>bar something_else();
Without the <INITIAL,example> qualifier, the bar pattern
in the second example wouldn't be active (i.e., couldn't
match) when in start condition example. If we just used
<example> to qualify bar, though, then it would only be
active in example and not in INITIAL, while in the first
example it's active in both, because in the first exam-
ple the example startion condition is an inclusive (%s)
start condition.
Also note that the special start-condition specifier <*>
matches every start condition. Thus, the above example
could also have been written;
%x example
%%
<example>foo do_something();
<*>bar something_else();
The default rule (to ECHO any unmatched character)
remains active in start conditions. It is equivalent
to:
<*>.|\n ECHO;
BEGIN(0) returns to the original state where only the
rules with no start conditions are active. This state
can also be referred to as the start-condition "INI-
TIAL", so BEGIN(INITIAL) is equivalent to BEGIN(0).
(The parentheses around the start condition name are not
required but are considered good style.)
BEGIN actions can also be given as indented code at the
beginning of the rules section. For example, the fol-
lowing will cause the scanner to enter the "SPECIAL"
start condition whenever yylex() is called and the
global variable enter_special is true:
int enter_special;
%x SPECIAL
%%
if ( enter_special )
BEGIN(SPECIAL);
<SPECIAL>blahblahblah
...more rules follow...
To illustrate the uses of start conditions, here is a
scanner which provides two different interpretations of
a string like "123.456". By default it will treat it as
three tokens, the integer "123", a dot ('.'), and the
integer "456". But if the string is preceded earlier in
the line by the string "expect-floats" it will treat it
as a single token, the floating-point number 123.456:
%{
#include <math.h>
%}
%s expect
%%
expect-floats BEGIN(expect);
<expect>[0-9]+"."[0-9]+ {
printf( "found a float, = %f\n",
atof( yytext ) );
}
<expect>\n {
/* that's the end of the line, so
* we need another "expect-number"
* before we'll recognize any more
* numbers
*/
BEGIN(INITIAL);
}
[0-9]+ {
printf( "found an integer, = %d\n",
atoi( yytext ) );
}
"." printf( "found a dot\n" );
Here is a scanner which recognizes (and discards) C com-
ments while maintaining a count of the current input
line.
%x comment
%%
int line_num = 1;
"/*" BEGIN(comment);
<comment>[^*\n]* /* eat anything that's not a '*' */
<comment>"*"+[^*/\n]* /* eat up '*'s not followed by '/'s */
<comment>\n ++line_num;
<comment>"*"+"/" BEGIN(INITIAL);
This scanner goes to a bit of trouble to match as much
text as possible with each rule. In general, when
attempting to write a high-speed scanner try to match as
much possible in each rule, as it's a big win.
Note that start-conditions names are really integer val-
ues and can be stored as such. Thus, the above could be
extended in the following fashion:
%x comment foo
%%
int line_num = 1;
int comment_caller;
"/*" {
comment_caller = INITIAL;
BEGIN(comment);
}
...
<foo>"/*" {
comment_caller = foo;
BEGIN(comment);
}
<comment>[^*\n]* /* eat anything that's not a '*' */
<comment>"*"+[^*/\n]* /* eat up '*'s not followed by '/'s */
<comment>\n ++line_num;
<comment>"*"+"/" BEGIN(comment_caller);
Furthermore, you can access the current start condition
using the integer-valued YY_START macro. For example,
the above assignments to comment_caller could instead be
written
comment_caller = YY_START;
Flex provides YYSTATE as an alias for YY_START (since
that is what's used by AT&T lex).
Note that start conditions do not have their own name-
space; %s's and %x's declare names in the same fashion
as #define's.
Finally, here's an example of how to match C-style
quoted strings using exclusive start conditions, includ-
ing expanded escape sequences (but not including check-
ing for a string that's too long):
%x str
%%
char string_buf[MAX_STR_CONST];
char *string_buf_ptr;
\" string_buf_ptr = string_buf; BEGIN(str);
<str>\" { /* saw closing quote - all done */
BEGIN(INITIAL);
*string_buf_ptr = '\0';
/* return string constant token type and
* value to parser
*/
}
<str>\n {
/* error - unterminated string constant */
/* generate error message */
}
<str>\\[0-7]{1,3} {
/* octal escape sequence */
int result;
(void) sscanf( yytext + 1, "%o", &result );
if ( result > 0xff )
/* error, constant is out-of-bounds */
*string_buf_ptr++ = result;
}
<str>\\[0-9]+ {
/* generate error - bad escape sequence; something
* like '\48' or '\0777777'
*/
}
<str>\\n *string_buf_ptr++ = '\n';
<str>\\t *string_buf_ptr++ = '\t';
<str>\\r *string_buf_ptr++ = '\r';
<str>\\b *string_buf_ptr++ = '\b';
<str>\\f *string_buf_ptr++ = '\f';
<str>\\(.|\n) *string_buf_ptr++ = yytext[1];
<str>[^\\\n\"]+ {
char *yptr = yytext;
while ( *yptr )
*string_buf_ptr++ = *yptr++;
}
Often, such as in some of the examples above, you wind
up writing a whole bunch of rules all preceded by the
same start condition(s). Flex makes this a little eas-
ier and cleaner by introducing a notion of start condi-
tion scope. A start condition scope is begun with:
<SCs>{
where SCs is a list of one or more start conditions.
Inside the start condition scope, every rule automati-
cally has the prefix <SCs> applied to it, until a '}'
which matches the initial '{'. So, for example,
<ESC>{
"\\n" return '\n';
"\\r" return '\r';
"\\f" return '\f';
"\\0" return '\0';
}
is equivalent to:
<ESC>"\\n" return '\n';
<ESC>"\\r" return '\r';
<ESC>"\\f" return '\f';
<ESC>"\\0" return '\0';
Start condition scopes may be nested.
Three routines are available for manipulating stacks of
start conditions:
void yy_push_state(int new_state)
pushes the current start condition onto the top
of the start condition stack and switches to
new_state as though you had used BEGIN new_state
(recall that start condition names are also inte-
gers).
void yy_pop_state()
pops the top of the stack and switches to it via
BEGIN.
int yy_top_state()
returns the top of the stack without altering the
stack's contents.
The start condition stack grows dynamically and so has
no built-in size limitation. If memory is exhausted,
program execution aborts.
To use start condition stacks, your scanner must include
a %option stack directive (see Options below).
MULTIPLE INPUT BUFFERS
Some scanners (such as those which support "include"
files) require reading from several input streams. As
flex scanners do a large amount of buffering, one cannot
control where the next input will be read from by simply
writing a YY_INPUT which is sensitive to the scanning
context. YY_INPUT is only called when the scanner
reaches the end of its buffer, which may be a long time
after scanning a statement such as an "include" which
requires switching the input source.
To negotiate these sorts of problems, flex provides a
mechanism for creating and switching between multiple
input buffers. An input buffer is created by using:
YY_BUFFER_STATE yy_create_buffer( FILE *file, int size )
which takes a FILE pointer and a size and creates a
buffer associated with the given file and large enough
to hold size characters (when in doubt, use YY_BUF_SIZE
for the size). It returns a YY_BUFFER_STATE handle,
which may then be passed to other routines (see below).
The YY_BUFFER_STATE type is a pointer to an opaque
struct yy_buffer_state structure, so you may safely ini-
tialize YY_BUFFER_STATE variables to ((YY_BUFFER_STATE)
0) if you wish, and also refer to the opaque structure
in order to correctly declare input buffers in source
files other than that of your scanner. Note that the
FILE pointer in the call to yy_create_buffer is only
used as the value of yyin seen by YY_INPUT; if you rede-
fine YY_INPUT so it no longer uses yyin, then you can
safely pass a nil FILE pointer to yy_create_buffer. You
select a particular buffer to scan from using:
void yy_switch_to_buffer( YY_BUFFER_STATE new_buffer )
switches the scanner's input buffer so subsequent tokens
will come from new_buffer. Note that
yy_switch_to_buffer() may be used by yywrap() to set
things up for continued scanning, instead of opening a
new file and pointing yyin at it. Note also that
switching input sources via either yy_switch_to_buffer()
or yywrap() does not change the start condition.
void yy_delete_buffer( YY_BUFFER_STATE buffer )
is used to reclaim the storage associated with a buffer.
( buffer can be nil, in which case the routine does
nothing.) You can also clear the current contents of a
buffer using:
void yy_flush_buffer( YY_BUFFER_STATE buffer )
This function discards the buffer's contents, so the
next time the scanner attempts to match a token from the
buffer, it will first fill the buffer anew using
YY_INPUT.
yy_new_buffer() is an alias for yy_create_buffer(), pro-
vided for compatibility with the C++ use of new and
delete for creating and destroying dynamic objects.
Finally, the YY_CURRENT_BUFFER macro returns a
YY_BUFFER_STATE handle to the current buffer.
Here is an example of using these features for writing a
scanner which expands include files (the <<EOF>> feature
is discussed below):
/* the "incl" state is used for picking up the name
* of an include file
*/
%x incl
%{
#define MAX_INCLUDE_DEPTH 10
YY_BUFFER_STATE include_stack[MAX_INCLUDE_DEPTH];
int include_stack_ptr = 0;
%}
%%
include BEGIN(incl);
[a-z]+ ECHO;
[^a-z\n]*\n? ECHO;
<incl>[ \t]* /* eat the whitespace */
<incl>[^ \t\n]+ { /* got the include file name */
if ( include_stack_ptr >= MAX_INCLUDE_DEPTH )
{
fprintf( stderr, "Includes nested too deeply" );
exit( 1 );
}
include_stack[include_stack_ptr++] =
YY_CURRENT_BUFFER;
yyin = fopen( yytext, "r" );
if ( ! yyin )
error( ... );
yy_switch_to_buffer(
yy_create_buffer( yyin, YY_BUF_SIZE ) );
BEGIN(INITIAL);
}
<<EOF>> {
if ( --include_stack_ptr < 0 )
{
yyterminate();
}
else
{
yy_delete_buffer( YY_CURRENT_BUFFER );
yy_switch_to_buffer(
include_stack[include_stack_ptr] );
}
}
Three routines are available for setting up input
buffers for scanning in-memory strings instead of files.
All of them create a new input buffer for scanning the
string, and return a corresponding YY_BUFFER_STATE han-
dle (which you should delete with yy_delete_buffer()
when done with it). They also switch to the new buffer
using yy_switch_to_buffer(), so the next call to yylex()
will start scanning the string.
yy_scan_string(const char *str)
scans a NUL-terminated string.
yy_scan_bytes(const char *bytes, int len)
scans len bytes (including possibly NUL's) start-
ing at location bytes.
Note that both of these functions create and scan a copy
of the string or bytes. (This may be desirable, since
yylex() modifies the contents of the buffer it is scan-
ning.) You can avoid the copy by using:
yy_scan_buffer(char *base, yy_size_t size)
which scans in place the buffer starting at base,
consisting of size bytes, the last two bytes of
which must be YY_END_OF_BUFFER_CHAR (ASCII NUL).
These last two bytes are not scanned; thus, scan-
ning consists of base[0] through base[size-2],
inclusive.
If you fail to set up base in this manner (i.e.,
forget the final two YY_END_OF_BUFFER_CHAR
bytes), then yy_scan_buffer() returns a nil
pointer instead of creating a new input buffer.
The type yy_size_t is an integral type to which
you can cast an integer expression reflecting the
size of the buffer.
END-OF-FILE RULES
The special rule "<<EOF>>" indicates actions which are
to be taken when an end-of-file is encountered and
yywrap() returns non-zero (i.e., indicates no further
files to process). The action must finish by doing one
of four things:
- assigning yyin to a new input file (in previous
versions of flex, after doing the assignment you
had to call the special action YY_NEW_FILE; this
is no longer necessary);
- executing a return statement;
- executing the special yyterminate() action;
- or, switching to a new buffer using
yy_switch_to_buffer() as shown in the example
above.
<<EOF>> rules may not be used with other patterns; they
may only be qualified with a list of start conditions.
If an unqualified <<EOF>> rule is given, it applies to
all start conditions which do not already have <<EOF>>
actions. To specify an <<EOF>> rule for only the ini-
tial start condition, use
<INITIAL><<EOF>>
These rules are useful for catching things like unclosed
comments. An example:
%x quote
%%
...other rules for dealing with quotes...
<quote><<EOF>> {
error( "unterminated quote" );
yyterminate();
}
<<EOF>> {
if ( *++filelist )
yyin = fopen( *filelist, "r" );
else
yyterminate();
}
MISCELLANEOUS MACROS
The macro YY_USER_ACTION can be defined to provide an
action which is always executed prior to the matched
rule's action. For example, it could be #define'd to
call a routine to convert yytext to lower-case. When
YY_USER_ACTION is invoked, the variable yy_act gives the
number of the matched rule (rules are numbered starting
with 1). Suppose you want to profile how often each of
your rules is matched. The following would do the
trick:
#define YY_USER_ACTION ++ctr[yy_act]
where ctr is an array to hold the counts for the differ-
ent rules. Note that the macro YY_NUM_RULES gives the
total number of rules (including the default rule, even
if you use -s), so a correct declaration for ctr is:
int ctr[YY_NUM_RULES];
The macro YY_USER_INIT may be defined to provide an
action which is always executed before the first scan
(and before the scanner's internal initializations are
done). For example, it could be used to call a routine
to read in a data table or open a logging file.
The macro yy_set_interactive(is_interactive) can be used
to control whether the current buffer is considered
interactive. An interactive buffer is processed more
slowly, but must be used when the scanner's input source
is indeed interactive to avoid problems due to waiting
to fill buffers (see the discussion of the -I flag
below). A non-zero value in the macro invocation marks
the buffer as interactive, a zero value as non-interac-
tive. Note that use of this macro overrides %option
always-interactive or %option never-interactive (see
Options below). yy_set_interactive() must be invoked
prior to beginning to scan the buffer that is (or is
not) to be considered interactive.
The macro yy_set_bol(at_bol) can be used to control
whether the current buffer's scanning context for the
next token match is done as though at the beginning of a
line. A non-zero macro argument makes rules anchored
with
The macro YY_AT_BOL() returns true if the next token
scanned from the current buffer will have '^' rules
active, false otherwise.
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. Redefining YY_BREAK allows, for
example, C++ users to #define YY_BREAK to do nothing
(while being very careful that every rule ends with a
"break" or a "return"!) to avoid suffering from unreach-
able statement warnings where because a rule's action
ends with "return", the YY_BREAK is inaccessible.
VALUES AVAILABLE TO THE USER
This section summarizes the various values available to
the user in the rule actions.
- char *yytext holds the text of the current token.
It may be modified but not lengthened (you cannot
append characters to the end).
If the special directive %array appears in the
first section of the scanner description, then
yytext is instead declared char yytext[YYLMAX],
where YYLMAX is a macro definition that you can
redefine in the first section if you don't like
the default value (generally 8KB). Using %array
results in somewhat slower scanners, but the
value of yytext becomes immune to calls to
input() and unput(), which potentially destroy
its value when yytext is a character pointer.
The opposite of %array is %pointer, which is the
default.
You cannot use %array when generating C++ scanner
classes (the -+ flag).
- int yyleng holds the length of the current token.
- FILE *yyin is the file which by default flex
reads from. It may be redefined but doing so
only makes sense before scanning begins or after
an EOF has been encountered. Changing it in the
midst of scanning will have unexpected results
since flex buffers its input; use yyrestart()
instead. Once scanning terminates because an
end-of-file has been seen, you can assign yyin at
the new input file and then call the scanner
again to continue scanning.
- void yyrestart( FILE *new_file ) may be called to
point yyin at the new input file. The switch-
over to the new file is immediate (any previously
buffered-up input is lost). Note that calling
yyrestart() with yyin as an argument thus throws
away the current input buffer and continues scan-
ning the same 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 han-
dle to the current buffer.
- YY_START returns an integer value corresponding
to the current start condition. You can subse-
quently use this value with BEGIN to return to
that start condition.
INTERFACING WITH YACC
One of the main uses of flex is as a companion to the
yacc parser-generator. yacc parsers expect to call a
routine named yylex() to find the next input token. The
routine is supposed to return the type of the next token
as well as putting any associated value in the global
yylval. To use flex with yacc, one specifies the -d
option to yacc to instruct it to generate the file
y.tab.h containing definitions of all the %tokens
appearing in the yacc input. This file is then included
in the flex scanner. For example, if one of the tokens
is "TOK_NUMBER", part of the scanner might look like:
%{
#include "y.tab.h"
%}
%%
[0-9]+ yylval = atoi( yytext ); return TOK_NUMBER;
OPTIONS
flex has the following options:
-b Generate backing-up information to lex.backup.
This is a list of scanner states which require
backing up and the input characters on which they
do so. By adding rules one can remove backing-up
states. If all backing-up states are eliminated
and -Cf or -CF is used, the generated scanner
will run faster (see the -p flag). Only users
who wish to squeeze every last cycle out of their
scanners need worry about this option. (See the
section on Performance Considerations below.)
-c is a do-nothing, deprecated option included for
POSIX compliance.
-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 backs up, accepts the
default rule, reaches the end of its input buffer
(or encounters a NUL; at this point, the two look
the same as far as the scanner's concerned), or
reaches an end-of-file.
-f specifies fast scanner. No table compression is
done and stdio is bypassed. The result is large
but fast. This option is equivalent to -Cfr (see
below).
-h generates a "help" summary of flex's options to
stdout and then exits. -? and --help are syn-
onyms for -h.
-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 pre-
served case (i.e., it will not be folded).
-l turns on maximum compatibility with the original
AT&T lex implementation. Note that this does not
mean full compatibility. Use of this option
costs a considerable amount of performance, and
it cannot be used with the -+, -f, -F, -Cf, or
-CF options. For details on the compatibilities
it provides, see the section "Incompatibilities
With Lex And POSIX" below. This option also
results in the name YY_FLEX_LEX_COMPAT being
#define'd in the generated scanner.
-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 serious
loss of performance in the resulting scanner. If
you give the flag twice, you will also get com-
ments regarding features that lead to minor per-
formance losses.
Note that the use of REJECT, %option yylineno,
and variable trailing context (see the Deficien-
cies / Bugs section below) entails a substantial
performance penalty; use of yymore(), the ^ oper-
ator, and the -I flag entail minor performance
penalties.
-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.
-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 sum-
mary of statistics regarding the scanner it gen-
erates. Most of the statistics are meaningless
to the casual flex user, but the first line iden-
tifies the version of flex (same as reported by
-V), and the next line the flags used when gener-
ating the scanner, including those that are on by
default.
-w suppresses warning messages.
-B instructs flex to generate a batch scanner, the
opposite of interactive scanners generated by -I
(see below). In general, you use -B when you are
certain that your scanner will never be used
interactively, and you want to squeeze a little
more performance out of it. If your goal is
instead to squeeze out a lot more performance,
you should be using the -Cf or -CF options (dis-
cussed below), which turn on -B automatically
anyway.
-F specifies that the fast scanner table representa-
tion should be used (and stdio bypassed). This
representation is about as fast as the full table
representation (-f), and for some sets of pat-
terns will be considerably smaller (and for oth-
ers, larger). In general, if the pattern set
contains both "keywords" and a catch-all, "iden-
tifier" 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 rep-
resentation. 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 -CFr (see below).
It cannot be used with -+.
-I instructs flex to generate an interactive scan-
ner. An interactive scanner is one that only
looks ahead to decide what token has been matched
if it absolutely must. It turns out that always
looking one extra character ahead, even if the
scanner has already seen enough text to disam-
biguate the current token, is a bit faster than
only looking ahead when necessary. But scanners
that always look ahead give dreadful interactive
performance; for example, when a user types a
newline, it is not recognized as a newline token
until they enter another token, which often means
typing in another whole line.
Flex scanners default to interactive unless you
use the -Cf or -CF table-compression options (see
below). That's because if you're looking for
high-performance you should be using one of these
options, so if you didn't, flex assumes you'd
rather trade off a bit of run-time performance
for intuitive interactive behavior. Note also
that you cannot use -I in conjunction with -Cf or
-CF. Thus, this option is not really needed; it
is on by default for all those cases in which it
is allowed.
You can force a scanner to not be interactive by
using -B (see above).
-L instructs flex not to generate #line directives.
Without this option, flex peppers the generated
scanner with #line directives so error messages
in the actions will be correctly located with
respect to either the original flex input file
(if the errors are due to code in the input
file), or lex.yy.c (if the errors are flex's
fault -- you should report these sorts of errors
to the email address given below).
-T makes flex run in trace mode. It will generate a
lot of messages to stderr concerning the form of
the input and the resultant non-deterministic and
deterministic finite automata. This option is
mostly for use in maintaining flex.
-V prints the version number to stdout and exits.
--version is a synonym for -V.
-7 instructs flex to generate a 7-bit scanner, i.e.,
one which can only recognized 7-bit characters in
its input. The advantage of using -7 is that the
scanner's tables can be up to half the size of
those generated using the -8 option (see below).
The disadvantage is that such scanners often hang
or crash if their input contains an 8-bit charac-
ter.
Note, however, that unless you generate your
scanner using the -Cf or -CF table compression
options, use of -7 will save only a small amount
of table space, and make your scanner consider-
ably less portable. Flex's default behavior is
to generate an 8-bit scanner unless you use the
-Cf or -CF, in which case flex defaults to gener-
ating 7-bit scanners unless your site was always
configured to generate 8-bit scanners (as will
often be the case with non-USA sites). You can
tell whether flex generated a 7-bit or an 8-bit
scanner by inspecting the flag summary in the -v
output as described above.
Note that if you use -Cfe or -CFe (those table
compression options, but also using equivalence
classes as discussed see below), flex still
defaults to generating an 8-bit scanner, since
usually with these compression options full 8-bit
tables are not much more expensive than 7-bit
tables.
-8 instructs flex to generate an 8-bit scanner,
i.e., one which can recognize 8-bit characters.
This flag is only needed for scanners generated
using -Cf or -CF, as otherwise flex defaults to
generating an 8-bit scanner anyway.
See the discussion of -7 above for flex's default
behavior and the tradeoffs between 7-bit and
8-bit scanners.
-+ specifies that you want flex to generate a C++
scanner class. See the section on Generating C++
Scanners below for details.
-C[aefFmr]
controls the degree of table compression and,
more generally, trade-offs between small scanners
and fast scanners.
-Ca ("align") instructs flex to trade off larger
tables in the generated scanner for faster per-
formance because the elements of the tables are
better aligned for memory access and computation.
On some RISC architectures, fetching and manipu-
lating longwords is more efficient than with
smaller-sized units such as shortwords. This
option can double the size of the tables used by
your scanner.
-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 equiva-
lence class). 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 above under the -F
flag) should be used. This option cannot be used
with -+.
-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).
-Cr causes the generated scanner to bypass use of
the standard I/O library (stdio) for input.
Instead of calling fread() or getc(), the scanner
will use the read() system call, resulting in a
performance gain which varies from system to sys-
tem, but in general is probably negligible unless
you are also using -Cf or -CF. Using -Cr can
cause strange behavior if, for example, you read
from yyin using stdio prior to calling the scan-
ner (because the scanner will miss whatever text
your previous reads left in the stdio input
buffer).
-Cr has no effect if you define YY_INPUT (see The
Generated Scanner above).
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, and are cumulative.
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}
-C{f,F}a
fastest & largest
Note that scanners with the smallest tables are
usually generated and compiled the quickest, so
during development you will usually want to use
the default, maximal compression.
-Cfe is often a good compromise between speed and
size for production scanners.
-ooutput
directs flex to write the scanner to the file
output instead of lex.yy.c. If you combine -o
with the -t option, then the scanner is written
to stdout but its #line directives (see the -L
option above) refer to the file output.
-Pprefix
changes the default yy prefix used by flex for
all globally-visible variable and function names
to instead be prefix. For example, -Pfoo changes
the name of yytext to footext. It also changes
the name of the default output file from lex.yy.c
to lex.foo.c. Here are all of the names
affected:
yy_create_buffer
yy_delete_buffer
yy_flex_debug
yy_init_buffer
yy_flush_buffer
yy_load_buffer_state
yy_switch_to_buffer
yyin
yyleng
yylex
yylineno
yyout
yyrestart
yytext
yywrap
(If you are using a C++ scanner, then only yywrap
and yyFlexLexer are affected.) Within your scan-
ner itself, you can still refer to the global
variables and functions using either version of
their name; but externally, they have the modi-
fied name.
This option lets you easily link together multi-
ple flex programs into the same executable.
Note, though, that using this option also renames
yywrap(), so you now must either provide your own
(appropriately-named) version of the routine for
your scanner, or use %option noyywrap, as linking
with -lfl no longer provides one for you by
default.
-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.
flex also provides a mechanism for controlling options
within the scanner specification itself, rather than
from the flex command-line. This is done by including
%option directives in the first section of the scanner
specification. You can specify multiple options with a
single %option directive, and multiple directives in the
first section of your flex input file.
Most options are given simply as names, optionally pre-
ceded by the word "no" (with no intervening whitespace)
to negate their meaning. A number are equivalent to
flex flags or their negation:
7bit -7 option
8bit -8 option
align -Ca option
backup -b option
batch -B option
c++ -+ option
caseful or
case-sensitive opposite of -i (default)
case-insensitive or
caseless -i option
debug -d option
default opposite of -s option
ecs -Ce option
fast -F option
full -f option
interactive -I option
lex-compat -l option
meta-ecs -Cm option
perf-report -p option
read -Cr option
stdout -t option
verbose -v option
warn opposite of -w option
(use "%option nowarn" for -w)
array equivalent to "%array"
pointer equivalent to "%pointer" (default)
Some %option's provide features otherwise not available:
always-interactive
instructs flex to generate a scanner which always
considers its input "interactive". Normally, on
each new input file the scanner calls isatty() in
an attempt to determine whether the scanner's
input source is interactive and thus should be
read a character at a time. When this option is
used, however, then no such call is made.
main directs flex to provide a default main() program
for the scanner, which simply calls yylex().
This option implies noyywrap (see below).
never-interactive
instructs flex to generate a scanner which never
considers its input "interactive" (again, no call
made to isatty()). This is the opposite of
always-interactive.
stack enables the use of start condition stacks (see
Start Conditions above).
stdinit
if set (i.e., %option stdinit) initializes yyin
and yyout to stdin and stdout, instead of the
default of nil. Some existing lex programs
depend on this behavior, even though it is not
compliant with ANSI C, which does not require
stdin and stdout to be compile-time constant.
yylineno
directs flex to generate a scanner that maintains
the number of the current line read from its
input in the global variable yylineno. This
option is implied by %option lex-compat.
yywrap if unset (i.e., %option noyywrap), makes the
scanner not call yywrap() upon an end-of-file,
but simply assume that there are no more files to
scan (until the user points yyin at a new file
and calls yylex() again).
flex scans your rule actions to determine whether you
use the REJECT or yymore() features. The reject and
yymore options are available to override its decision as
to whether you use the options, either by setting them
(e.g., %option reject) to indicate the feature is indeed
used, or unsetting them to indicate it actually is not
used (e.g., %option noyymore).
Three options take string-delimited values, offset with
'=':
%option outfile="ABC"
is equivalent to -oABC, and
%option prefix="XYZ"
is equivalent to -PXYZ. Finally,
%option yyclass="foo"
only applies when generating a C++ scanner ( -+ option).
It informs flex that you have derived foo as a subclass
of yyFlexLexer, so flex will place your actions in the
member function foo::yylex() instead of
yyFlexLexer::yylex(). It also generates a
yyFlexLexer::yylex() member function that emits a run-
time error (by invoking yyFlexLexer::LexerError()) if
called. See Generating C++ Scanners, below, for addi-
tional information.
A number of options are available for lint purists who
want to suppress the appearance of unneeded routines in
the generated scanner. Each of the following, if unset
(e.g., %option nounput ), results in the corresponding
routine not appearing in the generated scanner:
input, unput
yy_push_state, yy_pop_state, yy_top_state
yy_scan_buffer, yy_scan_bytes, yy_scan_string
(though yy_push_state() and friends won't appear anyway
unless you use %option stack).
PERFORMANCE CONSIDERATIONS
The main design goal of flex is that it generate high-
performance scanners. It has been optimized for dealing
well with large sets of rules. Aside from the effects
on scanner speed of the table compression -C options
outlined above, there are a number of options/actions
which degrade performance. These are, from most expen-
sive to least:
REJECT
%option yylineno
arbitrary trailing context
pattern sets that require backing up
%array
%option interactive
%option always-interactive
'^' beginning-of-line operator
yymore()
with the first three all being quite expensive and the
last two being quite cheap. Note also that unput() is
implemented as a routine call that potentially does
quite a bit of work, while yyless() is a quite-cheap
macro; so if just putting back some excess text you
scanned, use yyless().
REJECT should be avoided at all costs when performance
is important. It is a particularly expensive option.
Getting rid of backing up is messy and often may be an
enormous amount of work for a complicated scanner. In
principal, one begins by using the -b flag to generate a
lex.backup file. For example, on the input
%%
foo return TOK_KEYWORD;
foobar return TOK_KEYWORD;
the file looks like:
State #6 is non-accepting -
associated rule line numbers:
2 3
out-transitions: [ o ]
jam-transitions: EOF [ \001-n p-\177 ]
State #8 is non-accepting -
associated rule line numbers:
3
out-transitions: [ a ]
jam-transitions: EOF [ \001-` b-\177 ]
State #9 is non-accepting -
associated rule line numbers:
3
out-transitions: [ r ]
jam-transitions: EOF [ \001-q s-\177 ]
Compressed tables always back up.
The first few lines tell us that there's a scanner state
in which it can make a transition on an 'o' but not on
any other character, and that in that state the cur-
rently scanned text does not match any rule. The state
occurs when trying to match the rules found at lines 2
and 3 in the input file. If the scanner is in that
state and then reads something other than an 'o', it
will have to back up to find a rule which is matched.
With a bit of headscratching one can see that this must
be the state it's in when it has seen "fo". When this
has happened, if anything other than another 'o' is
seen, the scanner will have to back up to simply match
the 'f' (by the default rule).
The comment regarding State #8 indicates there's a prob-
lem when "foob" has been scanned. Indeed, on any char-
acter other than an 'a', the scanner will have to back
up to accept "foo". Similarly, the comment for State #9
concerns when "fooba" has been scanned and an 'r' does
not follow.
The final comment reminds us that there's no point going
to all the trouble of removing backing up from the rules
unless we're using -Cf or -CF, since there's no perfor-
mance gain doing so with compressed scanners.
The way to remove the backing up is to add "error"
rules:
%%
foo return TOK_KEYWORD;
foobar return TOK_KEYWORD;
fooba |
foob |
fo {
/* false alarm, not really a keyword */
return TOK_ID;
}
Eliminating backing up among a list of keywords can also
be done using a "catch-all" rule:
%%
foo return TOK_KEYWORD;
foobar return TOK_KEYWORD;
[a-z]+ return TOK_ID;
This is usually the best solution when appropriate.
Backing up messages tend to cascade. With a complicated
set of rules it's not uncommon to get hundreds of mes-
sages. If one can decipher them, though, it often only
takes a dozen or so rules to eliminate the backing up
(though it's easy to make a mistake and have an error
rule accidentally match a valid token. A possible
future flex feature will be to automatically add rules
to eliminate backing up).
It's important to keep in mind that you gain the bene-
fits of eliminating backing up only if you eliminate
every instance of backing up. Leaving just one means
you gain nothing.
Variable trailing context (where both the leading and
trailing parts do not have a fixed length) entails
almost the same performance loss as REJECT (i.e., sub-
stantial). So when possible a rule like:
%%
mouse|rat/(cat|dog) run();
is better written:
%%
mouse/cat|dog run();
rat/cat|dog run();
or as
%%
mouse|rat/cat run();
mouse|rat/dog run();
Note that here the special '|' action does not provide
any savings, and can even make things worse (see Defi-
ciencies / Bugs below).
Another area where the user can increase a scanner's
performance (and one that's easier to implement) arises
from the fact that the longer the tokens matched, the
faster the scanner will run. This is because with long
tokens the processing of most input characters takes
place in the (short) inner scanning loop, and does not
often have to go through the additional work of setting
up the scanning environment (e.g., yytext) for the
action. Recall the scanner for C comments:
%x comment
%%
int line_num = 1;
"/*" BEGIN(comment);
<comment>[^*\n]*
<comment>"*"+[^*/\n]*
<comment>\n ++line_num;
<comment>"*"+"/" BEGIN(INITIAL);
This could be sped up by writing it as:
%x comment
%%
int line_num = 1;
"/*" BEGIN(comment);
<comment>[^*\n]*
<comment>[^*\n]*\n ++line_num;
<comment>"*"+[^*/\n]*
<comment>"*"+[^*/\n]*\n ++line_num;
<comment>"*"+"/" BEGIN(INITIAL);
Now instead of each newline requiring the processing of
another action, recognizing the newlines is "distrib-
uted" over the other rules to keep the matched text as
long as possible. Note that adding rules does not slow
down the scanner! The speed of the scanner is indepen-
dent of the number of rules or (modulo the considera-
tions given at the beginning of this section) how com-
plicated the rules are with regard to operators such as
'*' and '|'.
A final example in speeding up a scanner: suppose you
want to scan through a file containing identifiers and
keywords, one per line and with no other extraneous
characters, and recognize all the keywords. A natural
first approach is:
%%
asm |
auto |
break |
... etc ...
volatile |
while /* it's a keyword */
.|\n /* it's not a keyword */
To eliminate the back-tracking, introduce a catch-all
rule:
%%
asm |
auto |
break |
... etc ...
volatile |
while /* it's a keyword */
[a-z]+ |
.|\n /* it's not a keyword */
Now, if it's guaranteed that there's exactly one word
per line, then we can reduce the total number of matches
by a half by merging in the recognition of newlines with
that of the other tokens:
%%
asm\n |
auto\n |
break\n |
... etc ...
volatile\n |
while\n /* it's a keyword */
[a-z]+\n |
.|\n /* it's not a keyword */
One has to be careful here, as we have now reintroduced
backing up into the scanner. In particular, while we
know that there will never be any characters in the
input stream other than letters or newlines, flex can't
figure this out, and it will plan for possibly needing
to back up when it has scanned a token like "auto" and
then the next character is something other than a new-
line or a letter. Previously it would then just match
the "auto" rule and be done, but now it has no "auto"
rule, only a "auto\n" rule. To eliminate the possibil-
ity of backing up, we could either duplicate all rules
but without final newlines, or, since we never expect to
encounter such an input and therefore don't how it's
classified, we can introduce one more catch-all rule,
this one which doesn't include a newline:
%%
asm\n |
auto\n |
break\n |
... etc ...
volatile\n |
while\n /* it's a keyword */
[a-z]+\n |
[a-z]+ |
.|\n /* it's not a keyword */
Compiled with -Cf, this is about as fast as one can get
a flex scanner to go for this particular problem.
A final note: flex is slow when matching NUL's, particu-
larly when a token contains multiple NUL's. It's best
to write rules which match short amounts of text if it's
anticipated that the text will often include NUL's.
Another final note regarding performance: as mentioned
above in the section How the Input is Matched, dynami-
cally resizing yytext to accommodate huge tokens is a
slow process because it presently requires that the
(huge) token be rescanned from the beginning. Thus if
performance is vital, you should attempt to match
"large" quantities of text but not "huge" quantities,
where the cutoff between the two is at about 8K charac-
ters/token.
GENERATING C++ SCANNERS
flex provides two different ways to generate scanners
for use with C++. The first way is to simply compile a
scanner generated by flex using a C++ compiler instead
of a C compiler. You should not encounter any compila-
tions errors (please report any you find to the email
address given in the Author section below). You can
then use C++ code in your rule actions instead of C
code. Note that the default input source for your scan-
ner remains yyin, and default echoing is still done to
yyout. Both of these remain FILE * variables and not
C++ streams.
You can also use flex to generate a C++ scanner class,
using the -+ option (or, equivalently, %option c++),
which is automatically specified if the name of the flex
executable ends in a '+', such as flex++. When using
this option, flex defaults to generating the scanner to
the file lex.yy.cc instead of lex.yy.c. The generated
scanner includes the header file FlexLexer.h, which
defines the interface to two C++ classes.
The first class, FlexLexer, provides an abstract base
class defining the general scanner class interface. It
provides the following member functions:
const char* YYText()
returns the text of the most recently matched
token, the equivalent of yytext.
int YYLeng()
returns the length of the most recently matched
token, the equivalent of yyleng.
int lineno() const
returns the current input line number (see
%option yylineno), or 1 if %option yylineno was
not used.
void set_debug( int flag )
sets the debugging flag for the scanner, equiva-
lent to assigning to yy_flex_debug (see the
Options section above). Note that you must build
the scanner using %option debug to include debug-
ging information in it.
int debug() const
returns the current setting of the debugging
flag.
Also provided are member functions equivalent to
yy_switch_to_buffer(), yy_create_buffer() (though the
first argument is an istream* object pointer and not a
FILE*), yy_flush_buffer(), yy_delete_buffer(), and
yyrestart() (again, the first argument is a istream*
object pointer).
The second class defined in FlexLexer.h is yyFlexLexer,
which is derived from FlexLexer. It defines the follow-
ing additional member functions:
yyFlexLexer( istream* arg_yyin = 0, ostream* arg_yyout =
0 )
constructs a yyFlexLexer object using the given
streams for input and output. If not specified,
the streams default to cin and cout, respec-
tively.
virtual int yylex()
performs the same role is yylex() does for ordi-
nary flex scanners: it scans the input stream,
consuming tokens, until a rule's action returns a
value. If you derive a subclass S from
yyFlexLexer and want to access the member func-
tions and variables of S inside yylex(), then you
need to use %option yyclass="S" to inform flex
that you will be using that subclass instead of
yyFlexLexer. In this case, rather than generat-
ing yyFlexLexer::yylex(), flex generates
S::yylex() (and also generates a dummy
yyFlexLexer::yylex() that calls
yyFlexLexer::LexerError() if called).
virtual void switch_streams(istream* new_in = 0,
ostream* new_out = 0) reassigns yyin to new_in
(if non-nil) and yyout to new_out (ditto), delet-
ing the previous input buffer if yyin is reas-
signed.
int yylex( istream* new_in, ostream* new_out = 0 )
first switches the input streams via
switch_streams( new_in, new_out ) and then
returns the value of yylex().
In addition, yyFlexLexer defines the following protected
virtual functions which you can redefine in derived
classes to tailor the scanner:
virtual int LexerInput( char* buf, int max_size )
reads up to max_size characters into buf and
returns the number of characters read. To indi-
cate end-of-input, return 0 characters. Note
that "interactive" scanners (see the -B and -I
flags) define the macro YY_INTERACTIVE. If you
redefine LexerInput() and need to take different
actions depending on whether or not the scanner
might be scanning an interactive input source,
you can test for the presence of this name via
#ifdef.
virtual void LexerOutput( const char* buf, int size )
writes out size characters from the buffer buf,
which, while NUL-terminated, may also contain
"internal" NUL's if the scanner's rules can match
text with NUL's in them.
virtual void LexerError( const char* msg )
reports a fatal error message. The default ver-
sion of this function writes the message to the
stream cerr and exits.
Note that a yyFlexLexer object contains its entire scan-
ning state. Thus you can use such objects to create
reentrant scanners. You can instantiate multiple
instances of the same yyFlexLexer class, and you can
also combine multiple C++ scanner classes together in
the same program using the -P option discussed above.
Finally, note that the %array feature is not available
to C++ scanner classes; you must use %pointer (the
default).
Here is an example of a simple C++ scanner:
// An example of using the flex C++ scanner class.
%{
int mylineno = 0;
%}
string \"[^\n"]+\"
ws [ \t]+
alpha [A-Za-z]
dig [0-9]
name ({alpha}|{dig}|\$)({alpha}|{dig}|[_.\-/$])*
num1 [-+]?{dig}+\.?([eE][-+]?{dig}+)?
num2 [-+]?{dig}*\.{dig}+([eE][-+]?{dig}+)?
number {num1}|{num2}
%%
{ws} /* skip blanks and tabs */
"/*" {
int c;
while((c = yyinput()) != 0)
{
if(c == '\n')
++mylineno;
else if(c == '*')
{
if((c = yyinput()) == '/')
break;
else
unput(c);
}
}
}
{number} cout << "number " << YYText() << '\n';
\n mylineno++;
{name} cout << "name " << YYText() << '\n';
{string} cout << "string " << YYText() << '\n';
%%
int main( int /* argc */, char** /* argv */ )
{
FlexLexer* lexer = new yyFlexLexer;
while(lexer->yylex() != 0)
;
return 0;
}
If you want to create multiple (different) lexer
classes, you use the -P flag (or the prefix= option) to
rename each yyFlexLexer to some other xxFlexLexer. You
then can include <FlexLexer.h> in your other sources
once per lexer class, first renaming yyFlexLexer as fol-
lows:
#undef yyFlexLexer
#define yyFlexLexer xxFlexLexer
#include <FlexLexer.h>
#undef yyFlexLexer
#define yyFlexLexer zzFlexLexer
#include <FlexLexer.h>
if, for example, you used %option prefix="xx" for one of
your scanners and %option prefix="zz" for the other.
IMPORTANT: the present form of the scanning class is
experimental and may change considerably between major
releases.
INCOMPATIBILITIES WITH LEX AND POSIX
flex is a rewrite of the AT&T Unix lex tool (the two
implementations do not share any code, though), with
some extensions and incompatibilities, both of which are
of concern to those who wish to write scanners accept-
able to either implementation. Flex is fully compliant
with the POSIX lex specification, except that when using
%pointer (the default), a call to unput() destroys the
contents of yytext, which is counter to the POSIX
specification.
In this section we discuss all of the known areas of
incompatibility between flex, AT&T lex, and the POSIX
specification.
flex's -l option turns on maximum compatibility with the
original AT&T lex implementation, at the cost of a major
loss in the generated scanner's performance. We note
below which incompatibilities can be overcome using the
-l option.
flex is fully compatible with lex with the following
exceptions:
- The undocumented lex scanner internal variable
yylineno is not supported unless -l or %option
yylineno is used.
yylineno should be maintained on a per-buffer
basis, rather than a per-scanner (single global
variable) basis.
yylineno is not part of the POSIX specification.
- The input() routine is not redefinable, though it
may be called to read characters following what-
ever has been matched by a rule. If input()
encounters an end-of-file the normal yywrap()
processing is done. A ``real'' end-of-file is
returned by input() as EOF.
Input is instead controlled by defining the
YY_INPUT macro.
The flex restriction that input() cannot be rede-
fined is in accordance with the POSIX specifica-
tion, which simply does not specify any way of
controlling the scanner's input other than by
making an initial assignment to yyin.
- The unput() routine is not redefinable. This
restriction is in accordance with POSIX.
- flex scanners are not as reentrant as lex scan-
ners. In particular, if you have an interactive
scanner and an interrupt handler which long-jumps
out of the scanner, and the scanner is subse-
quently called again, you may get the following
message:
fatal flex scanner internal error--end of buffer missed
To reenter the scanner, first use
yyrestart( yyin );
Note that this call will throw away any buffered
input; usually this isn't a problem with an
interactive scanner.
Also note that flex C++ scanner classes are reen-
trant, so if using C++ is an option for you, you
should use them instead. See "Generating C++
Scanners" above for details.
- output() is not supported. Output from the ECHO
macro is done to the file-pointer yyout (default
stdout).
output() is not part of the POSIX specification.
- lex does not support exclusive start conditions
(%x), though they are in the POSIX specification.
- 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.
Note that if the definition begins with ^ or ends
with $ then it is not expanded with parentheses,
to allow these operators to appear in definitions
without losing their special meanings. But the
<s>, /, and <<EOF>> operators cannot be used in a
flex definition.
Using -l results in the lex behavior of no paren-
theses around the definition.
The POSIX specification is that the definition be
enclosed in parentheses.
- Some implementations of lex allow a rule's action
to begin on a separate line, if the rule's pat-
tern has trailing whitespace:
%%
foo|bar<space here>
{ foobar_action(); }
flex does not support this feature.
- The lex %r (generate a Ratfor scanner) option is
not supported. It is not part of the POSIX spec-
ification.
- After a call to unput(), yytext is undefined
until the next token is matched, unless the scan-
ner was built using %array. This is not the case
with lex or the POSIX specification. The -l
option does away with this incompatibility.
- The precedence of the {} (numeric range) operator
is different. lex interprets "abc{1,3}" as
"match one, two, or three occurrences of 'abc'",
whereas flex interprets it as "match 'ab' fol-
lowed by one, two, or three occurrences of 'c'".
The latter is in agreement with the POSIX speci-
fication.
- The precedence of the ^ operator is different.
lex interprets "^foo|bar" as "match either 'foo'
at the beginning of a line, or 'bar' anywhere",
whereas flex interprets it as "match either 'foo'
or 'bar' if they come at the beginning of a
line". The latter is in agreement with the POSIX
specification.
- The special table-size declarations such as %a
supported by lex are not required by flex scan-
ners; flex ignores them.
- The name FLEX_SCANNER is #define'd so scanners
may be written for use with either flex or lex.
Scanners also include YY_FLEX_MAJOR_VERSION and
YY_FLEX_MINOR_VERSION indicating which version of
flex generated the scanner (for example, for the
2.5 release, these defines would be 2 and 5
respectively).
The following flex features are not included in lex or
the POSIX specification:
C++ scanners
%option
start condition scopes
start condition stacks
interactive/non-interactive scanners
yy_scan_string() and friends
yyterminate()
yy_set_interactive()
yy_set_bol()
YY_AT_BOL()
<<EOF>>
<*>
YY_DECL
YY_START
YY_USER_ACTION
YY_USER_INIT
#line directives
%{}'s around actions
multiple actions on a line
plus almost all of the flex flags. The last feature in
the list refers to the fact that with flex you can put
multiple actions on the same line, separated with semi-
colons, while with lex, the following
foo handle_foo(); ++num_foos_seen;
is (rather surprisingly) truncated to
foo handle_foo();
flex does not truncate the action. Actions that are not
enclosed in braces are simply terminated at the end of
the line.
DIAGNOSTICS
warning, rule cannot be matched indicates that the given
rule cannot be matched because it follows other rules
that will always match the same text as it. For exam-
ple, in the following "foo" cannot be matched because it
comes after an identifier "catch-all" rule:
[a-z]+ got_identifier();
foo got_foo();
Using REJECT in a scanner suppresses this warning.
warning, -s option given but default rule can be matched
means that it is possible (perhaps only in a particular
start condition) that the default rule (match any single
character) is the only one that will match a particular
input. Since -s was given, presumably this is not
intended.
reject_used_but_not_detected undefined or
yymore_used_but_not_detected undefined - These errors
can occur at compile time. They indicate that the scan-
ner 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). Use %option reject or
%option yymore to indicate to flex that you really do
use these features.
flex scanner jammed - a scanner compiled with -s has
encountered an input string which wasn't matched by any
of its rules. This error can also occur due to internal
problems.
token too large, exceeds YYLMAX - your scanner uses
%array and one of its rules matched a string longer than
the YYLMAX constant (8K bytes by default). You can
increase the value by #define'ing YYLMAX in the defini-
tions section of your flex input.
scanner requires -8 flag to use the character 'x' - Your
scanner specification includes recognizing the 8-bit
character 'x' and you did not specify the -8 flag, and
your scanner defaulted to 7-bit because you used the -Cf
or -CF table compression options. See the discussion of
the -7 flag for details.
flex scanner push-back overflow - you used unput() to
push back so much text that the scanner's buffer could
not hold both the pushed-back text and the current token
in yytext. Ideally the scanner should dynamically
resize the buffer in this case, but at present it does
not.
input buffer overflow, can't enlarge buffer because
scanner uses REJECT - the scanner was working on match-
ing an extremely large token and needed to expand the
input buffer. This doesn't work with scanners that use
REJECT.
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 acti-
vation frame. Before reentering the scanner, use:
yyrestart( yyin );
or, as noted above, switch to using the C++ scanner
class.
too many start conditions in <> construct! - you listed
more start conditions in a <> construct than exist (so
you must have listed at least one of them twice).
FILES
-lfl library with which scanners must be linked.
lex.yy.c
generated scanner (called lexyy.c on some sys-
tems).
lex.yy.cc
generated C++ scanner class, when using -+.
<FlexLexer.h>
header file defining the C++ scanner base class,
FlexLexer, and its derived class, yyFlexLexer.
flex.skl
skeleton scanner. This file is only used when
building flex, not when flex executes.
lex.backup
backing-up information for -b flag (called
lex.bck on some systems).
DEFICIENCIES / BUGS
Some trailing context patterns cannot be properly
matched and generate warning messages ("dangerous trail-
ing 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 actu-
ally 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 exam-
ple, in the following:
%%
abc |
xyz/def
Use of unput() invalidates yytext and yyleng, unless the
%array directive or the -l option has been used.
Pattern-matching of NUL's is substantially slower than
matching other characters.
Dynamic resizing of the input buffer is slow, as it
entails rescanning all the text matched so far by the
current (generally huge) token.
Due to both buffering of input and read-ahead, you can-
not 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.
The flex internal algorithms need documentation.
SEE ALSO
lex(1), yacc(1), sed(1), awk(1).
John Levine, Tony Mason, and Doug Brown, Lex & Yacc,
O'Reilly and Associates. Be sure to get the 2nd edi-
tion.
M. E. Lesk and E. Schmidt, LEX - Lexical Analyzer Gener-
ator
Alfred Aho, Ravi Sethi and Jeffrey Ullman, Compilers:
Principles, Techniques and Tools, Addison-Wesley (1986).
Describes the pattern-matching techniques used by flex
(deterministic finite automata).
AUTHOR
Vern Paxson, with the help of many ideas and much inspi-
ration from Van Jacobson. Original version by Jef
Poskanzer. The fast table representation is a partial
implementation of a design done by Van Jacobson. The
implementation was done by Kevin Gong and Vern Paxson.
Thanks to the many flex beta-testers, feedbackers, and
contributors, especially Francois Pinard, Casey Leedom,
Robert Abramovitz, Stan Adermann, Terry Allen, David
Barker-Plummer, John Basrai, Neal Becker, Nelson H.F.
Beebe, benson@odi.com, Karl Berry, Peter A. Bigot, Simon
Blanchard, Keith Bostic, Frederic Brehm, Ian Brockbank,
Kin Cho, Nick Christopher, Brian Clapper, J.T. Conklin,
Jason Coughlin, Bill Cox, Nick Cropper, Dave Curtis,
Scott David Daniels, Chris G. Demetriou, Theo Deraadt,
Mike Donahue, Chuck Doucette, Tom Epperly, Leo Eskin,
Chris Faylor, Chris Flatters, Jon Forrest, Jeffrey
Friedl, Joe Gayda, Kaveh R. Ghazi, Wolfgang Glunz, Eric
Goldman, Christopher M. Gould, Ulrich Grepel, Peer
Griebel, Jan Hajic, Charles Hemphill, NORO Hideo, Jarkko
Hietaniemi, Scott Hofmann, Jeff Honig, Dana Hudes, Eric
Hughes, John Interrante, Ceriel Jacobs, Michal
Jaegermann, Sakari Jalovaara, Jeffrey R. Jones, Henry
Juengst, Klaus Kaempf, Jonathan I. Kamens, Terrence O
Kane, Amir Katz, ken@ken.hilco.com, Kevin B. Kenny,
Steve Kirsch, Winfried Koenig, Marq Kole, Ronald Lam-
precht, Greg Lee, Rohan Lenard, Craig Leres, John
Levine, Steve Liddle, David Loffredo, Mike Long, Mohamed
el Lozy, Brian Madsen, Malte, Joe Marshall, Bengt
Martensson, Chris Metcalf, Luke Mewburn, Jim Meyering,
R. Alexander Milowski, Erik Naggum, G.T. Nicol, Landon
Noll, James Nordby, Marc Nozell, Richard Ohnemus,
Karsten Pahnke, Sven Panne, Roland Pesch, Walter Pelis-
sero, Gaumond Pierre, Esmond Pitt, Jef Poskanzer, Joe
Rahmeh, Jarmo Raiha, Frederic Raimbault, Pat Rankin,
Rick Richardson, Kevin Rodgers, Kai Uwe Rommel, Jim
Roskind, Alberto Santini, Andreas Scherer, Darrell
Schiebel, Raf Schietekat, Doug Schmidt, Philippe Schnoe-
belen, Andreas Schwab, Larry Schwimmer, Alex Siegel,
Eckehard Stolz, Jan-Erik Strvmquist, Mike Stump, Paul
Stuart, Dave Tallman, Ian Lance Taylor, Chris Thewalt,
Richard M. Timoney, Jodi Tsai, Paul Tuinenga, Gary Weik,
Frank Whaley, Gerhard Wilhelms, Kent Williams, Ken Yap,
Ron Zellar, Nathan Zelle, David Zuhn, and those whose
names have slipped my marginal mail-archiving skills but
whose contributions are appreciated all the same.
Thanks to Keith Bostic, Jon Forrest, Noah Friedman, John
Gilmore, Craig Leres, John Levine, Bob Mulcahy, G.T.
Nicol, Francois Pinard, Rich Salz, and Richard Stallman
for help with various distribution headaches.
Thanks to Esmond Pitt and Earle Horton for 8-bit charac-
ter support; to Benson Margulies and Fred Burke for C++
support; to Kent Williams and Tom Epperly for C++ class
support; to Ove Ewerlid for support of NUL's; and to
Eric Hughes for support of multiple buffers.
This work was primarily done when I was with the Real
Time Systems Group at the Lawrence Berkeley Laboratory
in Berkeley, CA. Many thanks to all there for the sup-
port I received.
Send comments to vern@ee.lbl.gov.
Version 2.5 April 1995 FLEX(1)
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