flex - fast lexical analyzer generator
flex [-78BbcdFfhIiLlnpsTtVvw+?] [-C[aeFfmr]] [--help]
[--version]
[-ooutput] [-Pprefix] [-Sskeleton] [filename...]
This manual describes flex, a tool for generating programs
that perform
pattern-matching on text. The manual includes both tutorial
and reference
sections:
Description [Toc] [Back]
A brief overview of the tool.
Some Simple Examples [Toc] [Back]
Format of the Input File [Toc] [Back]
Patterns [Toc] [Back]
The extended regular expressions used by flex.
How the Input is Matched [Toc] [Back]
The rules for determining what has been matched.
Actions [Toc] [Back]
How to specify what to do when a pattern is matched.
The Generated Scanner [Toc] [Back]
Details regarding the scanner that flex produces; how to
control the input
source.
Start Conditions [Toc] [Back]
Introducing context into scanners, and managing "mini-scanners".
Multiple Input Buffers [Toc] [Back]
How to manipulate multiple input sources; how to scan from
strings instead
of files.
End-of-File Rules [Toc] [Back]
Special rules for matching the end of the input.
Miscellaneous Macros [Toc] [Back]
A summary of macros available to the actions.
Values Available to the User [Toc] [Back]
A summary of values available to the actions.
Interfacing with Yacc [Toc] [Back]
Connecting flex scanners together with yacc(1) parsers.
Options [Toc] [Back]
flex command-line options, and the ``%option'' directive.
Performance Considerations [Toc] [Back]
How to make scanners go as fast as possible.
Generating C++ Scanners
The (experimental) facility for generating C++ scanner
classes.
Incompatibilities with Lex and POSIX [Toc] [Back]
How flex differs from AT&T lex and the POSIX lex standard.
Files [Toc] [Back]
Files used by flex.
Diagnostics [Toc] [Back]
Those error messages produced by flex (or scanners it generates) whose
meanings might not be apparent.
See Also [Toc] [Back]
Other documentation, related tools.
Authors [Toc] [Back]
Includes contact information.
Bugs [Toc] [Back]
Known problems with flex.
flex is a tool for generating scanners: programs which recognize lexical
patterns in text. flex reads the given input files, or its
standard input
if no file names are given, for a description of a scanner to generate.
The description is in the form of pairs of regular expressions and
C code, called rules. flex generates as output a C source
file,
lex.yy.c, which defines a routine yylex(). This file is
compiled and
linked with the -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;
%%
++num_lines; ++num_chars;
. ++num_chars;
%%
main()
{
yylex();
printf("# of lines = %d, # of chars = %d0,
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 ("0) 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)0, yytext,
atoi(yytext));
}
{DIGIT}+"."{DIGIT}* {
printf("A float: %s (%g)0, yytext,
atof(yytext));
}
if|then|begin|end|procedure|function {
printf("A keyword: %s0, yytext);
}
{ID} printf("An identifier: %s0, yytext);
"+"|"-"|"*"|"/" printf("An operator: %s0, yytext);
"{"[^}0*"}" /* eat up one-line comments */
[ 0+ /* eat up whitespace */
. printf("Unrecognized character: %s0, yytext);
%%
main(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.
FORMAT OF THE INPUT FILE [Toc] [Back] 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-whitespace 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]*
This 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
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 welldefined
and it may well cause compile-time errors (this feature is present
for POSIX compliance; see below for other such features).
In the definitions section (but not in the rules section),
an unindented
comment (i.e., a line beginning with "/*") is also copied
verbatim to the
output up to the next "*/".
The patterns in the input are written using an extended set
of regular
expressions. These are:
x Match the character `x'.
. Any character (byte) except newline.
[xyz] A "character class"; in this case, the pattern
matches either
an `x', a `y', or a `z'.
[abj-oZ] A "character class" with a range in it; matches an
`a', a `b',
any letter from `j' through `o', or a `Z'.
[^A-Z] A "negated character class", i.e., any character
but those in
the class. In this case, any character EXCEPT an
uppercase
letter.
[^A-Z0 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]
The literal string: [xyz]"foo.
If `X' is an `a', `b', `f', `n', `r', `t', or `v',
then the ANSI-C
interpretation of ` (used to
escape operators such as `*').
A NUL character (ASCII code 0).
123 The character with octal value 123.
a The character with hexadecimal value 2a.
(r) Match an `r'; parentheses are used to override
precedence (see
below).
rs The regular expression `r' followed by the regular
expression
`s'; called "concatenation".
r|s Either an `r' or an `s'.
r/s An `r', but only if it is followed by an `s'. The
text matched
by `s' is included when determining whether this
rule is the
"longest match", but is then returned to the input
before the
action is executed. So the action only sees the
text matched
by `r'. This type of pattern is called "trailing
context".
(There are some combinations of r/s that flex cannot match correctly;
see notes in the BUGS section below regarding
"dangerous trailing context".)
^r An `r', but only at the beginning of a line (i.e.,
just starting
to scan, or right after a newline has been
scanned).
r$ An `r', but only at the end of a line (i.e., just
before a
newline). Equivalent to "r/0.
Note that flex's notion of "newline" is exactly
whatever the C
compiler used to compile flex interprets `0 as.
<s>r An `r', but only in start condition `s' (see below
for
discussion of start conditions).
<s1,s2,s3>r
The 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 zeroor-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(3) returns true -
i.e., any alphabetic
or numeric. Some systems don't provide isblank(3),
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 the scanner is case-insensitive (the -i flag), then [:upper:] and
[:lower:] are equivalent to [:alpha:].
Some notes on patterns:
- A negated character class such as the example "[^A-Z]"
above will
match a newline unless "0 (or an equivalent escape sequence) is one
of the characters explicitly present in the negated
character class
(e.g., "[^A-Z0"). This is unlike how many other regular
expression
tools treat negated character classes, but unfortunately
the inconsistency
is historically entrenched. Matching newlines
means that a
pattern like "[^"]*" can match the entire input unless
there's another
quote in the input.
- A rule can have at most one instance of trailing context
(the `/' operator
or the `$' operator). The start condition, `^',
and "<<EOF>>"
patterns can only occur at the beginning of a pattern,
and, as well
as with `/' and `$', cannot be grouped inside parentheses. A `^'
which does not occur at the beginning of a rule or a `$'
which does
not occur at the end of a rule loses its special properties and is
treated as a normal character.
- The following are illegal:
foo/bar$
<sc1>foo<sc2>bar
Note that the first of these, can be written "foo/bar0.
- The following will result in `$' or `^' being treated as
a normal
character:
foo|(bar$)
foo|^bar
If what's wanted is a "foo" or a bar-followed-by-a-newline, the following
could be used (the special `|' action is explained below):
foo |
bar$ /* action goes here */
A similar trick will work for matching a foo or a barat-the-beginning-of-a-line.
HOW THE INPUT IS MATCHED [Toc] [Back] 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. Which definition
flex uses can be
controlled by including one of the special directives
``%pointer'' or
``%array'' in the first (definitions) section of flex input.
The default
is ``%pointer'', unless the -l lex compatibility option is
used, 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 not enough dynamic memory is available). The disadvantage
is that actions are restricted in how they can modify yytext (see
the next section), and calls to the unput() function 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 yytext can be modified
as much as
wanted, 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. The size can be changed by
simply #define'ing
YYLMAX to a different value in the first section of
flex input.
As mentioned above, with ``%pointer'' yytext grows dynamically to accommodate
large tokens. While this means a ``%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 ``%array'' cannot be used 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:
%%
[ ]+ putchar(' ');
[ ]+$ /* ignore this token */
If the action contains a `{', then the action spans till the
balancing
`}' is found, and the action may cross multiple lines. flex
knows about
C strings and comments and won't be fooled by braces found
within them,
but also allows actions to begin with `%{' and will consider
the action
to be all the text up to the next `%}' (regardless of ordinary braces
inside the action).
An action consisting solely of a vertical bar (`|') means
"same as the
action for the next rule". See below for an illustration.
Actions can include arbitrary C code, including return
statements to return
a value to whatever routine called yylex(). Each time
yylex() is
called, it continues processing tokens from where it last
left off until
it either reaches the end of the file or executes a return.
Actions are free to modify yytext except for lengthening it
(adding characters
to its end - these will overwrite later characters in
the input
stream). This, however, does not apply when using ``%array'' (see
above); in that case, yytext may be freely modified in any
way.
Actions are free to modify yyleng except they should not do
so if the action
also includes use of yymore() (see below).
There are a number of special directives which can be included within an
action:
ECHO Copies yytext to the scanner's output.
BEGIN Followed by the name of a start condition, places
the scanner in
the corresponding start condition (see below).
REJECT Directs the scanner to proceed on to the "second
best" rule which
matched the input (or a prefix of the input). The
rule is chosen
as described above in HOW THE INPUT IS MATCHED, and
yytext and
yyleng set up appropriately. It may either be one
which matched
as much text as the originally chosen rule but came
later in the
flex input file, or one which matched less text.
For example,
the following will both count the words in the input
and call the
routine special() whenever "frob" is seen:
int word_count = 0;
%%
frob special(); REJECT;
[^ 0+ ++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;
.| /* eat up any unmatched character */
(The first three rules share the fourth's action
since they use
the special `|' action.) REJECT is a particularly
expensive feature
in terms of scanner performance; if it is used
in any of the
scanner's actions it will slow down all of the scanner's matching.
Furthermore, REJECT cannot be used with the
-Cf or -CF options
(see below).
Note also that unlike the other special actions,
REJECT is a
branch; code immediately following it in the action
will not be
executed.
yymore()
Tells the scanner that the next time it matches a
rule, the corresponding
token should be appended onto the current
value of
yytext rather than replacing it. For example, given
the input
"mega-kludge" the following will write "mega-megakludge" 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 yyleng must not be modified when using
yymore(). Second,
the presence of yymore() in the scanner's action entails a
minor performance penalty in the scanner's matching
speed.
yyless(n)
Returns all but the first n characters of the current token back
to the input stream, where they will be rescanned
when the scanner
looks for the next match. yytext and yyleng are
adjusted appropriately
(e.g., yyleng will now be equal to n).
For example,
on the input "foobar" the following will write out
"foobarbar":
%%
foobar ECHO; yyless(3);
[a-z]+ ECHO;
An argument of 0 to yyless will cause the entire
current input
string to be scanned again. Unless how the scanner
will subsequently
process its input has been changed (using
BEGIN, for example),
this will result in an endless loop.
Note that yyless is a macro and can only be used in
the flex input
file, not from other source files.
unput(c)
Puts the character c back into 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;
char *yycopy;
/* Copy yytext because unput() trashes
yytext */
if ((yycopy = strdup(yytext)) == NULL)
err(1, NULL);
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 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 the value
of yytext should be preserved after a call to
unput() (as in the
above example), it must either first be copied elsewhere, or the
scanner must be built using ``%array'' instead (see
HOW THE INPUT
IS MATCHED).
Finally, note that EOF cannot be put back to attempt
to mark the
input stream with an end-of-file.
input()
Reads the next character from the input stream. For
example, the
following is one way to eat up C comments:
%%
"/*" {
int c;
for (;;) {
while ((c = input()) != '*' &&
c != EOF)
; /* eat up text of
comment */
if (c == '*') {
while ((c = input())
== '*')
;
if (c == '/')
break; /*
found the end */
}
if (c == EOF) {
errx(1, "EOF in comment");
break;
}
}
}
(Note that if the scanner is compiled using C++,
then input() is
instead referred to as yyinput(), in order to avoid
a name clash
with the C++ stream by the name of input.)
YY_FLUSH_BUFFER
Flushes the scanner's internal buffer so that the
next time the
scanner attempts to match a token, it will first refill the
buffer using YY_INPUT (see THE GENERATED SCANNER,
below). This
action is a special case of the more general
yy_flush_buffer()
function, described below in the section MULTIPLE
INPUT BUFFERS.
yyterminate()
Can be used in lieu of a return statement in an action. It terminates
the scanner and returns a 0 to the scanner's
caller, indicating
"all done". By default, yyterminate() is
also called
when an end-of-file is encountered. It is a macro
and may be redefined.
THE GENERATED SCANNER [Toc] [Back] 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 the environment supports function prototypes, then it
will be "int
yylex(void)".) This definition may be changed by defining
the YY_DECL
macro. For example:
#define YY_DECL float lexscan(a, b) float a, b;
would give the scanning routine the name lexscan, returning
a float, and
taking two floats as arguments. Note that if arguments are
given to the
scanning routine using a K&R-style/non-prototyped function
declaration,
the definition must be terminated 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 YY_INPUT has been
set up 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(3) 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'' start condition is an inclusive (`%s')
start condition.
Also note that the special start-condition specifier `<*>'
matches every
start condition. Thus, the above example could also have
been written:
%x example
%%
<example>foo do_something();
<*>bar something_else();
The default rule (to ECHO any unmatched character) remains
active in
start conditions. It is equivalent to:
<*>.| ECHO;
``BEGIN(0)'' returns to the original state where only the
rules with no
start conditions are active. This state can also be referred to as the
start-condition INITIAL, so ``BEGIN(INITIAL)'' is equivalent
to
``BEGIN(0)''. (The parentheses around the start condition
name are not
required but are considered good style.)
BEGIN actions can also be given as indented code at the beginning of the
rules section. For example, the following will cause the
scanner to enter
the "SPECIAL" start condition whenever yylex() is called
and the
global variable enter_special is true:
int enter_special;
%x SPECIAL
%%
if (enter_special)
BEGIN(SPECIAL);
<SPECIAL>blahblahblah
...more rules follow...
To illustrate the uses of start conditions, here is a scanner which provides
two different interpretations of a string like
"123.456". By default
it will treat it as three tokens: the integer "123", a
dot (`.'),
and the integer "456". But if the string is preceded earlier in the line
by the string "expect-floats" it will treat it as a single
token, the
floating-point number 123.456:
%{
#include <math.h>
%}
%s expect
%%
expect-floats BEGIN(expect);
<expect>[0-9]+"."[0-9]+ {
printf("found a float, = %f0,
atof(yytext));
}
<expect>{
/*
* 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, = %d0,
atoi(yytext));
}
"." printf("found a dot0);
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>[^*0* /* eat anything that's not a '*'
*/
<comment>"*"+[^*/0* /* eat up '*'s not followed by
'/'s */
<comment> ++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 as possible in each rule, as it's a
big win.
Note that start-condition 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>[^*0* /* eat anything that's not a '*'
*/
<comment>"*"+[^*/0* /* eat up '*'s not followed by
'/'s */
<comment> ++line_num;
<comment>"*"+"/" BEGIN(comment_caller);
Furthermore, the current start condition can be accessed by
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
%%
#define MAX_STR_CONST 1024
char string_buf[MAX_STR_CONST];
char *string_buf_ptr;
<str>
BEGIN(INITIAL);
*string_buf_ptr = ' ';
/*
* return string constant token type and
* value to parser
*/
}
<str>{
/* 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 */
} else
*string_buf_ptr++ = result;
}
<str>\[0-9]+ {
/*
* generate error - bad escape sequence; something
* like '48' or ' 777777'
*/
}
<str>\n *string_buf_ptr++ = '0;
<str>\t *string_buf_ptr++ = '';
<str>\r *string_buf_ptr++ = '
<str>\b *string_buf_ptr++ = ';
<str>\f *string_buf_ptr++ = ';
<str>\(.|0 *string_buf_ptr++ = yytext[1];
<str>[^\
char *yptr = yytext;
while (*yptr)
*string_buf_ptr++ = *yptr++;
}
Often, such as in some of the examples above, a whole bunch
of rules are
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 '';
"\r" return '
"\f" return ';
"\0" return ' ';
}
is equivalent to:
<ESC>"\n" return '';
<ESC>"\r" return '
<ESC>"\f" return ';
<ESC>"\0" return ' ';
Start condition scopes may be nested.
Three routines are available for manipulating stacks of
start conditions:
void yy_push_state(int new_state)
Pushes the current start condition onto the top of
the start condition
stack and switches to new_state as though
``BEGIN
new_state'' had been used (recall that start condition names are
also integers).
void yy_pop_state()
Pops the top of the stack and switches to it via
BEGIN.
int yy_top_state()
Returns the top of the stack without altering the
stack's contents.
The start condition stack grows dynamically and so has no
built-in size
limitation. If memory is exhausted, program execution
aborts.
To use start condition stacks, scanners must include a
``%option stack''
directive (see OPTIONS below).
MULTIPLE INPUT BUFFERS [Toc] [Back] 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 YY_BUFFER_STATE variables may be safely initialized to
``((YY_BUFFER_STATE) 0)'' if desired, and the opaque structure can also
be referred to in order to correctly declare input buffers
in source
files other than that of scanners. 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 YY_INPUT is redefined so that it no longer uses
yyin, then a
nil FILE pointer can safely be passed to yy_create_buffer().
To select a
particular buffer to scan:
void yy_switch_to_buffer(YY_BUFFER_STATE new_buffer)
It switches the scanner's input buffer so subsequent tokens
will come
from new_buffer. Note that yy_switch_to_buffer() may be
used by yywrap()
to set things up for continued scanning, instead of opening
a new file
and pointing yyin at it. Note also that switching input
sources via either
yy_switch_to_buffer() or yywrap() does not change the
start condition.
void yy_delete_buffer(YY_BUFFER_STATE buffer)
is used to reclaim the storage associated with a buffer.
(buffer can be
nil, in which case the routine does nothing.) To clear the
current contents
of a buffer:
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-z0*0 ECHO;
<incl>[ ]* /* eat the whitespace */
<incl>[^ 0+ { /* got the include file name */
if (include_stack_ptr >= MAX_INCLUDE_DEPTH)
errx(1, "Includes nested too deeply");
include_stack[include_stack_ptr++] =
YY_CURRENT_BUFFER;
yyin = fopen(yytext, "r");
if (yyin == NULL)
err(1, NULL);
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 should be deleted afterwards
using
yy_delete_buffer()). They also switch to the new buffer using
yy_switch_to_buffer(), so the next call to yylex() will
start scanning
the string.
yy_scan_string(const char *str)
Scans a NUL-terminated string.
yy_scan_bytes(const char *bytes, int len)
Scans len bytes (including possibly NUL's) starting
at location
bytes.
Note that both of these functions create and scan a copy of
the string or
bytes. (This may be desirable, since yylex() modifies the
contents of
the buffer it is scanning.) The copy can be avoided by using:
yy_scan_buffer(char *base, yy_size_t size)
Which scans the buffer starting at base, consisting
of size
bytes, the last two bytes of which must be
YY_END_OF_BUFFER_CHAR
(ASCII NUL). These last two bytes are not scanned;
thus, scanning
consists of base[0] through base[size-2], inclusive.
If base is not set up 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 which can be
cast to an
integer expression reflecting the size of the
buffer.
The special rule "<<EOF>>" indicates actions which are to be
taken when
an end-of-file is encountered and yywrap() returns non-zero
(i.e.,
indicates no further files to process). The action must
finish by doing
one of four things:
- Assigning yyin to a new input file (in previous versions
of flex, after
doing the assignment, it was necessary to call the
special action
YY_NEW_FILE; this is no longer necessary).
- Executing a return statement.
- Executing the special yyterminate() action.
- Switching to a new buffer using yy_switch_to_buffer() as
shown in the
example above.
<<EOF>> rules may not be used with other patterns; they may
only be qualified
with a list of start conditions. If an unqualified
<<EOF>> rule is
given, it applies to all start conditions which do not already have
<<EOF>> actions. To specify an <<EOF>> rule for only the
initial start
condition, use
<INITIAL><<EOF>>
These rules are useful for catching things like unclosed
comments. An
example:
%x quote
%%
...other rules for dealing with quotes...
<quote><<EOF>> {
error("unterminated quote");
yyterminate();
}
<<EOF>> {
if (*++filelist)
yyin = fopen(*filelist, "r");
else
yyterminate();
}
The macro YY_USER_ACTION can be defined to provide an action
which is always
executed prior to the matched rule's action. For example, it could
be #define'd to call a routine to convert yytext to lowercase. When
YY_USER_ACTION is invoked, the variable yy_act gives the
number of the
matched rule (rules are numbered starting with 1). For example, to profile
how often each rule 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 -s is used), 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
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