FLEX(1) Version 2.5 (April 1995) FLEX(1)
NAME [Toc] [Back]
flex - fast lexical analyzer generator
SYNOPSIS [Toc] [Back]
flex [-bcdfhilnpstvwBFILTV78+? -C[aefFmr] -ooutput -Pprefix
-Sskeleton] [--help --version] [filename ...]
OVERVIEW [Toc] [Back]
This manual describes flex, a tool for generating programs
that perform pattern-matching on text. The manual includes
both tutorial and reference sections:
Description
a brief overview of the tool
Some Simple Examples
Format Of The Input File
Patterns
the extended regular expressions used by flex
How The Input Is Matched
the rules for determining what has been matched
Actions
how to specify what to do when a pattern is matched
The Generated Scanner
details regarding the scanner that flex produces;
how to control the input source
Start Conditions
introducing context into your scanners, and
managing "mini-scanners"
Multiple Input Buffers
how to manipulate multiple input sources; how to
scan from strings instead of files
End-of-file Rules
special rules for matching the end of the input
Miscellaneous Macros
a summary of macros available to the actions
Values Available To The User
a summary of values available to the actions
Interfacing With Yacc
connecting flex scanners together with yacc parsers
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Options
flex command-line options, and the "%option"
directive
Performance Considerations
how to make your scanner go as fast as possible
Generating C++ Scanners
the (experimental) facility for generating C++
scanner classes
Incompatibilities With Lex And POSIX
how flex differs from AT&T lex and the POSIX lex
standard
Diagnostics
those error messages produced by flex (or scanners
it generates) whose meanings might not be apparent
Files
files used by flex
Deficiencies / Bugs
known problems with flex
See Also
other documentation, related tools
Author
includes contact information
DESCRIPTION [Toc] [Back]
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.
SOME SIMPLE EXAMPLES [Toc] [Back]
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:
%%
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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 ) );
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}
{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.
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
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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
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(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 compiletime
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 "*/".
PATTERNS [Toc] [Back]
The patterns in the input are written using an extended set
of regular expressions. These are:
x match the character 'x'
. any character (byte) except newline
[xyz] a "character class"; in this case, the pattern
matches either an 'x', a 'y', or a 'z'
[abj-oZ] a "character class" with a range in it; matches
an 'a', a 'b', any letter from 'j' through 'o',
or a 'Z'
[^A-Z] a "negated character class", i.e., any character
but those in the class. In this case, any
character EXCEPT an uppercase letter.
[^A-Z\n] any character EXCEPT an uppercase letter or
a newline
r* zero or more r's, where r is any regular expression
r+ one or more r's
r? zero or one r's (that is, "an optional r")
r{2,5} anywhere from two to five r's
r{2,} two or more r's
r{4} exactly 4 r's
{name} the expansion of the "name" definition
(see above)
"[xyz]\"foo"
the literal string: [xyz]"foo
\X if X is an 'a', 'b', 'f', 'n', 'r', 't', or 'v',
then the ANSI-C interpretation of \x.
Otherwise, a literal 'X' (used to escape
operators such as '*')
\0 a NUL character (ASCII code 0)
\123 the character with octal value 123
\x2a the character with hexadecimal value 2a
(r) match an r; parentheses are used to override
precedence (see below)
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rs the regular expression r followed by the
regular expression s; called "concatenation"
r|s either an r or an s
r/s an r but only if it is followed by an s. The
text matched by s is included when determining
whether this rule is the "longest match",
but is then returned to the input before
the action is executed. So the action only
sees the text matched by r. This type
of pattern is called trailing context".
(There are some combinations of r/s that flex
cannot match correctly; see notes in the
Deficiencies / Bugs section below regarding
"dangerous trailing context".)
^r an r, but only at the beginning of a line (i.e.,
which just starting to scan, or right after a
newline has been scanned).
r$ an r, but only at the end of a line (i.e., just
before a newline). Equivalent to "r/\n".
Note that flex's notion of "newline" is exactly
whatever the C compiler used to compile flex
interprets '\n' as; in particular, on some DOS
systems you must either filter out \r's in the
input yourself, or explicitly use r/\r\n for "r$".
<s>r an r, but only in start condition s (see
below for discussion of start conditions)
<s1,s2,s3>r
same, but in any of start conditions s1,
s2, or s3
<*>r an r in any start condition, even an exclusive one.
<<EOF>> an end-of-file
<s1,s2><<EOF>>
an end-of-file when in start condition s1 or s2
Note that inside of a character class, all regular
expression operators lose their special meaning except
escape ('\') and the character class operators, '-', ']',
and, at the beginning of the class, '^'.
The regular expressions listed above are grouped according
to precedence, from highest precedence at the top to lowest
at the bottom. Those grouped together have equal
precedence. For example,
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foo|bar*
is the same as
(foo)|(ba(r*))
since the '*' operator has higher precedence than
concatenation, and concatenation higher than alternation
('|'). This pattern therefore matches either the string
"foo" or the string "ba" followed by zero-or-more r's. To
match "foo" or zero-or-more "bar"'s, use:
foo|(bar)*
and to match zero-or-more "foo"'s-or-"bar"'s:
(foo|bar)*
In addition to characters and ranges of characters,
character classes can also contain character class
expressions. These are expressions enclosed inside [: and :]
delimiters (which themselves must appear between the '[' and
']' of the character class; other elements may occur inside
the character class, too). The valid expressions are:
[:alnum:] [:alpha:] [:blank:]
[:cntrl:] [:digit:] [:graph:]
[:lower:] [:print:] [:punct:]
[:space:] [:upper:] [:xdigit:]
These expressions all designate a set of characters
equivalent to the corresponding standard C isXXX function.
For example, [:alnum:] designates those characters for which
isalnum() returns true - i.e., any alphabetic or numeric.
Some systems don't provide isblank(), so flex defines
[:blank:] as a blank or a tab.
For example, the following character classes are all
equivalent:
[[:alnum:]]
[[:alpha:][:digit:]
[[:alpha:]0-9]
[a-zA-Z0-9]
If your scanner is case-insensitive (the -i flag), then
[:upper:] and [:lower:] are equivalent to [:alpha:].
Some notes on patterns:
- A negated character class such as the example "[^A-Z]"
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above will match a newline unless "\n" (or an
equivalent escape sequence) is one of the characters
explicitly present in the negated character class
(e.g., "[^A-Z\n]"). This is unlike how many other
regular expression tools treat negated character
classes, but unfortunately the inconsistency is
historically entrenched. Matching newlines means that
a pattern like [^"]* can match the entire input unless
there's another quote in the input.
- A rule can have at most one instance of trailing
context (the '/' operator or the '$' operator). The
start condition, '^', and "<<EOF>>" patterns can only
occur at the beginning of a pattern, and, as well as
with '/' and '$', cannot be grouped inside parentheses.
A '^' which does not occur at the beginning of a rule
or a '$' which does not occur at the end of a rule
loses its special properties and is treated as a normal
character.
The following are illegal:
foo/bar$
<sc1>foo<sc2>bar
Note that the first of these, can be written
"foo/bar\n".
The following will result in '$' or '^' being treated
as a normal character:
foo|(bar$)
foo|^bar
If what's wanted is a "foo" or a bar-followed-by-anewline,
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
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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
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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).
ACTIONS [Toc] [Back]
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
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(adding characters to its end--these will overwrite later
characters in the input stream). This however does not
apply when using %array (see above); in that case, yytext
may be freely modified in any way.
Actions are free to modify yyleng except they should not do
so if the action also includes use of yymore() (see below).
There are a number of special directives which can be
included within an action:
- ECHO copies yytext to the scanner's output.
- BEGIN followed by the name of a start condition places
the scanner in the corresponding start condition (see
below).
- REJECT directs the scanner to proceed on to the "second
best" rule which matched the input (or a prefix of the
input). The rule is chosen as described above in "How
the Input is Matched", and yytext and yyleng set up
appropriately. It may either be one which matched as
much text as the originally chosen rule but came later
in the flex input file, or one which matched less text.
For example, the following will both count the words in
the input and call the routine special() whenever
"frob" is seen:
int word_count = 0;
%%
frob special(); REJECT;
[^ \t\n]+ ++word_count;
Without the REJECT, any "frob"'s in the input would not
be counted as words, since the scanner normally
executes only one action per token. Multiple REJECT's
are allowed, each one finding the next best choice to
the currently active rule. For example, when the
following scanner scans the token "abcd", it will write
"abcdabcaba" to the output:
%%
a |
ab |
abc |
abcd ECHO; REJECT;
.|\n /* eat up any unmatched character */
(The first three rules share the fourth's action since
they use the special '|' action.) REJECT is a
particularly expensive feature in terms of scanner
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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 "megakludge"
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 "megakludge".
Two notes regarding use of yymore(). First, yymore() depends
on the value of yyleng correctly reflecting the size of the
current token, so you must not modify yyleng if you are
using yymore(). Second, the presence of yymore() in the
scanner's action entails a minor performance penalty in the
scanner's matching speed.
- yyless(n) returns all but the first n characters of the
current token back to the input stream, where they will
be rescanned when the scanner looks for the next match.
yytext and yyleng are adjusted appropriately (e.g.,
yyleng will now be equal to n ). For example, on the
input "foobar" the following will write out
"foobarbar":
%%
foobar ECHO; yyless(3);
[a-z]+ ECHO;
An argument of 0 to yyless will cause the entire
current input string to be scanned again. Unless
you've changed how the scanner will subsequently
process its input (using BEGIN, for example), this will
result in an endless loop.
Note that yyless is a macro and can only be used in the flex
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input file, not from other source files.
- unput(c) puts the character c back onto the input
stream. It will be the next character scanned. The
following action will take the current token and cause
it to be rescanned enclosed in parentheses.
{
int i;
/* Copy yytext because unput() trashes yytext */
char *yycopy = strdup( yytext );
unput( ')' );
for ( i = yyleng - 1; i >= 0; --i )
unput( yycopy[i] );
unput( '(' );
free( yycopy );
}
Note that since each unput() puts the given character
back at the beginning of the input stream, pushing back
strings must be done back-to-front.
An important potential problem when using unput() is that if
you are using %pointer (the default), a call to unput()
destroys the contents of yytext, starting with its rightmost
character and devouring one character to the left with each
call. If you need the value of yytext preserved after a
call to unput() (as in the above example), you must either
first copy it elsewhere, or build your scanner using %array
instead (see How The Input Is Matched).
Finally, note that you cannot put back EOF to attempt to
mark the input stream with an end-of-file.
- input() reads the next character from the input stream.
For example, the following is one way to eat up C
comments:
%%
"/*" {
register int c;
for ( ; ; )
{
while ( (c = input()) != '*' &&
c != EOF )
; /* eat up text of comment */
if ( c == '*' )
{
while ( (c = input()) == '*' )
;
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if ( c == '/' )
break; /* found the end */
}
if ( c == EOF )
{
error( "EOF in comment" );
break;
}
}
}
(Note that if the scanner is compiled using C++, then
input() is instead referred to as yyinput(), in order
to avoid a name clash with the C++ stream by the name
of input.)
- YY_FLUSH_BUFFER flushes the scanner's internal buffer
so that the next time the scanner attempts to match a
token, it will first refill the buffer using YY_INPUT
(see The Generated Scanner, below). This action is a
special case of the more general yy_flush_buffer()
function, described below in the section Multiple Input
Buffers.
- yyterminate() can be used in lieu of a return statement
in an action. It terminates the scanner and returns a
0 to the scanner's caller, indicating "all done". By
default, yyterminate() is also called when an end-offile
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 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
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give arguments to the scanning routine using a K&Rstyle/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); \
}
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FLEX(1) Version 2.5 (April 1995) FLEX(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 (nonzero),
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.
START CONDITIONS [Toc] [Back]
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
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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 "miniscanners"
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
startion 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();
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FLEX(1) Version 2.5 (April 1995) FLEX(1)
<*>bar something_else();
The default rule (to ECHO any unmatched character) remains
active in start conditions. It is equivalent to:
<*>.|\n ECHO;
BEGIN(0) returns to the original state where only the rules
with no start conditions are active. This state can also be
referred to as the start-condition "INITIAL", so
BEGIN(INITIAL) is equivalent to BEGIN(0). (The parentheses
around the start condition name are not required but are
considered good style.)
BEGIN actions can also be given as indented code at the
beginning of the rules section. For example, the following
will cause the scanner to enter the "SPECIAL" start
condition whenever yylex() is called and the global variable
enter_special is true:
int enter_special;
%x SPECIAL
%%
if ( enter_special )
BEGIN(SPECIAL);
<SPECIAL>blahblahblah
...more rules follow...
To illustrate the uses of start conditions, here is a
scanner which provides two different interpretations of a
string like "123.456". By default it will treat it as three
tokens, the integer "123", a dot ('.'), and the integer
"456". But if the string is preceded earlier in the line by
the string "expect-floats" it will treat it as a single
token, the floating-point number 123.456:
%{
#include <math.h>
%}
%s expect
%%
expect-floats BEGIN(expect);
<expect>[0-9]+"."[0-9]+ {
printf( "found a float, = %f\n",
atof( yytext ) );
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FLEX(1) Version 2.5 (April 1995) FLEX(1)
}
<expect>\n {
/* that's the end of the line, so
* we need another "expect-number"
* before we'll recognize any more
* numbers
*/
BEGIN(INITIAL);
}
[0-9]+ {
printf( "found an integer, = %d\n",
atoi( yytext ) );
}
"." printf( "found a dot\n" );
Here is a scanner which recognizes (and discards) C comments
while maintaining a count of the current input line.
%x comment
%%
int line_num = 1;
"/*" BEGIN(comment);
<comment>[^*\n]* /* eat anything that's not a '*' */
<comment>"*"+[^*/\n]* /* eat up '*'s not followed by '/'s */
<comment>\n ++line_num;
<comment>"*"+"/" BEGIN(INITIAL);
This scanner goes to a bit of trouble to match as much text
as possible with each rule. In general, when attempting to
write a high-speed scanner try to match as much possible in
each rule, as it's a big win.
Note that start-conditions names are really integer values
and can be stored as such. Thus, the above could be
extended in the following fashion:
%x comment foo
%%
int line_num = 1;
int comment_caller;
"/*" {
comment_caller = INITIAL;
BEGIN(comment);
}
...
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FLEX(1) Version 2.5 (April 1995) FLEX(1)
<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 */
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FLEX(1) Version 2.5 (April 1995) FLEX(1)
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';
}
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FLEX(1) Version 2.5 (April 1995) FLEX(1)
is equivalent to:
<ESC>"\\n" return '\n';
<ESC>"\\r" return '\r';
<ESC>"\\f" return '\f';
<ESC>"\\0" return '\0';
Start condition scopes may be nested.
Three routines are available for manipulating stacks of
start conditions:
void yy_push_state(int new_state)
pushes the current start condition onto the top of the
start condition stack and switches to new_state as
though you had used BEGIN new_state (recall that start
condition names are also integers).
void yy_pop_state()
pops the top of the stack and switches to it via BEGIN.
int yy_top_state()
returns the top of the stack without altering the
stack's contents.
The start condition stack grows dynamically and so has no
built-in size limitation. If memory is exhausted, program
execution aborts.
To use start condition stacks, your scanner must include a
%option stack directive (see Options below).
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).
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FLEX(1) Version 2.5 (April 1995) FLEX(1)
It returns a YY_BUFFER_STATE handle, which may then be
passed to other routines (see below). The YY_BUFFER_STATE
type is a pointer to an opaque struct yy_buffer_state
structure, so you may safely initialize YY_BUFFER_STATE
variables to ((YY_BUFFER_STATE) 0) if you wish, and also
refer to the opaque structure in order to correctly declare
input buffers in source files other than that of your
scanner. Note that the FILE pointer in the call to
yy_create_buffer is only used as the value of yyin seen by
YY_INPUT; if you redefine YY_INPUT so it no longer uses
yyin, then you can safely pass a nil FILE pointer to
yy_create_buffer. You select a particular buffer to scan
from using:
void yy_switch_to_buffer( YY_BUFFER_STATE new_buffer )
switches the scanner's input buffer so subsequent tokens
will come from new_buffer. Note that yy_switch_to_buffer()
may be used by yywrap() to set things up for continued
scanning, instead of opening a new file and pointing yyin at
it. Note also that switching input sources via either
yy_switch_to_buffer() or yywrap() does not change the start
condition.
void yy_delete_buffer( YY_BUFFER_STATE buffer )
is used to reclaim the storage associated with a buffer. (
buffer can be nil, in which case the routine does nothing.)
You can also clear the current contents of a buffer using:
void yy_flush_buffer( YY_BUFFER_STATE buffer )
This function discards the buffer's contents, so the next
time the scanner attempts to match a token from the buffer,
it will first fill the buffer anew using YY_INPUT.
yy_new_buffer() is an alias for yy_create_buffer(), provided
for compatibility with the C++ use of new and delete for
creating and destroying dynamic objects.
Finally, the YY_CURRENT_BUFFER macro returns a
YY_BUFFER_STATE handle to the current buffer.
Here is an example of using these features for writing a
scanner which expands include files (the <<EOF>> feature is
discussed below):
/* the "incl" state is used for picking up the name
* of an include file
*/
%x incl
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FLEX(1) Version 2.5 (April 1995) FLEX(1)
%{
#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
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should delete with yy_delete_buffer() when done with it).
They also switch to the new buffer using
yy_switch_to_buffer(), so the next call to yylex() will
start scanning the string.
yy_scan_string(const char *str)
scans a NUL-terminated string.
yy_scan_bytes(const char *bytes, int len)
scans len bytes (including possibly NUL's) starting at
location bytes.
Note that both of these functions create and scan a copy of
the string or bytes. (This may be desirable, since yylex()
modifies the contents of the buffer it is scanning.) You
can avoid the copy by using:
yy_scan_buffer(char *base, yy_size_t size)
which scans in place the buffer starting at base,
consisting of size bytes, the last two bytes of which
must be YY_END_OF_BUFFER_CHAR (ASCII NUL). These last
two bytes are not scanned; thus, scanning consists of
base[0] through base[size-2], inclusive.
If you fail to set up base in this manner (i.e., forget
the final two YY_END_OF_BUFFER_CHAR bytes), then
yy_scan_buffer() returns a nil pointer instead of
creating a new input buffer.
The type yy_size_t is an integral type to which you can
cast an integer expression reflecting the size of the
buffer.
END-OF-FILE RULES [Toc] [Back]
The special rule "<<EOF>>" indicates actions which are to be
taken when an end-of-file is encountered and yywrap()
returns non-zero (i.e., indicates no further files to
process). The action must finish by doing one of four
things:
- assigning yyin to a new input file (in previous
versions of flex, after doing the assignment you had to
call the special action YY_NEW_FILE; this is no longer
necessary);
- executing a return statement;
- executing the special yyterminate() action;
- or, switching to a new buffer using
yy_switch_to_buffer() as shown in the example above.
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FLEX(1) Version 2.5 (April 1995) FLEX(1)
<<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();
}
MISCELLANEOUS MACROS [Toc] [Back]
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
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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 nonzero
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.
VALUES AVAILABLE TO THE USER [Toc] [Back]
This section summarizes the various values available to the
user in the rule actions.
- char *yytext holds the text of the current token. It
may be modified but not lengthened (you cannot append
characters to the end).
If the special directive %array appears in the first
section of the scanner description, then yytext is
instead declared char yytext[YYLMAX], where YYLMAX is a
macro definition that you can redefine in the first
section if you don't like the default value (generally
8KB). Using %array results in somewhat slower
scanners, but the value of yytext becomes immune to
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calls to input() and unput(), which potentially destroy
its value when yy
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