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 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 "*/".
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
<*>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)*
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.
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 input() and unput() functions destroy 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 input() and 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 accomodate large tokens. While this means your %pointer scanner can accomodate 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 -+ option; see below).
%%
"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). Modifying the final character of yytext may alter whether when scanning resumes rules anchored with '^' are active. Specifically, changing the final character of yytext to a newline will activate such rules on the next scan, and changing it to anything else will deactivate the rules. Users should not rely on this behavior being present in future releases. Finally, note that none of this paragraph applies when using %array (see above).
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:
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
-Cf
or
-CF
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.
Note that yyless is a macro and can only be used in the flex input file, not from other source files.
{
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.
Also note that you cannot put back
EOF
to attempt to mark the input stream with an end-of-file.
%%
"/*" {
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 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, 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.
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.
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.
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 */
Also note that the special start-condition specifier <*> matches every start condition. Thus, the above example could also have been written;
%x 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);
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;
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 *yytext_ptr = yytext;
while ( *yytext_ptr )
*string_buf_ptr++ = *yytext_ptr++;
}
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 set 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_delete_buffer( YY_CURRENT_BUFFER );
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" );
else
yyterminate();
}
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.
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;
--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.
"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}
-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.
yyFlexLexer
yy_create_buffer
yy_delete_buffer
yy_flex_debug
yy_init_buffer
yy_load_buffer_state
yy_switch_to_buffer
yyin
yyleng
yylex
yyout
yyrestart
yytext
yywrap
Within your scanner itself, you can still refer to the global variables
and functions using either version of their name; but eternally, they
have the modified name.
REJECT
pattern sets that require backing up
arbitrary trailing context
yymore()
'^' beginning-of-line operator
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).
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
A final note regarding performance: as mentioned above in the section How the Input is Matched, dynamically resizing yytext to accomodate 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.
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 a "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.
You can also use flex to generate a C++ scanner class, using the -+ option, 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:
Also provided are member functions equivalent to yy_switch_to_buffer(), yy_create_buffer() (though the first argument is an istream* object pointer and not a FILE*), yy_delete_buffer(), and yyrestart() (again, the first argument is a istream* object pointer).
The second class defined in FlexLexer.h is yyFlexLexer, which is derived from FlexLexer. It defines the following additional member functions:
In addition, yyFlexLexer defines the following protected virtual functions which you can redefine in derived classes to tailor the scanner:
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;
}
IMPORTANT: the present form of the scanning class is
experimental
and may change considerably between major releases.
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:
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.
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.
The following flex features are not included in lex or the POSIX specification:
yyterminate()
<<EOF>>
<*>
YY_DECL
YY_START
YY_USER_ACTION
#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). Make an explicit reference to the action in your flex input file. (Note that previously flex supported a %used/%unused mechanism for dealing with this problem; this feature is still supported but now deprecated, and will go away soon unless the author hears from people who can argue compellingly that they need it.)
flex scanner jammed - a scanner compiled with -s has 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 an scanner which is reentered after a long-jump has jumped out (or over) the scanner's activation frame. Before reentering the scanner, use:
yyrestart( yyin );
or, as noted above, switch to using the C++ scanner class.
too many start conditions in <> construct! - you listed more start conditions in a <> construct than exist (so you must have listed at least one of them twice).
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 Francois Pinard, Casey Leedom, Nelson H.F. Beebe, benson@odi.com, Peter A. Bigot, Keith Bostic, Frederic Brehm, Nick Christopher, Jason Coughlin, Bill Cox, Dave Curtis, Scott David Daniels, Chris G. Demetriou, Mike Donahue, Chuck Doucette, Tom Epperly, Leo Eskin, Chris Faylor, Jon Forrest, Kaveh R. Ghazi, Eric Goldman, Ulrich Grepel, Jan Hajic, Jarkko Hietaniemi, Eric Hughes, John Interrante, Ceriel Jacobs, Jeffrey R. Jones, Henry Juengst, Amir Katz, ken@ken.hilco.com, Kevin B. Kenny, Marq Kole, Ronald Lamprecht, Greg Lee, Craig Leres, John Levine, Steve Liddle, Mohamed el Lozy, Brian Madsen, Chris Metcalf, Luke Mewburn, Jim Meyering, G.T. Nicol, Landon Noll, Marc Nozell, Richard Ohnemus, Sven Panne, Roland Pesch, Walter Pelissero, Gaumond Pierre, Esmond Pitt, Jef Poskanzer, Joe Rahmeh, Frederic Raimbault, Rick Richardson, Kevin Rodgers, Jim Roskind, Doug Schmidt, Philippe Schnoebelen, Andreas Schwab, Alex Siegel, Mike Stump, Paul Stuart, Dave Tallman, Chris Thewalt, Paul Tuinenga, Gary Weik, Frank Whaley, Gerhard Wilhelms, Kent Williams, Ken Yap, 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 Paxson
Systems Engineering
Bldg. 46A, Room 1123
Lawrence Berkeley Laboratory
University of California
Berkeley, CA 94720
vern@ee.lbl.gov