Application
Development
Michael K. Johnson
Erik W. Troan
|
|
Linux
Application Development |
Michael K. Johnson Erik W. Troan |
flex - 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:
flex is a tool for generating scanners: programs which
recognized lexical patterns in text. flex reads the given
input files, or its standard input if no file names are
given, for a description of a scanner to generate. The
description is in the form of pairs of regular expressions
and C code, called rules. flex generates as output a C
source file, `lex.yy.c', which defines a routine `yylex()'.
This file is compiled and linked with the `-lfl' library to
produce an executable. When the executable is run, it
analyzes its input for occurrences of the regular
expressions. Whenever it finds one, it executes the
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:
2a
flex
cannot match correctly; see notes in the Deficiencies / Bugs section
below regarding "dangerous trailing context".)
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:
foo/bar$ <sc1>foo<sc2>barNote 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|^barIf 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:
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.
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.
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.
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.
%%
"/*" {
register int c;
for ( ; ; )
{
while ( (c = input()) != '*' &&
c != EOF )
; /* eat up text of comment */
if ( c == '*' )
{
while ( (c = input()) == '*' )
;
if ( c == '/' )
break; /* found the end */
}
if ( c == EOF )
{
error( "EOF in comment" );
break;
}
}
}
(Note that if the scanner is compiled using `C++',
then `input()' is instead referred to as `yyinput()',
in order to avoid a name clash with the `C++' stream
by the name of input.)
YY_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.
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 `-lfl' to obtain the default version of the routine, which always returns 1.
Three routines are available for scanning from in-memory buffers rather than files: `yy_scan_string()', `yy_scan_bytes()', and `yy_scan_buffer()'. See the discussion of them below in the section Multiple Input Buffers.
The scanner writes its `ECHO' output to the yyout global
(default, 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'
starting 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 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]+ {
Version 2.5 December 1994 18
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:
BEGIN.
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. 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.
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_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:
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);
return statement;
<<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 always-interactive' or `%option never-interactive' (see Options below). `yy_set_interactive()' must be invoked prior to beginning to scan the buffer that is (or is not) to be considered interactive.
The macro `yy_set_bol(at_bol)' can be used to control whether the current buffer's scanning context for the next token match is done as though at the beginning of a line. A non-zero macro argument makes rules anchored with
The macro `YY_AT_BOL()' returns true if the next token scanned from the current buffer will have '^' rules active, false otherwise.
In the generated scanner, the actions are all gathered in
one large switch statement and separated using YY_BREAK,
which may be redefined. By default, it is simply a
"break", to separate each rule's action from the following
rule's. Redefining YY_BREAK allows, for example, C++
users to #define YY_BREAK to do nothing (while being very
careful that every rule ends with a "break" or a
"return"!) to avoid suffering from 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.
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).
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.
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.
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.
yacc
One of the main uses of flex is as a companion to the yacc
parser-generator. yacc parsers expect to call a routine
named `yylex()' to find the next input token. The routine
is supposed to return the type of the next token as well
as putting any associated value in the global yylval. To
use flex with yacc, one specifies the `-d' option to yacc to
instruct it to generate the file `y.tab.h' containing
definitions of all the `%tokens' appearing in the yacc input.
This file is then included in the flex scanner. For
example, if one of the tokens is "TOK_NUMBER", part of the
scanner might look like:
%{
#include "y.tab.h"
%}
%%
[0-9]+ yylval = atoi( yytext ); return TOK_NUMBER;
flex has the following options:
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.
flex's options to
stdout and then exits. `-?' and `--help' are synonyms
for `-h'.
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).
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.
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.
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.
flex to write the scanner it generates to
standard output instead of `lex.yy.c'.
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.
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.
"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 `-+'.
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.
You can force a scanner to not be interactive by
using `-B' (see above).
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).
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.
stdout and exits.
`--version' is a synonym for `-V'.
flex to generate a 7-bit scanner, i.e.,
one which can only recognized 7-bit characters in
its input. The advantage of using `-7' is that the
scanner's tables can be up to half the size of
those generated using the `-8' option (see below).
The disadvantage is that such scanners often hang
or crash if their input contains an 8-bit
character.
Note, however, that unless you generate your
scanner using the `-Cf' or `-CF' table compression options,
use of `-7' will save only a small amount of table
space, and make your scanner 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.
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.
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 alternate fast scanner
representation (described above under the `-F' flag)
should be used. This option cannot be used with
`-+'.
`-Cm' directs flex to construct meta-equivalence
classes, which are sets of equivalence classes (or
characters, if equivalence classes are not being
used) that are commonly used together.
Meta-equivalence classes are often a big win when using
compressed tables, but they have a moderate
performance impact (one or two "if" tests and one array
look-up per character scanned).
`-Cr' causes the generated scanner to bypass use of
the standard I/O library (stdio) for input.
Instead of calling `fread()' or `getc()', the scanner
will use the `read()' system call, resulting in a
performance gain which varies from system to
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.
put 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.
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 `-lfl' no longer provides one for you by
default.
flex
constructs its scanners. You'll never need this
option unless you are doing flex maintenance or
development.
flex also provides a mechanism for controlling options
within the scanner specification itself, rather than from
the flex command-line. This is done by including `%option'
directives in the first section of the scanner
specification. You can specify multiple options with a single
`%option' directive, and multiple directives in the first
section of your flex input file. Most options are given
simply as names, optionally preceded by the word "no"
(with no intervening whitespace) to negate their meaning.
A number are equivalent to flex flags or their negation:
7bit -7 option
8bit -8 option
align -Ca option
backup -b option
batch -B option
c++ -+ option
caseful or
case-sensitive opposite of -i (default)
case-insensitive or
caseless -i option
debug -d option
default opposite of -s option
ecs -Ce option
fast -F option
full -f option
interactive -I option
lex-compat -l option
meta-ecs -Cm option
perf-report -p option
read -Cr option
stdout -t option
verbose -v option
warn opposite of -w option
(use "%option nowarn" for -w)
array equivalent to "%array"
pointer equivalent to "%pointer" (default)
Some `%option's' provide features otherwise not available:
noyywrap (see below).
yyin
and yyout to nil FILE pointers, instead of stdin
and stdout.
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'.
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, 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 head-scratching 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 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.
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:
yytext.
yyleng.
yy_flex_debug (see the Options section above). Note that you
must build the scanner using `%option debug' to include debugging
information in it.
Also provided are member functions equivalent to `yy_switch_to_buffer(), yy_create_buffer()' (though the first argument is an `istream*' object pointer and not a `FILE*', `yy_flush_buffer()', `yy_delete_buffer()', and `yyrestart()' (again, the first argument is a `istream*' object pointer).
The second class defined in `FlexLexer.h' is yyFlexLexer,
which is derived from FlexLexer. It defines the following
additional member functions:
yyFlexLexer object using the given
streams for input and output. If not specified,
the streams default to cin and cout, respectively.
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).
yyin to new_in
(if non-nil)
and yyout to new_out
(ditto), deleting the previous input buffer if yyin
is reassigned.
In addition, yyFlexLexer defines the following protected
virtual functions which you can redefine in derived
classes to tailor the scanner:
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'.
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';
%%
Version 2.5 December 1994 44
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.
lex and POSIX
flex is a rewrite of the AT&T Unix lex tool (the two
implementations do not share any code, though), with some
extensions and incompatibilities, both of which are of
concern to those who wish to write scanners 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:
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.
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.
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 missedTo 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.
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.
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.
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.
lex `%r' (generate a Ratfor scanner) option is
not supported. It is not part of the POSIX
specification.
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.
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.
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.
lex are not required by flex scanners;
flex ignores them.
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
semicolons, 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.
[a-z]+ got_identifier(); foo got_foo();Using
REJECT in a scanner suppresses this warning.
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.
MAX constant (8K bytes by default). You can increase the
value by #define'ing YYLMAX in the definitions section of
your flex input.
yytext.
Ideally the scanner should dynamically resize the buffer
in this case, but at present it does not.
REJECT.
yyrestart( yyin );or, as noted above, switch to using the C++ scanner class.
FlexLexer, and its derived class, yyFlexLexer.
Some trailing context patterns cannot be properly matched and generate warning messages ("dangerous trailing context"). These are patterns where the ending of the first part of the rule matches the beginning of the second part, such as "zx*/xy*", where the 'x*' matches the 'x' at the beginning of the trailing context. (Note that the POSIX draft states that the text matched by such patterns is undefined.)
For some trailing context rules, parts which are actually fixed-length are not recognized as such, leading to the abovementioned performance loss. In particular, parts using '|' or {n} (such as "foo{3}") are always considered variable-length.
Combining trailing context with the special '|' action can result in fixed trailing context being turned into the more expensive variable trailing context. For example, in the following:
%% abc | xyz/def
Use of `unput()' invalidates yytext and yyleng, unless the `%array' 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, Stan
Adermann, Terry Allen, David Barker-Plummer, John Basrai, Nelson
H.F. Beebe, `benson@odi.com', Karl Berry, Peter A. Bigot,
Simon Blanchard, Keith Bostic, Frederic Brehm, Ian Brockbank, Kin
Cho, Nick Christopher, Brian Clapper, J.T. Conklin, Jason Coughlin,
Bill Cox, Nick Cropper, Dave Curtis, Scott David Daniels, Chris
G. Demetriou, Theo Deraadt, Mike Donahue, Chuck Doucette, Tom Epperly,
Leo Eskin, Chris Faylor, Chris Flatters, Jon Forrest, Joe Gayda, Kaveh
R. Ghazi, 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, 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, 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'.
This document was generated on 16 September 1996 using the texi2html translator version 1.51.