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.
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 definitionThe "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 actionwhere 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, an unindented comment (i.e., a line beginning with "/*") is also copied verbatim to the output up to the next "*/". Also, any line in the definitions section beginning with '#' is ignored, though this style of comment is deprecated and may go away in the future.
x match the character 'x'
. any character except newline
[xyz] a "character class"; in this case, the pattern
matches either an 'x', a 'y', or a 'z'
[abj-oZ] a "character class" with a range in it; matches
an 'a', a 'b', any letter from 'j' through 'o',
or a 'Z'
[^A-Z] a "negated character class", i.e., any character
but those in the class. In this case, any
character EXCEPT an uppercase letter.
[^A-Z\n] any character EXCEPT an uppercase letter or
a newline
r* zero or more r's, where r is any regular expression
r+ one or more r's
r? zero or one r's (that is, "an optional r")
r{2,5} anywhere from two to five r's
r{2,} two or more r's
r{4} exactly 4 r's
{name} the expansion of the "name" definition
(see above)
"[xyz]\"foo"
the literal string: [xyz]"foo
\X if X is an 'a', 'b', 'f', 'n', 'r', 't', or 'v',
then the ANSI-C interpretation of \x.
Otherwise, a literal 'X' (used to escape
operators such as '*')
\123 the character with octal value 123
\x2a the character with hexadecimal value 2a
(r) match an r; parentheses are used to override
precedence (see below)
rs the regular expression r followed by the
regular expression s; called "concatenation"
r|s either an r or an s
r/s an r but only if it is followed by an s. The
s is not part of the matched text. This type
of pattern is called as "trailing context".
^r an r, but only at the beginning of a line
r$ an r, but only at the end of a line. Equivalent
to "r/\n".
<s>r an r, but only in start condition s (see
below for discussion of start conditions)
<s1,s2,s3>r
same, but in any of start conditions s1,
s2, or s3
<<EOF>> an end-of-file
<s1,s2><<EOF>>
an end-of-file when in start condition s1 or s2
The regular expressions listed above are grouped according to
precedence, from highest precedence at the top to lowest at the bottom.
Those grouped together have equal precedence. 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)*
Some notes on patterns:
foo/bar$
<sc1>foo<sc2>bar
Note that the first of these, can be written "foo/bar\n".
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.
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.
%%
"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. Once it reaches an end-of-file, however, then any subsequent call to yylex() will simply immediately return, unless yyrestart() is first called (see below).
Actions are not allowed to modify yytext or yyleng.
There are a number of special directives which can be included within an action:
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 scanner performance;
if it is used in
any
of the scanner's actions it will slow down
all
of the scanner's matching. Furthermore,
REJECT
cannot be used with the
-f
or
-F
options (see below).
%%
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".
The presence of
yymore()
in the scanner's action entails a minor performance penalty in the
scanner's matching speed.
%%
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.
{
int i;
unput( ')' );
for ( i = yyleng - 1; i >= 0; --i )
unput( yytext[i] );
unput( '(' );
}
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.
%%
"/*" {
register int c;
for ( ; ; )
{
while ( (c = input()) != '*' &&
c != EOF )
; /* eat up text of comment */
if ( c == '*' )
{
while ( (c = input()) == '*' )
;
if ( c == '/' )
break; /* found the end */
}
if ( c == EOF )
{
error( "EOF in comment" );
break;
}
}
}
(Note that if the scanner is compiled using
C++,
then
input()
is instead referred to as
yyinput(),
in order to avoid a name clash with the
C++
stream by the name of
input.)
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 redefining
the "YY_DECL" macro. For example, you could use:
#undef YY_DECL
#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. In the former case, when called again the scanner will immediately return unless yyrestart() is called to point yyin at the new input file. ( yyrestart() takes one argument, a FILE * pointer.) In the latter case (i.e., when an action executes a return), 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 redefining the YY_INPUT macro. YY_INPUT's calling sequence is "YY_INPUT(buf,result,max_size)". Its action is to place up to max_size characters in the character array buf and return in the integer variable result either the number of characters read or the constant YY_NULL (0 on Unix systems) to indicate EOF. The default YY_INPUT reads from the global file-pointer "yyin".
A sample redefinition of YY_INPUT (in the definitions section of the input file):
%{
#undef YY_INPUT
#define YY_INPUT(buf,result,max_size) \
{ \
int c = getchar(); \
result = (c == EOF) ? YY_NULL : (buf[0] = c, 1); \
}
%}
This definition will change the input processing to occur
one character at a time.
You also can add in things like keeping track of the input line number this way; but don't expect your scanner to go very fast.
When the scanner receives an end-of-file indication from YY_INPUT, it then checks the yywrap() function. If yywrap() returns false (zero), then it is assumed that the function has gone ahead and set up yyin to point to another input file, and scanning continues. If it returns true (non-zero), then the scanner terminates, returning 0 to its caller.
The default yywrap() always returns 1. Presently, to redefine it you must first "#undef yywrap", as it is currently implemented as a macro. As indicated by the hedging in the previous sentence, it may be changed to a true function in the near future.
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.
<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 */
is equivalent to
%x example
%%
<INITIAL,example>foo /* do something */
The default rule (to ECHO any unmatched character) remains active in start conditions.
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 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);
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);
One can then implement a "stack" of start conditions using an
array of integers. (It is likely that such stacks will become
a full-fledged
flex
feature in the future.) Note, though, that
start conditions do not have their own name-space; %s's and %x's
declare names in the same fashion as #define's.
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:
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 sets things up for continued scanning, instead of opening a new file and pointing yyin at it.
void yy_delete_buffer( YY_BUFFER_STATE buffer )is used to reclaim the storage associated with a buffer.
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_switch_to_buffer(
include_stack[include_stack_ptr] );
}
<<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" );
YY_NEW_FILE;
}
else
yyterminate();
}
The macro YY_USER_INIT may be redefined 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.
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.
%{
#include "y.tab.h"
%}
%%
[0-9]+ yylval = atoi( yytext ); return TOK_NUMBER;
%t
1 abcd
2 ABCDEFGHIJKLMNOPQRSTUVWXYZ
52 0123456789
6 \t\ \n
%t
This example specifies that the characters 'a', 'b', 'c', and 'd'
are to all be lumped into group #1, upper-case letters
in group #2, digits in group #52, tabs, blanks, and newlines into
group #6, and
no other characters will appear in the patterns.
The group numbers are actually disregarded by
flex;
%t
serves, though, to lump characters together. Given the above
table, for example, the pattern "a(AA)*5" is equivalent to "d(ZQ)*0".
They both say, "match any character in group #1, followed by
zero-or-more pairs of characters
from group #2, followed by a character from group #52." Thus
%t
provides a crude way for introducing equivalence classes into
the scanner specification.
Note that the -i option (see below) coupled with the equivalence classes which flex automatically generates take care of virtually all the instances when one might consider using %t. But what the hell, it's there if you want it.
--accepting rule at line 53 ("the matched text")
The line number refers to the location of the rule in the file
defining the scanner (i.e., the file that was fed to flex). Messages
are also generated when the scanner backtracks, accepts the
default rule, reaches the end of its input buffer (or encounters
a NUL; at this point, the two look the same as far as the scanner's concerned),
or reaches an end-of-file.
"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.
slowest & smallest
-Cem
-Cm
-Ce
-C
-C{f,F}e
-C{f,F}
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.
REJECT
pattern sets that require backtracking
arbitrary trailing context
'^' beginning-of-line operator
yymore()
with the first three all being quite expensive and the last two
being quite cheap.
REJECT should be avoided at all costs when performance is important. It is a particularly expensive option.
Getting rid of backtracking 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.backtrack 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 backtrack.
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 backtrack 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 a 'b', the scanner will have to back up to accept "foo". Similarly, the comment for State #9 concerns when "fooba" has been scanned.
The final comment reminds us that there's no point going to all the trouble of removing backtracking from the rules unless we're using -f or -F, since there's no performance gain doing so with compressed scanners.
The way to remove the backtracking 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 backtracking 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.
Backtracking 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 backtracking (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 backtracking).
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
BUGS
in flex(1)).
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 backtracking
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 backtracking
when it has scanned a token like "auto" and then the next character
is something other than a newline or a letter. Previously it would
then just match the "auto" rule and be done, but now it has no "auto"
rule, only a "auto\n" rule. To eliminate the possibility of backtracking,
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.
flex is fully compatible with lex with the following exceptions:
fatal flex scanner internal error--end of buffer missedTo reenter the scanner, first use
yyrestart( yyin );
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 because of this, the
^, $, <s>, /,
and
<<EOF>>
operators cannot be used in a
flex
definition.
The following flex features are not included in lex or the POSIX draft standard:
yyterminate()
<<EOF>>
YY_DECL
#line directives
%{}'s around actions
yyrestart()
comments beginning with '#' (deprecated)
multiple actions on a line
This last feature 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.
flex scanner jammed - a scanner compiled with -s has encountered an input string which wasn't matched by any of its rules.
flex input buffer overflowed - a scanner rule matched a string long enough to overflow the scanner's internal input buffer (16K bytes by default - controlled by YY_BUF_SIZE in "flex.skel". Note that to redefine this macro, you must first #undefine it).
scanner requires -8 flag - Your scanner specification includes recognizing 8-bit characters and you did not specify the -8 flag (and your site has not installed flex with -8 as the default).
fatal flex scanner internal error--end of buffer missed - This can occur in an scanner which is reentered after a long-jump has jumped out (or over) the scanner's activation frame. Before reentering the scanner, use:
yyrestart( yyin );
too many %t classes! - You managed to put every single character into its own %t class. flex requires that at least one of the classes share characters.
flex(1), lex(1), yacc(1), sed(1), awk(1).
M. E. Lesk and E. Schmidt, LEX - Lexical Analyzer Generator
Thanks to the many flex beta-testers, feedbackers, and contributors, especially Casey Leedom, benson@odi.com, Keith Bostic, Frederic Brehm, Nick Christopher, Jason Coughlin, Scott David Daniels, Leo Eskin, Chris Faylor, Eric Goldman, Eric Hughes, Jeffrey R. Jones, Kevin B. Kenny, Ronald Lamprecht, Greg Lee, Craig Leres, Mohamed el Lozy, Jim Meyering, Marc Nozell, Esmond Pitt, Jef Poskanzer, Jim Roskind, Dave Tallman, Frank Whaley, Ken Yap, and those whose names have slipped my marginal mail-archiving skills but whose contributions are appreciated all the same.
Thanks to Keith Bostic, John Gilmore, Craig Leres, Bob Mulcahy, 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 Ove Ewerlid for the basics of support for NUL's; and to Eric Hughes for the basics of support for multiple buffers.
Work is being done on extending flex to generate scanners in which the state machine is directly represented in C code rather than tables. These scanners may well be substantially faster than those generated using -f or -F. If you are working in this area and are interested in comparing notes and seeing whether redundant work can be avoided, contact Ove Ewerlid (ewerlid@mizar.DoCS.UU.SE).
This work was primarily done when I was at 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 Paxson
Computer Systems Engineering
Bldg. 46A, Room 1123
Lawrence Berkeley Laboratory
University of California
Berkeley, CA 94720
vern@ee.lbl.gov
ucbvax!ee.lbl.gov!vern