flex, lex - fast lexical analyzer generator
flex [-bcdfhilnpstvwBFILTV78+? -C[aefFmr] -ooutput -Pprefix
-Sskeleton] [--help --version] [filename ...]
This manual describes
flex, a tool for generating programs that perform
pattern-matching on text. The manual includes both tutorial and reference
sections:
Description
a brief overview of the tool
Some Simple Examples
Format Of The Input File
Patterns
the extended regular expressions used by flex
How The Input Is Matched
the rules for determining what has been matched
Actions
how to specify what to do when a pattern is matched
The Generated Scanner
details regarding the scanner that flex produces;
how to control the input source
Start Conditions
introducing context into your scanners, and
managing "mini-scanners"
Multiple Input Buffers
how to manipulate multiple input sources; how to
scan from strings instead of files
End-of-file Rules
special rules for matching the end of the input
Miscellaneous Macros
a summary of macros available to the actions
Values Available To The User
a summary of values available to the actions
Interfacing With Yacc
connecting flex scanners together with yacc parsers
Options
flex command-line options, and the "%option"
directive
Performance Considerations
how to make your scanner go as fast as possible
Generating C++ Scanners
the (experimental) facility for generating C++
scanner classes
Incompatibilities With Lex And POSIX
how flex differs from AT&T lex and the POSIX lex
standard
Diagnostics
those error messages produced by flex (or scanners
it generates) whose meanings might not be apparent
Files
files used by flex
Deficiencies / Bugs
known problems with flex
See Also
other documentation, related tools
Author
includes contact information
flex is a tool for generating
scanners: programs which recognize
lexical patterns in text.
flex reads the given input files, or its
standard input if no file names are given, for a description of a scanner to
generate. The description is in the form of pairs of regular expressions and C
code, called
rules. flex generates as output a C source file,
lex.yy.c, which defines a routine
yylex(). This file is compiled
and linked with the
-ll library to produce an executable. When the
executable is run, it analyzes its input for occurrences of the regular
expressions. Whenever it finds one, it executes the corresponding C code.
First some simple examples to get the flavor of how one uses
flex. The
following
flex input specifies a scanner which whenever it encounters
the string "username" will replace it with the user's login name:
%%
username printf( "%s", getlogin() );
By default, any text not matched by a
flex scanner is copied to the
output, so the net effect of this scanner is to copy its input file to its
output with each 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 sections.
The
flex input file consists of three sections, separated by a line with
just
%% in it:
definitions
%%
rules
%%
user code
The
definitions section contains declarations of simple
name
definitions to simplify the scanner specification, and declarations of
start conditions, which are explained in a later section.
Name definitions have the form:
name definition
The "name" is a word beginning with a letter or an underscore ('_')
followed by zero or more letters, 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 definition can subsequently be
referred to using "{name}", which will expand to
"(definition)". For example,
DIGIT [0-9]
ID [a-z][a-z0-9]*
defines "DIGIT" to be a regular expression which matches a single
digit, and "ID" to be a regular expression which matches a letter
followed by zero-or-more letters-or-digits. A subsequent reference to
{DIGIT}+"."{DIGIT}*
is identical to
([0-9])+"."([0-9])*
and matches one-or-more digits followed by a '.' 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 executed whenever the scanning
routine is entered. Other indented or %{} text in the rule section is still
copied to the output, but its meaning is not well-defined and it may well
cause compile-time errors (this feature is present for
POSIX
compliance; see below for other such features).
In the definitions section (but not in the rules section), an unindented comment
(i.e., a line beginning with "/*") is also copied verbatim to the
output up to the next "*/".
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.,
when just starting to scan, or right after a
newline has been scanned).
r$ an r, but only at the end of a line (i.e., just
before a newline). Equivalent to "r/\n".
Note that flex's notion of "newline" is exactly
whatever the C compiler used to compile flex
interprets '\n' as; in particular, on some DOS
systems you must either filter out \r's in the
input yourself, or explicitly use r/\r\n for "r$".
<s>r an r, but only in start condition s (see
below for discussion of start conditions)
<s1,s2,s3>r
same, but in any of start conditions s1,
s2, or s3
<*>r an r in any start condition, even an exclusive one.
<<EOF>> an end-of-file
<s1,s2><<EOF>>
an end-of-file when in start condition s1 or s2
Note that inside of a character class, all regular expression operators lose
their special meaning except escape ('\') and the character class operators,
'-', ']', and, at the beginning of the class, '^'.
The regular expressions listed above are grouped according to precedence, from
highest precedence at the top to lowest at the bottom. Those grouped together
have equal precedence. For example,
foo|bar*
is the same as
(foo)|(ba(r*))
since the '*' operator has higher precedence than 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:]
designates 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 '$', cannot be grouped inside
parentheses. A '^' which does not occur at the beginning of a rule or a
'$' which does not occur at the end of a rule loses its special properties
and is treated as a normal character.
- The following are illegal:
foo/bar$
<sc1>foo<sc2>bar
Note that the first of these, can be written "foo/bar\n".
- The following will result in '$' or '^' being treated as a normal
character:
foo|(bar$)
foo|^bar
If what's wanted is a "foo" or a bar-followed-by-a-newline, the
following could be used (the 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.
When the generated scanner is run, it analyzes its input looking for strings
which match any of its patterns. If it finds more than one match, it takes the
one matching the most text (for trailing context rules, this includes the
length of the trailing part, even though it will then be returned to the
input). If it finds two or more matches of the same length, the rule listed
first in the
flex input file is chosen.
Once the match is determined, the text corresponding to the match (called the
token) is made available in the global character pointer
yytext,
and its length in the global integer
yyleng. The
action
corresponding to the matched pattern is then executed (a more detailed
description of actions follows), and then the remaining input is scanned for
another match.
If no match is found, then the
default rule is executed: the next
character in the input is considered matched and copied to the standard
output. Thus, the simplest legal
flex input is:
%%
which generates a scanner that simply copies its input (one character at a time)
to its output.
Note that
yytext can be defined in two different ways: either as a
character
pointer or as a character
array. You can control which
definition
flex uses by including one of the special directives
%pointer or
%array in the first (definitions) section of your
flex input. The default is
%pointer, unless you use the
-l lex
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()
function destroys the present contents of
yytext, which can be a
considerable porting headache when moving between different
lex
versions.
The advantage of
%array is that you can then modify
yytext to your
heart's content, and calls to
unput() do not destroy
yytext (see
below). Furthermore, existing
lex programs sometimes access
yytext externally using declarations of the form:
extern char yytext[];
This definition is erroneous when used with
%pointer, but correct for
%array.
%array defines
yytext to be an array of
YYLMAX 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
dynamically 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).
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 program which deletes all occurrences of "zap
me" from its input:
%%
"zap me"
(It will copy all other characters in the input to the output since they will be
matched by the default rule.)
Here is a program which compresses multiple blanks and tabs down to a single
blank, and throws away whitespace found at the end of a line:
%%
[ \t]+ putchar( ' ' );
[ \t]+$ /* ignore this token */
If the action contains a '{', then the action spans till the balancing '}' is
found, and the action may cross multiple lines.
flex knows about C
strings and comments and won't be fooled by braces found within them, but also
allows actions to begin with
%{ and will consider the action to be all
the text up to the next
%} (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 chosen rule but came
later in the flex input file, or one which matched less text. For
example, the following will both count the words in the input and call the
routine special() whenever "frob" is seen:
int word_count = 0;
%%
frob special(); REJECT;
[^ \t\n]+ ++word_count;
Without the REJECT, any "frob"'s in the input would not be
counted as words, since the scanner normally executes only one action per
token. Multiple REJECT's are allowed, each one finding the next
best choice to the currently active rule. For example, when the following
scanner scans the token "abcd", it will write
"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 current value
of yytext rather than replacing it. For example, given the input
"mega-kludge" the following will write
"mega-mega-kludge" to the output:
%%
mega- ECHO; yymore();
kludge ECHO;
First "mega-" is matched and echoed to the 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
performance 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:
%%
"/*" {
int c;
for ( ; ; )
{
while ( (c = input()) != '*' &&
c != EOF )
; /* eat up text of comment */
if ( c == '*' )
{
while ( (c = input()) == '*' )
;
if ( c == '/' )
break; /* found the end */
}
if ( c == EOF )
{
error( "EOF in comment" );
break;
}
}
}
(Note that if the scanner is compiled using C++, then input()
is instead referred to as yyinput(), in order to avoid a name clash
with the C++ stream by the name of input.)
- -
- YY_FLUSH_BUFFER flushes the scanner's internal buffer so that the
next time the scanner attempts to match a token, it will first refill the
buffer using YY_INPUT (see The Generated Scanner, below). This
action is a special case of the more general yy_flush_buffer()
function, described below in the section Multiple Input Buffers.
- -
- yyterminate() can be used in lieu of a return statement in an
action. It terminates the scanner and returns a 0 to the scanner's caller,
indicating "all done". By default, yyterminate() is also
called when an end-of-file is encountered. It is a macro and may be
redefined.
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 declaration,
you must terminate the definition with a semi-colon (;).
Whenever
yylex() is called, it scans tokens from the global input file
yyin (which defaults to stdin). It continues until it either reaches an
end-of-file (at which point it returns the value 0) or one of its actions
executes a
return statement.
If the scanner reaches an end-of-file, subsequent calls are undefined unless
either
yyin is pointed at a new input file (in which case scanning
continues from that file), or
yyrestart() is called.
yyrestart()
takes one argument, a
FILE * pointer (which can be nil, if you've set
up
YY_INPUT to scan from a source other than
yyin), and
initializes
yyin for scanning from that file. Essentially there is no
difference between just assigning
yyin to a new input file or using
yyrestart() to do so; the latter is available for compatibility with
previous versions of
flex, and because it can be used to switch input
files in the middle of scanning. It can also be used to throw away the current
input buffer, by calling it with an argument of
yyin; but better is to
use
YY_FLUSH_BUFFER (see above). Note that
yyrestart() does
not reset the start condition to
INITIAL (see Start Conditions,
below).
If
yylex() stops scanning due to executing a
return statement in
one of the actions, the scanner may then be called again and it will resume
scanning where it left off.
By default (and for purposes of efficiency), the scanner uses block-reads rather
than simple
getc() calls to read characters from
yyin. The
nature of how it gets its input can be controlled by defining the
YY_INPUT macro. YY_INPUT's calling sequence is
"YY_INPUT(buf,result,max_size)". Its action is to place up to
max_size characters in the character array
buf and return in the
integer variable
result either the number of characters read or the
constant YY_NULL (0 on Unix systems) to indicate EOF. The default YY_INPUT
reads from the global file-pointer "yyin".
A sample definition of YY_INPUT (in the definitions 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 character at a
time.
When the scanner receives an end-of-file indication from YY_INPUT, it then
checks the
yywrap() function. If
yywrap() returns false (zero),
then it is assumed that the function has gone ahead and set up
yyin to
point to another input file, and scanning continues. If it returns true
(non-zero), then the scanner terminates, returning 0 to its caller. Note that
in either case, the start condition remains unchanged; it does
not
revert to
INITIAL.
If you do not supply your own version of
yywrap(), then you must either
use
%option noyywrap (in which case the scanner behaves as though
yywrap() returned 1), or you must link with
-ll 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,
stdout), which may be redefined by the user simply by assigning it to some
other
FILE pointer.
flex provides a mechanism for conditionally activating rules. Any rule
whose pattern is prefixed with "<sc>" will only be active when
the scanner is in the start condition named "sc". For example,
<STRING>[^"]* { /* eat up the string body ... */
...
}
will be active only when the scanner is in the "STRING" start
condition, and
<INITIAL,STRING,QUOTE>\. { /* handle an escape ... */
...
}
will be active only when the current start condition is either
"INITIAL", "STRING", or "QUOTE".
Start conditions are declared in the definitions (first) section of the input
using unindented lines beginning with either
%s or
%x followed
by a list of names. The former declares
inclusive start conditions, the
latter
exclusive start conditions. A start condition is activated using
the
BEGIN action. Until the next
BEGIN action is executed, rules
with the given start condition will be active and rules with other start
conditions will be inactive. If the start condition is
inclusive, then
rules with no start conditions at all will also be active. If it is
exclusive, then
only rules qualified with the start condition
will be active. A set of rules contingent on the same exclusive start
condition describe a scanner which is independent of any of the other rules in
the
flex input. Because of this, 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 connection between the
two. The set of rules:
%s example
%%
<example>foo do_something();
bar something_else();
is equivalent to
%x example
%%
<example>foo do_something();
<INITIAL,example>bar something_else();
Without the
<INITIAL,example> qualifier, the
bar pattern in
the second example wouldn't be active (i.e., couldn't match) when in start
condition
example. If we just used
<example> to qualify
bar, though, then it would only be active in
example and not in
INITIAL, while in the first example it's active in both, because in the
first example the
example start condition is an
inclusive
(%s) start condition.
Also note that the special start-condition specifier
<*> matches
every start condition. Thus, the above example could also have been written;
%x example
%%
<example>foo do_something();
<*>bar something_else();
The default rule (to
ECHO any unmatched character) remains active in
start conditions. It is equivalent to:
<*>.|\n ECHO;
BEGIN(0) returns to the original state where only the rules with no start
conditions are active. This state can also be referred to as the
start-condition "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 possible with
each rule. In general, when attempting to write a high-speed scanner try to
match as much possible in each rule, as it's a big win.
Note that start-conditions names are really integer 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 conditions:
- void yy_push_state(int new_state)
- pushes the current start condition onto the top of the start condition
stack and switches to new_state as though you had used BEGIN
new_state (recall that start condition names are also 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 contents.
The start condition stack grows dynamically and so has no built-in size
limitation. If memory is exhausted, program execution aborts.
To use start condition stacks, your scanner must include a
%option stack
directive (see Options below).
Some scanners (such as those which support "include" files) require
reading from several input streams. As
flex scanners do a large amount
of buffering, one cannot control where the next input will be read from by
simply writing a
YY_INPUT which is sensitive to the scanning context.
YY_INPUT is only called when the scanner reaches the end of its buffer,
which may be a long time after scanning a statement such as an
"include" which requires switching the input source.
To negotiate these sorts of problems,
flex provides a mechanism for
creating and switching between multiple input buffers. An input buffer is
created by using:
YY_BUFFER_STATE yy_create_buffer( FILE *file, int size )
which takes a
FILE pointer and a size and creates a buffer associated
with the given file and large enough to hold
size characters (when in
doubt, use
YY_BUF_SIZE for the size). It returns a
YY_BUFFER_STATE handle, which may then be passed to other routines (see
below). The
YY_BUFFER_STATE type is a pointer to an opaque
struct
yy_buffer_state structure, so you may safely initialize YY_BUFFER_STATE
variables to
((YY_BUFFER_STATE) 0) if you wish, and also refer to the
opaque structure in order to correctly declare input buffers in source files
other than that of your scanner. Note that the
FILE pointer in the call
to
yy_create_buffer is only used as the value of
yyin seen by
YY_INPUT; if you 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 particular buffer to scan from using:
void yy_switch_to_buffer( YY_BUFFER_STATE new_buffer )
switches the scanner's input buffer so subsequent tokens will come from
new_buffer. Note that
yy_switch_to_buffer() may be used by
yywrap() to set things up for continued scanning, instead of opening a new
file and pointing
yyin at it. Note also that switching input sources
via either
yy_switch_to_buffer() or
yywrap() does
not
change the start condition.
void yy_delete_buffer( YY_BUFFER_STATE buffer )
is used to reclaim the storage associated with a buffer. (
buffer can be
nil, in which case the routine does nothing.) You can also clear the current
contents of a buffer using:
void yy_flush_buffer( YY_BUFFER_STATE buffer )
This function discards the buffer's contents, so the next time the scanner
attempts to match a token from the buffer, it will first fill the buffer anew
using
YY_INPUT.
yy_new_buffer() is an alias for
yy_create_buffer(), provided for
compatibility with the C++ use of
new and
delete for creating
and destroying dynamic objects.
Finally, the
YY_CURRENT_BUFFER macro returns a
YY_BUFFER_STATE
handle to the current buffer.
Here is an example of using these features for writing a scanner which expands
include files (the
<<EOF>> feature is discussed below):
/* the "incl" state is used for picking up the name
* of an include file
*/
%x incl
%{
#define MAX_INCLUDE_DEPTH 10
YY_BUFFER_STATE include_stack[MAX_INCLUDE_DEPTH];
int include_stack_ptr = 0;
%}
%%
include BEGIN(incl);
[a-z]+ ECHO;
[^a-z\n]*\n? ECHO;
<incl>[ \t]* /* eat the whitespace */
<incl>[^ \t\n]+ { /* got the include file name */
if ( include_stack_ptr >= MAX_INCLUDE_DEPTH )
{
fprintf( stderr, "Includes nested too deeply" );
exit( 1 );
}
include_stack[include_stack_ptr++] =
YY_CURRENT_BUFFER;
yyin = fopen( yytext, "r" );
if ( ! yyin )
error( ... );
yy_switch_to_buffer(
yy_create_buffer( yyin, YY_BUF_SIZE ) );
BEGIN(INITIAL);
}
<<EOF>> {
if ( --include_stack_ptr < 0 )
{
yyterminate();
}
else
{
yy_delete_buffer( YY_CURRENT_BUFFER );
yy_switch_to_buffer(
include_stack[include_stack_ptr] );
}
}
Three routines are available for setting up input buffers for scanning in-memory
strings instead of files. All of them create a new input buffer for scanning
the string, and return a corresponding
YY_BUFFER_STATE 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.
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();
}
The macro
YY_USER_ACTION can be defined to provide an action which is
always executed prior to the matched rule's action. For example, it could be
#define'd to call a routine to convert yytext to lower-case. When
YY_USER_ACTION is invoked, the variable
yy_act gives the number
of the matched rule (rules are numbered starting with 1). Suppose you want to
profile how often each of your rules is matched. The following would do the
trick:
#define YY_USER_ACTION ++ctr[yy_act]
where
ctr is an array to hold the counts for the 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 declaration for
ctr is:
int ctr[YY_NUM_RULES];
The macro
YY_USER_INIT may be defined to provide an action which is
always executed before the first scan (and before the scanner's internal
initializations are done). For example, it could be used to call a routine to
read in a data table or open a logging file.
The macro
yy_set_interactive(is_interactive) can be used to control
whether the current buffer is considered
interactive. An interactive
buffer is processed more slowly, but must be used when the scanner's input
source is indeed interactive to avoid problems due to waiting to fill buffers
(see the discussion of the
-I flag below). A non-zero value in the
macro invocation marks the buffer as interactive, a zero value as
non-interactive. Note that use of this macro overrides
%option interactive
, %option always-interactive or
%option never-interactive
(see Options below).
yy_set_interactive() must be invoked prior to
beginning to scan the buffer that is (or is not) to be considered interactive.
The macro
yy_set_bol(at_bol) can be used to control whether the current
buffer's scanning context for the next token match is done as though at the
beginning of a line. A non-zero macro argument makes rules anchored with
'^' active, while a zero argument makes '^' rules inactive.
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.
This section summarizes the various values available to the user in the rule
actions.
- -
- char *yytext holds the text of the current token. It may be
modified but not lengthened (you cannot append characters to the
end).
- If the special directive %array appears in the first section of the
scanner description, then yytext is instead declared char
yytext[YYLMAX], where YYLMAX is a macro definition that you can
redefine in the first section if you don't like the default value
(generally 8KB). Using %array results in somewhat slower scanners,
but the value of yytext becomes immune to calls to input()
and unput(), which potentially destroy its value when yytext
is a character pointer. The opposite of %array is %pointer,
which is the default.
- You cannot use %array when generating C++ scanner classes (the
-+ flag).
- -
- int yyleng holds the length of the current token.
- -
- FILE *yyin is the file which by default flex reads from. It
may be redefined but doing so only makes sense before scanning begins or
after an EOF has been encountered. Changing it in the midst of scanning
will have unexpected results since flex buffers its input; use
yyrestart() instead. Once scanning terminates because an
end-of-file has been seen, you can assign yyin at the new input
file and then call the scanner again to continue scanning.
- -
- void yyrestart( FILE *new_file ) may be called to point yyin
at the new input file. The switch-over to the new file is immediate (any
previously buffered-up input is lost). Note that calling
yyrestart() with yyin as an argument thus throws away the
current input buffer and continues 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
current 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.
One of the main uses of
flex is as a companion to the
yacc
parser-generator.
yacc parsers expect to call a routine named
yylex() to find the next input token. The routine is supposed to return
the type of the next token as well as putting any associated value in the
global
yylval. To use
flex with
yacc, one specifies the
-d option to
yacc to instruct it to generate the file
y.tab.h containing definitions of all the
%tokens appearing in
the
yacc input. This file is then included in the
flex scanner.
For example, if one of the tokens is "TOK_NUMBER", part of the
scanner might look like:
%{
#include "y.tab.h"
%}
%%
[0-9]+ yylval = atoi( yytext ); return TOK_NUMBER;
flex has the following options:
- -b, --backup
- Generate backing-up information to lex.backup. This is a list of
scanner states which require backing up and the input characters on which
they do so. By adding rules one can remove backing-up states. If
all backing-up states are eliminated and -Cf or -CF
is used, the generated scanner will run faster (see the -p flag).
Only users who wish to squeeze every last cycle out of their scanners need
worry about this option. (See the section on Performance Considerations
below.)
- -c
- is a do-nothing, deprecated option included for POSIX compliance.
- -d, --debug
- makes the generated scanner run in debug mode. Whenever a pattern
is recognized and the global yy_flex_debug is non-zero (which is
the default), the scanner will write to stderr a line of the form:
--accepting rule at line 53 ("the matched text")
The line number refers to the location of the rule in the file defining the
scanner (i.e., the file that was fed to flex). Messages are also generated
when the scanner backs up, accepts the default rule, reaches the end of
its input buffer (or encounters a NUL; at this point, the two look the
same as far as the scanner's concerned), or reaches an end-of-file.
- -f, --full
- 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, --help
- generates a "help" summary of flex's options to
stdout and then exits. -? and --help are synonyms for
-h.
- -i, --case-insensitive
- 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, --lex-compat
- turns on maximum compatibility with the original AT&T lex
implementation. Note that this does not mean full compatibility.
Use of this option costs a considerable amount of performance, and it
cannot be used with the -+, -f, -F, -Cf, or -CF options. For
details on the compatibilities it provides, see the section
"Incompatibilities With Lex And POSIX" below. This option also
results in the name YY_FLEX_LEX_COMPAT being #define'd in the
generated scanner.
- -n
- is another do-nothing, deprecated option included only for POSIX
compliance.
- -p, --perf-report
- generates a performance report to stderr. The report consists of comments
regarding features of the flex input file which will cause a
serious loss of performance in the resulting scanner. If you give the flag
twice, you will also get 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, --no-default
- 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, --stdout
- instructs flex to write the scanner it generates to standard output
instead of lex.yy.c.
- -v, --verbose
- specifies that flex should write to stderr a summary of
statistics regarding the scanner it generates. Most of the statistics 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, --nowarn
- suppresses warning messages.
- -B, --batch
- instructs flex to generate a batch scanner, the opposite of
interactive scanners generated by -I (see below). In
general, you use -B when you are certain that your scanner
will never be used interactively, and you want to squeeze a little
more performance out of it. If your goal is instead to squeeze out a
lot more performance, you should be using the -Cf or
-CF options (discussed below), which turn on -B
automatically anyway.
- -F, --fast
- 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 "keywords" 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, --interactive
- instructs flex to generate an interactive scanner. An
interactive scanner is one that only looks ahead to decide what token has
been matched if it absolutely must. It turns out that always looking one
extra character ahead, even if the scanner has already seen enough text to
disambiguate the current token, is a bit faster than only looking ahead
when necessary. But scanners that always look ahead give dreadful
interactive performance; for example, when a user types a newline, it is
not recognized as a newline token until they enter another token,
which often means typing in another whole line.
- Flex scanners default to interactive unless you use the
-Cf or -CF table-compression options (see below). That's
because if you're looking for high-performance you should be using one of
these options, so if you didn't, flex assumes you'd rather trade
off a bit of run-time performance for intuitive interactive behavior. Note
also that you cannot use -I in conjunction with -Cf
or -CF. Thus, this option is not really needed; it is on by default
for all those cases in which it is allowed.
- Note that if isatty() returns false for the scanner input, flex
will revert to batch mode, even if -I was specified. To force
interactive mode no matter what, use %option always-interactive
(see Options below).
- You can force a scanner to not be interactive by using -B
(see above).
- -L, --noline
- instructs flex not to generate #line directives. Without
this option, flex peppers the generated scanner with #line
directives so error messages in the actions will be correctly located with
respect to either the original flex input file (if the errors are
due to code in the input file), or lex.yy.c (if the errors are
flex's fault -- you should report these sorts of errors to the
email address given below).
- -T, --trace
- makes flex run in trace mode. It will generate a lot of
messages to stderr concerning the form of the input and the
resultant non-deterministic and deterministic finite automata. This option
is mostly for use in maintaining flex.
- -V, --version
- prints the version number to stdout and exits. --version is
a synonym for -V.
- -7, --7bit
- instructs flex to generate a 7-bit scanner, i.e., one which can
only recognize 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 considerably 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 generated a 7-bit or an 8-bit
scanner by inspecting the flag summary in the -v output as
described above.
- Note that if you use -Cfe or -CFe (those table compression
options, but also using equivalence classes as discussed see below), flex
still defaults to generating an 8-bit scanner, since usually with these
compression options full 8-bit tables are not much more expensive than
7-bit tables.
- -8, --8bit
- 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.
- -+, --c++
- 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 ("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
computation. 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, --ecs 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).
Equivalence classes usually give dramatic reductions in the final
table/object file sizes (typically a factor of 2-5) and are pretty cheap
performance-wise (one array look-up per character scanned).
- -Cf specifies that the full scanner tables should be
generated - flex should not compress the tables by taking
advantages of similar transition functions for different states.
- -CF specifies that the alternative fast scanner representation
(described above under the -F flag) should be used. This option
cannot be used with -+.
- -Cm, --meta-ecs directs flex to construct
meta-equivalence classes, which are sets of equivalence classes (or
characters, if equivalence classes are not being used) that are commonly
used together. Meta-equivalence classes are often a big win when using
compressed tables, but they have a moderate performance impact (one or two
"if" tests and one array look-up per character scanned).
- -Cr, --read 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 compressed
but neither equivalence classes nor meta-equivalence classes should be
used.
- The options -Cf or -CF and -Cm do not make sense
together - there is no opportunity for meta-equivalence classes if the
table is not being compressed. Otherwise the options may be freely mixed,
and are cumulative.
- The default setting is -Cem, which specifies that flex
should generate equivalence classes and meta-equivalence classes. This
setting provides the highest degree of table compression. You can trade
off faster-executing scanners at the cost of larger tables with the
following generally being true:
slowest & smallest
-Cem
-Cm
-Ce
-C
-C{f,F}e
-C{f,F}
-C{f,F}a
fastest & largest
Note that scanners with the smallest tables are usually generated and
compiled the quickest, so during development you will usually want to use
the default, maximal compression.
- -Cfe is often a good compromise between speed and size for
production scanners.
- -ooutput, --outputfile=FILE
- directs flex to write the scanner to the file output instead of
lex.yy.c. If you combine -o with the -t option, then
the scanner is written to stdout but its #line directives
(see the -L option above) refer to the file output.
- -Pprefix, --prefix=STRING
- 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 programs
into the same executable. Note, though, that using this option also
renames yywrap(), so you now must either provide your own
(appropriately-named) version of the routine for your scanner, or use
%option noyywrap, as linking with -ll no longer provides one
for you by default.
- -Sskeleton_file, --skel=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.
- -X, --posix-compat
- maximal compatibility with POSIX lex.
- --yylineno
- track line count in yylineno.
- --yyclass=NAME
- name of C++ class.
- --header-file=FILE
- create a C header file in addition to the scanner.
- --tables-file[=FILE]
- write tables to FILE.
- -Dmacro[=defn]
- #define macro defn (default defn is '1').
- -R, --reentrant
- generate a reentrant C scanner
- --bison-bridge
- scanner for bison pure parser.
- --bison-locations
- include yylloc support.
- --stdinit
- initialize yyin/yyout to stdin/stdout.
- --noansi-definitions old-style function definitions.
- --noansi-prototypes
- empty parameter list in prototypes.
- --nounistd
- do not include <unistd.h>.
- --noFUNCTION
- do not generate a particular FUNCTION.
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 scanner,
which simply calls yylex(). This option implies noyywrap
(see below).
- never-interactive
- instructs flex to generate a scanner which never considers its input
"interactive" (again, no call made to isatty()). This is
the opposite of always-interactive.
- stack
- enables the use of start condition stacks (see Start Conditions
above).
- stdinit
- if set (i.e., %option stdinit) initializes yyin and
yyout to stdin and stdout, instead of the default of
nil. Some existing lex programs depend on this behavior,
even though it is not compliant with ANSI C, which does not require
stdin and stdout to be compile-time constant.
- yylineno
- directs flex to generate a scanner that maintains the number of the
current line read from its input in the global variable yylineno.
This option is implied by %option lex-compat.
- yywrap
- if unset (i.e., %option noyywrap), makes the scanner not call
yywrap() upon an end-of-file, but simply assume that there are no
more files to scan (until the user points yyin at a new file and
calls yylex() again).
flex scans your rule actions to determine whether you use the
REJECT or
yymore() features. The
reject and
yymore
options are available to override its decision as to whether you use the
options, either by setting them (e.g.,
%option reject) to indicate the
feature is indeed used, or unsetting them to indicate it actually is not used
(e.g.,
%option noyymore).
Three options take string-delimited values, offset with '=':
%option outfile="ABC"
is equivalent to
-oABC, and
%option prefix="XYZ"
is equivalent to
-PXYZ. Finally,
%option yyclass="foo"
only applies when generating a C++ scanner (
-+ option). It informs
flex that you have derived
foo as a subclass of
yyFlexLexer, so
flex will place your actions in the member
function
foo::yylex() instead of
yyFlexLexer::yylex(). It also
generates a
yyFlexLexer::yylex() member function that emits a run-time
error (by invoking
yyFlexLexer::LexerError()) if called. See Generating
C++ Scanners, below, for additional information.
A number of options are available for lint purists who want to suppress the
appearance of unneeded routines in the generated scanner. Each of the
following, if unset (e.g.,
%option nounput ), results in the
corresponding routine not appearing in the generated scanner:
input, unput
yy_push_state, yy_pop_state, yy_top_state
yy_scan_buffer, yy_scan_bytes, yy_scan_string
(though
yy_push_state() and friends won't appear anyway unless you use
%option stack).
The main design goal of
flex is that it generate high-performance
scanners. It has been optimized for dealing well with large sets of rules.
Aside from the effects on scanner speed of the table compression
-C
options outlined above, there are a number of options/actions which degrade
performance. These are, from most 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 happened, if anything other than another 'o' is
seen, the scanner will have to back up to simply match the 'f' (by the default
rule).
The comment regarding State #8 indicates there's a 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 eliminating backing
up only if you eliminate
every instance of backing up. Leaving just one
means you gain nothing.
Variable trailing context (where both the leading and trailing parts do
not have a fixed length) entails almost the same performance loss as
REJECT (i.e., 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 independent 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 an "auto\n" rule. To eliminate the
possibility of backing up, we could either duplicate all rules but without
final newlines, or, since we never expect to encounter such an input and
therefore don't how it's classified, we can introduce one more catch-all rule,
this one which doesn't include a newline:
%%
asm\n |
auto\n |
break\n |
... etc ...
volatile\n |
while\n /* it's a keyword */
[a-z]+\n |
[a-z]+ |
.|\n /* it's not a keyword */
Compiled with
-Cf, this is about as fast as one can get a
flex
scanner to go for this particular problem.
A final note:
flex is slow when matching NUL's, 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" quantities of text but not "huge"
quantities, where the cutoff between the two is at about 8K characters/token.
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 echoing is still done to
yyout. Both of these
remain
FILE * variables and not C++
streams.
You can also use
flex to generate a C++ scanner class, using the
-+ option (or, equivalently,
%option c++), which is
automatically specified if the name of the flex executable ends in a '+', such
as
flex++. When using this option, flex defaults to generating the
scanner to the file
lex.yy.cc instead of
lex.yy.c. The generated
scanner includes the header file
FlexLexer.h, which defines the
interface to two C++ classes.
The first class,
FlexLexer, provides an abstract base class defining the
general scanner class interface. It provides the following member functions:
- const char* YYText()
- returns the text of the most recently matched token, the equivalent of
yytext.
- int YYLeng()
- returns the length of the most recently matched token, the equivalent of
yyleng.
- int lineno() const
- returns the current input line number (see %option yylineno), or
1 if %option yylineno was not used.
- void set_debug( int flag )
- sets the debugging flag for the scanner, equivalent to assigning 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
std::istream* object pointer and not a
FILE*),
yy_flush_buffer(), yy_delete_buffer(), and
yyrestart()
(again, the first argument is a
std::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( std::istream* arg_yyin = 0, std::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 scanners:
it scans the input stream, consuming tokens, until a rule's action returns
a value. If you derive a subclass S from yyFlexLexer and
want to access the member functions and variables of S inside
yylex(), then you need to use %option yyclass="S"
to inform flex that you will be using that subclass instead of
yyFlexLexer. In this case, rather than generating
yyFlexLexer::yylex(), flex generates S::yylex() (and
also generates a dummy yyFlexLexer::yylex() that calls
yyFlexLexer::LexerError() if called).
- virtual void switch_streams(std::istream* new_in = 0,
- std::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( std::istream* new_in, std::ostream* new_out = 0 )
- first switches the input streams via switch_streams( new_in, new_out
) and then returns the value of yylex().
In addition,
yyFlexLexer defines the following protected virtual
functions which you can redefine in derived classes to tailor the scanner:
- virtual int LexerInput( char* buf, int max_size )
- reads up to max_size characters into buf and returns the
number of characters read. To indicate end-of-input, return 0 characters.
Note that "interactive" scanners (see the -B and
-I flags) define the macro YY_INTERACTIVE. If you redefine
LexerInput() and need to take different actions depending on
whether or not the scanner might be scanning an interactive input source,
you can test for the presence of this name via #ifdef.
- virtual void LexerOutput( const char* buf, int size )
- writes out size characters from the buffer buf, which, while
NUL-terminated, may also contain "internal" NUL's if the
scanner's rules can match text with NUL's in them.
- virtual void LexerError( const char* msg )
- reports a fatal error message. The default 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.
flex is a rewrite of the AT&T Unix
lex tool (the two
implementations do not share any code, though), with some extensions and
incompatibilities, both of which are of concern to those who wish to write
scanners 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 scanner's performance. We note below which incompatibilities can be
overcome using the
-l option.
flex is fully compatible with
lex with the following exceptions:
- -
- The undocumented lex scanner internal variable yylineno is
not supported unless -l or %option yylineno is used.
- yylineno should be maintained on a per-buffer basis, rather than a
per-scanner (single global variable) basis.
- yylineno is not part of the POSIX specification.
- -
- The input() routine is not redefinable, though it may be called to
read characters following 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
particular, if you have an interactive scanner and an interrupt handler
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 parentheses.
With lex, the following:
NAME [A-Z][A-Z0-9]*
%%
foo{NAME}? printf( "Found it\n" );
%%
will not match the string "foo" because when the macro is expanded
the rule is equivalent to "foo[A-Z][A-Z0-9]*?" and the
precedence is such that the '?' is associated with "[A-Z0-9]*".
With flex, the rule will be expanded to
"foo([A-Z][A-Z0-9]*)?" and so the string "foo" will
match.
- Note that if the definition begins with ^ or ends with $
then it is not expanded with parentheses, to allow these operators
to appear in definitions without losing their special meanings. But the
<s>, /, and <<EOF>> operators cannot be
used in a flex definition.
- Using -l results in the lex behavior of no 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 occurrences 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
specification:
C++ scanners
%option
start condition scopes
start condition stacks
interactive/non-interactive scanners
yy_scan_string() and friends
yyterminate()
yy_set_interactive()
yy_set_bol()
YY_AT_BOL()
<<EOF>>
<*>
YY_DECL
YY_START
YY_USER_ACTION
YY_USER_INIT
#line directives
%{}'s around actions
multiple actions on a line
plus almost all of the flex flags. The last feature in the list refers to the
fact that with
flex you can put multiple actions on the same line,
separated with semi-colons, while with
lex, the following
foo handle_foo(); ++num_foos_seen;
is (rather surprisingly) truncated to
foo handle_foo();
flex does not truncate the action. Actions that are not enclosed in
braces are simply terminated at the end of the line.
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
specification includes recognizing the 8-bit character
'x' and you did
not specify the -8 flag, and your scanner defaulted to 7-bit because you used
the
-Cf or
-CF table compression options. See the discussion of
the
-7 flag for details.
flex scanner push-back overflow - you used
unput() to push back so
much text that the scanner's buffer could not hold both the pushed-back text
and the current token in
yytext. Ideally the scanner should dynamically
resize the buffer in this case, but at present it does not.
input buffer overflow, can't enlarge buffer because scanner uses REJECT -
the scanner was working on matching an extremely large token and needed to
expand the input buffer. This doesn't work with scanners that use
REJECT.
fatal flex scanner internal error--end of buffer missed - This can occur
in a 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).
- -ll
- 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
systems).
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 above mentioned 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
directive or the
-l option has been used.
Pattern-matching of NUL's is substantially slower than matching other
characters.
Dynamic resizing of the input buffer is slow, as it entails rescanning all the
text matched so far by the current (generally huge) token.
Due to both buffering of input and read-ahead, you 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.
lex(1), yacc(1), sed(1), awk(1).
John Levine, Tony Mason, and Doug Brown,
Lex & Yacc, O'Reilly and
Associates. Be sure to get the 2nd 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).
Vern Paxson, with the help of many ideas and much inspiration from Van Jacobson.
Original version by Jef Poskanzer. The fast table representation is a partial
implementation of a design done by Van Jacobson. The implementation was done
by Kevin Gong and Vern Paxson.
Thanks to the many
flex beta-testers, feedbackers, and contributors,
especially Francois Pinard, Casey Leedom, Robert Abramovitz, Stan Adermann,
Terry Allen, David Barker-Plummer, John Basrai, Neal Becker, Nelson H.F.
Beebe, benson@odi.com, Karl Berry, Peter A. Bigot, Simon Blanchard, Keith
Bostic, Frederic Brehm, Ian Brockbank, Kin Cho, Nick Christopher, Brian
Clapper, J.T. Conklin, Jason Coughlin, Bill Cox, Nick Cropper, Dave Curtis,
Scott David Daniels, Chris G. Demetriou, Theo de Raadt, 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 Rahmeh, Jarmo Raiha, Frederic Raimbault, Pat Rankin, Rick Richardson,
Kevin Rodgers, Kai Uwe Rommel, Jim Roskind, Alberto Santini, Andreas Scherer,
Darrell Schiebel, Raf Schietekat, Doug Schmidt, Philippe Schnoebelen, 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.