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:
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
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
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 -ll 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.
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-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-ormore
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:
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)
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,
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-ormore
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]" 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-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-thebeginning-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. 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:
- 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 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
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 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()) == '*' )
;
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-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 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 blockreads
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 -ll 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 inclu-
sive, 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:
<*>.|\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 ) );
}
<expect>\n {
/* that's the end of the line, so
* we need another "expect-number"
* before we'll recognize any more
* numbers
*/
BEGIN(INITIAL);
}
[0-9]+ {
printf( "found an integer, = %d\n",
atoi( yytext ) );
}
"." printf( "found a dot\n" );
Here is a scanner which recognizes (and discards) C comments while
maintaining a count of the current input line.
%x comment
%%
int line_num = 1;
"/*" BEGIN(comment);
<comment>[^*\n]* /* eat anything that's not a '*' */
<comment>"*"+[^*/\n]* /* eat up '*'s not followed by '/'s */
<comment>\n ++line_num;
<comment>"*"+"/" BEGIN(INITIAL);
This scanner goes to a bit of trouble to match as much text as possible
with each rule. In general, when attempting to write a high-speed
scanner try to match as much possible in each rule, as it's a big win.
Note that start-conditions names are really integer values and can be
stored as such. Thus, the above could be extended in the following
fashion:
%x comment foo
%%
int line_num = 1;
int comment_caller;
"/*" {
comment_caller = INITIAL;
BEGIN(comment);
}
...
<foo>"/*" {
comment_caller = foo;
BEGIN(comment);
}
<comment>[^*\n]* /* eat anything that's not a '*' */
<comment>"*"+[^*/\n]* /* eat up '*'s not followed by '/'s */
<comment>\n ++line_num;
<comment>"*"+"/" BEGIN(comment_caller);
Furthermore, you can access the current start condition using the integer-valued
YY_START macro. For example, the above assignments to com-
ment_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:
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). 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
%{
#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.
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.
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.
<<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 interactive , %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 '^' active, while a zero argument makes '^' rules inactive.
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 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).
- int yyleng holds the length of the current token.
- FILE *yyin is the file which by default 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.
- void yyrestart( FILE *new_file ) may be called to point 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.
- FILE *yyout is the file to which ECHO actions are done. It can
be reassigned by the user.
- 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.
INTERFACING WITH YACC [Toc] [Back] 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:
-b Generate backing-up information to lex.backup. This is a list
of scanner states which require backing up and the input characters
on which they do so. By adding rules one can remove backing-up
states. If all backing-up states are eliminated and -Cf
or -CF is used, the generated scanner will run faster (see the
-p flag). Only users who wish to squeeze every last cycle out
of their scanners need worry about this option. (See the section
on Performance Considerations below.)
-c is a do-nothing, deprecated option included for POSIX compliance.
-d makes the generated scanner run in debug mode. Whenever a pattern
is recognized and the global 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 (
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