flex++
FLEX(1) FLEX(1)
NAME
flex - fast lexical analyzer generator
SYNOPSIS
flex [-bcdfhilnpstvwBFILTV78+? -C[aefFmr] -ooutput -Pprefix -Sskele-
ton] [--help --version] [filename ...]
OVERVIEW
This manual describes flex, a tool for generating programs that per-
form 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 lex-
ical patterns in text. flex reads the given input files, or its stan-
dard input if no file names are given, for a description of a scanner
to generate. The description is in the form of pairs of regular
expressions and C code, called rules. flex generates as output a C
source file, lex.yy.c, which defines a routine yylex(). This file is
compiled and linked with the -lfl library to produce an executable.
When the executable is run, it analyzes its input for occurrences of
the regular expressions. Whenever it finds one, it executes the cor-
responding 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 encoun-
ters 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 occurrence 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 language 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 sec-
tions.
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 defini-
tions 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, digits, ’_’, 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 defini-
tion 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 fol-
lowed 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 ’.’ followed 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 second %% 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 exe-
cuted 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 section), an unin-
dented comment (i.e., a line beginning with "/*") is also copied ver-
batim 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 opera-
tors 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 according to prece-
dence, 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 concatenation, 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 function. For example, [:alnum:] des-
ignates those characters for which isalnum() returns true - i.e., any
alphabetic 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 unfortunately the inconsistency is historically
entrenched. Matching newlines means that a pattern 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 ’$’, cannot be grouped
inside parentheses. A ’^’ which does not occur at the begin-
ning of a rule or a ’$’ 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 ’$’ or ’^’ being treated as a nor-
mal character:
foo|(bar$)
foo|^bar
If what’s wanted is a "foo" or a bar-followed-by-a-newline, the
following could be used (the special ’|’ 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 cor-
responding 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 stan-
dard 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 def-
inition 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 compatibility 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 disadvantage is that you are restricted in how your actions can
modify yytext (see the next section), and calls to the unput() func-
tion destroys the present contents of yytext, which can be a consider-
able 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 characters, which
defaults to a fairly large value. You can change the size by simply
#define’ing YYLMAX to a different 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 remainder of the line is its action. If the
action is empty, then when the pattern is matched the input token is
simply discarded. For example, here is the specification for a pro-
gram 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 con-
sider the action to be all the text up to the next %} (regardless 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 processing 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 cho-
sen 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 "abcdabcaba" 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 cur-
rent 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 output. 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
parentheses.
{
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 character 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 Scan-
ner, below). This action is a special case of the more general
yy_flush_buffer() function, described below in the section Mul-
tiple Input Buffers.
- yyterminate() can be used in lieu of a return statement in an
action. It terminates the scanner and returns a 0 to the scan-
ner’s caller, indicating "all done". By default, yyterminate()
is also called when an end-of-file is encountered. It is a
macro and may be redefined.
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 routines 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 decla-
ration, 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 scan-
ning 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 assigning yyin to a new input file or using yyrestart()
to do so; the latter is available for compatibility with previous ver-
sions 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 inte-
ger variable result either the number of characters read or the con-
stant 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 section 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 charac-
ter 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 discussion of them below in the section
Multiple Input Buffers.
The scanner writes its ECHO output to the yyout global (default, std-
out), 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 condi-
tion, and
<INITIAL,STRING,QUOTE>\. { /* handle an escape ... */
...
}
will be active only when the current start condition is either "INI-
TIAL", "STRING", or "QUOTE".
Start conditions are declared in the definitions (first) section of
the input using unindented lines beginning with either %s or %x fol-
lowed by a list of names. The former declares inclusive start condi-
tions, the latter exclusive start conditions. A start condition is
activated using the BEGIN action. Until the next BEGIN action is exe-
cuted, 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, exclusive
start conditions make it easy to specify "mini-scanners" which scan
portions of the input that are syntactically 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 connec-
tion 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 condi-
tion 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 example 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 writ-
ten;
%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 "INITIAL", 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 following 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 comments 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 possi-
ble 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 values 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, including expanded escape sequences
(but not including checking 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 easier and cleaner by introducing a notion of
start condition 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 automatically 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 condi-
tions:
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 integers).
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 con-
tents.
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 scan-
ning 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 initialize YY_BUFFER_STATE variables to
((YY_BUFFER_STATE) 0) if you wish, and also refer to the opaque struc-
ture 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 redefine YY_INPUT so it no longer uses yyin, then you can
safely pass a nil FILE pointer to yy_create_buffer. You select a par-
ticular 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(), provided for com-
patibility 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 handle (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) starting 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 scanning.) 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, scanning 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 initial
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 follow-
ing would do the trick:
#define YY_USER_ACTION ++ctr[yy_act]
where ctr is an array to hold the counts for the different 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 decla-
ration 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 inter-
nal 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-interactive. 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 cur-
rent 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 unreachable 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 ter-
minates 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 immedi-
ate (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 scanning 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 handle to the cur-
rent buffer.
- YY_START returns an integer value corresponding to the current
start condition. You can subsequently 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-gen-
erator. 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_NUM-
BER", 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 char-
acters 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 compli-
ance.
-d makes the generated scanner run in debug mode. Whenever a pat-
tern 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 synonyms 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 preserved
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 compatibil-
ity. Use of this option costs a considerable amount of perfor-
mance, 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 scan-
ner. If you give the flag twice, you will also get comments
regarding features that lead to minor performance losses.
Note that the use of REJECT, %option yylineno, and variable
trailing context (see the Deficiencies / Bugs section below)
entails a substantial performance penalty; use of yymore(), the
^ operator, 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 summary of statis-
tics regarding the scanner it generates. Most of the statis-
tics are meaningless to the casual flex user, but the first
line identifies the version of flex (same as reported by -V),
and the next line the flags used when generating 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 per-
formance out of it. If your goal is instead to squeeze out a
lot more performance, you should be using the -Cf or -CF
options (discussed below), which turn on -B automatically any-
way.
-F specifies that the fast scanner table representation should be
used (and stdio bypassed). This representation is about as
fast as the full table representation (-f), and for some sets
of patterns will be considerably smaller (and for others,
larger). In general, if the pattern set contains both "key-
words" and a catch-all, "identifier" 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 representation. 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 scanner. An interac-
tive 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 disambiguate the current token,
is a bit faster than only looking ahead when necessary. But
scanners that always look ahead give dreadful interactive per-
formance; 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 interac-
tive 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 direc-
tives 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 mes-
sages to stderr concerning the form of the input and the resul-
tant 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 character.
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 generating 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 gener-
ated 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 performance because the elements
of the tables are better aligned for memory access and computa-
tion. On some RISC architectures, fetching and manipulating
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 equivalence class). Equiva-
lence classes usually give dramatic reductions in the final ta-
ble/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 com-
pressed 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 system, 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 scanner
(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 com-
pressed 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 ta-
ble 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 gener-
ated 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 pro-
duction 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 scan-
ner 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 scanner itself, you can
still refer to the global variables and functions using either
version of their name; but externally, they have the modified
name.
This option lets you easily link together multiple flex pro-
grams 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 preceded 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 scan-
ner, 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::Lexer-
Error()) if called. See Generating C++ Scanners, below, for addi-
tional information.
A number of options are available for lint purists who want to sup-
press 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 compres-
sion -C options outlined above, there are a number of options/actions
which degrade performance. These are, from most expensive 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 currently 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 hap-
pened, 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 problem when "foob"
has been scanned. Indeed, on any character 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 performance 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 messages. 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 benefits of eliminat-
ing 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., substantial). 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 Deficiencies / 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 "distributed" 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 considerations given at the
beginning of this section) how complicated 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 newline 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 possibility of
backing up, we could either duplicate all rules but without final new-
lines, or, since we never expect to encounter such an input and there-
fore 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, particularly 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, dynamically 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" quanti-
ties of text but not "huge" quantities, where the cutoff between the
two is at about 8K characters/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 compilations 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 scanner remains yyin, and default echo-
ing 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 speci-
fied 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 equiv-
alent 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, equivalent to assign-
ing to yy_flex_debug (see the Options section above). Note
that you must build the scanner using %option debug to include
debugging 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 following 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, respectively.
virtual int yylex()
performs the same role is yylex() does for ordinary flex scan-
ners: 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 functions and vari-
ables 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 generating
yyFlexLexer::yylex(), flex generates S::yylex() (and also gen-
erates 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), deleting the previous input buffer if
yyin is reassigned.
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 func-
tions 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 indicate end-of-input, return 0 charac-
ters. Note that "interactive" scanners (see the -B and -I
flags) define the macro YY_INTERACTIVE. If you redefine Lex-
erInput() 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 scan-
ner’s rules can match text with NUL’s in them.
virtual void LexerError( const char* msg )
reports a fatal error message. The default version of this
function writes the message to the stream cerr and exits.
Note that a yyFlexLexer object contains its entire scanning 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 follows:
#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 incompatibil-
ities, both of which are of concern to those who wish to write scan-
ners acceptable 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 scan-
ner’s performance. We note below which incompatibilities can be over-
come 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 whatever 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 redefined is in
accordance with the POSIX specification, 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 scanners. In partic-
ular, if you have an interactive scanner and an interrupt han-
dler which long-jumps out of the scanner, and the scanner is
subsequently 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 reentrant, 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 parenthe-
ses. 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 parentheses 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 pattern 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 specification.
- After a call to unput(), yytext is undefined until the next
token is matched, unless the scanner 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 occur-
rences of ’abc’", whereas flex interprets it as "match ’ab’
followed by one, two, or three occurrences of ’c’". The latter
is in agreement with the POSIX specification.
- 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 scanners; 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 spec-
ification:
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 example, 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 scanner uses REJECT or yymore() but that flex failed to
notice the fact, meaning that flex scanned the first two sections
looking for occurrences of these actions and failed to find any, but
somehow you snuck some in (via a #include file, for example). 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
definitions section of your flex input.
scanner requires -8 flag to use the character ’x’ - Your scanner spec-
ification 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 matching an extremely large token
and needed to expand the input buffer. This doesn’t work with scan-
ners 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 activation 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 systems).
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 sys-
tems).
DEFICIENCIES / BUGS
Some trailing context patterns cannot be properly matched and generate
warning messages ("dangerous trailing context"). These are patterns
where the ending of the first part of the rule matches the beginning
of the second part, such as "zx*/xy*", where the ’x*’ matches the ’x’
at the beginning of the trailing context. (Note that the POSIX draft
states that the text matched by such patterns is undefined.)
For some trailing context rules, parts which are actually fixed-length
are not recognized as such, leading to the abovementioned performance
loss. In particular, parts using ’|’ or {n} (such as "foo{3}") are
always considered variable-length.
Combining trailing context with the special ’|’ action can result in
fixed trailing context being turned into the more expensive variable
trailing context. For example, in the following:
%%
abc |
xyz/def
Use of unput() invalidates yytext and yyleng, unless the %array direc-
tive 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 cannot intermix
calls to <stdio.h> routines, such as, for example, getchar(), with
flex rules and expect it to work. Call input() instead.
The total table entries listed by the -v flag excludes the number of
table entries needed to determine what rule has been matched. The
number of entries is equal to the number of DFA states if the scanner
does not use REJECT, and somewhat greater than the number of states if
it does.
REJECT cannot be used with the -f or -F options.
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 As-
sociates. Be sure to get the 2nd edition.
M. E. Lesk and E. Schmidt, LEX - Lexical Analyzer Generator
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 inspiration from Van
Jacobson. Original version by Jef Poskanzer. The fast table repre-
sentation is a partial implementation of a design done by Van Jacob-
son. 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 Lamprecht, 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 Pelissero, Gaumond Pierre, Esmond Pitt, Jef Poskanzer, Joe Rah-
meh, 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 Sch-
noebelen, 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 character 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 support I received.
Send comments to vern@ee.lbl.gov.
Version 2.5 April 1995 FLEX(1)
