flex, lex - fast lexical analyzer generator
flex [-bcdfhilnpstvwBFILTV78+? -C[aefFmr] -ooutput -Ppre-
fix -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 ATT 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
recognized lexical patterns in text. flex reads the given
input files, or its standard input if no file names are
given, for a description of a scanner to generate. The
description is in the form of pairs of regular expressions
and C code, called rules. flex generates as output a C
source file, lex.yy.c, which defines a routine yylex().
This file is compiled and linked with the -lfl library to
produce an executable. When the executable is run, it
analyzes its input for occurrences of the regular expressions.
Whenever it finds one, it executes the corresponding
C code.
First some simple examples to get the flavor of how one
uses flex. The following flex input specifies a scanner
which whenever it encounters the string "username" will
replace it with the user's login name:
%%
username printf( "%s", getlogin() );
By default, any text not matched by a flex scanner is
copied to the output, so the net effect of this scanner is
to copy its input file to its output with each occurrence
of "username" expanded. In this input, there is just one
rule. "username" is the pattern and the "printf" is the
action. The "%%" marks the beginning of the rules.
Here's another simple example:
int num_lines = 0, num_chars = 0;
%%
\n ++num_lines; ++num_chars;
. ++num_chars;
%%
main()
{
yylex();
printf( "# of lines = %d, # of chars = %d\n",
num_lines, num_chars );
}
This scanner counts the number of characters and the number
of lines in its input (it produces no output other
than the final report on the counts). The first line
declares two globals, "num_lines" and "num_chars", which
are accessible both inside yylex() and in the main() routine
declared after the second "%%". There are two rules,
one which matches a newline ("\n") and increments both the
line count and the character count, and one which matches
any character other than a newline (indicated by the "."
regular expression).
A somewhat more complicated example:
/* scanner for a toy Pascal-like language */
%{
/* need this for the call to atof() below */
#include math.h
%}
DIGIT [0-9]
ID [a-z][a-z0-9]*
%%
{DIGIT}+ {
printf( "An integer: %s (%d)\n", yytext,
atoi( yytext ) );
}
{DIGIT}+"."{DIGIT}* {
printf( "A float: %s (%g)\n", yytext,
atof( yytext ) );
}
if|then|begin|end|procedure|function {
printf( "A keyword: %s\n", yytext );
}
{ID} printf( "An identifier: %s\n", yytext );
"+"|"-"|"*"|"/" printf( "An operator: %s\n", yytext );
"{"[^}\n]*"}" /* eat up one-line comments */
[ \t\n]+ /* eat up whitespace */
. printf( "Unrecognized character: %s\n", yytext );
%%
main( argc, argv )
int argc;
char **argv;
{
++argv, --argc; /* skip over program name */
if ( argc 0 )
yyin = fopen( argv[0], "r" );
else
yyin = stdin;
yylex();
}
This is the beginnings of a simple scanner for a language
like Pascal. It identifies different types of tokens and
reports on what it has seen.
The details of this example will be explained in the following
sections.
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-or-more digits.
The rules section of the flex input contains a series of
rules of the form:
pattern action
where the pattern must be unindented and the action must
begin on the same line.
See below for a further description of patterns and
actions.
Finally, the user code section is simply copied to
lex.yy.c verbatim. It is used for companion routines
which call or are called by the scanner. The presence of
this section is optional; if it is missing, the second %%
in the input file may be skipped, too.
In the definitions and rules sections, any indented text
or text enclosed in %{ and %} is copied verbatim to the
output (with the %{}'s removed). The %{}'s must appear
unindented on lines by themselves.
In the rules section, any indented or %{} text appearing
before the first rule may be used to declare variables
which are local to the scanning routine and (after the
declarations) code which is to be executed whenever the
scanning routine is entered. Other indented or %{} text
in the rule section is still copied to the output, but its
meaning is not well-defined and it may well cause 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 "*/".
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$".
sr an r, but only in start condition s (see
below for discussion of start conditions)
s1,s2,s3r
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,s2EOF
an end-of-file when in start condition s1 or s2
Note that inside of a character class, all regular expression
operators lose their special meaning except escape
('\') and the character class operators, '-', ']', and, at
the beginning of the class, '^'.
The regular expressions listed above are grouped according
to precedence, from highest precedence at the top to lowest
at the bottom. Those grouped together have equal
precedence. For example,
foo|bar*
is the same as
(foo)|(ba(r*))
since the '*' operator has higher precedence than concatenation,
and concatenation higher than alternation ('|').
This pattern therefore matches either the string "foo" or
the string "ba" followed by zero-or-more r's. To match
"foo" or zero-or-more "bar"'s, use:
foo|(bar)*
and to match zero-or-more "foo"'s-or-"bar"'s:
(foo|bar)*
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 "[^AZ]"
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$
sc1foosc2bar
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
bar-at-the-beginning-of-a-line.
HOW THE INPUT IS MATCHED [Toc] [Back] When the generated scanner is run, it analyzes its input
looking for strings which match any of its patterns. If
it finds more than one match, it takes the one matching
the most text (for trailing context rules, this includes
the length of the trailing part, even though it will then
be returned to the input). If it finds two or more
matches of the same length, the rule listed first in the
flex input file is chosen.
Once the match is determined, the text corresponding to
the match (called the token) is made available in the
global character pointer yytext, and its length in the
global integer yyleng. The action corresponding to the
matched pattern is then executed (a more detailed description
of actions follows), and then the remaining input is
scanned for another match.
If no match is found, then the default rule is executed:
the next character in the input is considered matched and
copied to the standard output. Thus, the simplest legal
flex input is:
%%
which generates a scanner that simply copies its input
(one character at a time) to its output.
Note that yytext can be defined in two different ways:
either as a character pointer or as a character array.
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 KRstyle/non-prototyped
function declaration, you must terminate
the definition with a semi-colon (;).
Whenever yylex() is called, it scans tokens from the
global input file yyin (which defaults to stdin). It continues
until it either reaches an end-of-file (at which
point it returns the value 0) or one of its actions executes
a return statement.
If the scanner reaches an end-of-file, subsequent calls
are undefined unless either yyin is pointed at a new input
file (in which case scanning continues from that file), or
yyrestart() is called. yyrestart() takes one argument, a
FILE * pointer (which can be nil, if you've set up
YY_INPUT to scan from a source other than yyin), and initializes
yyin for scanning from that file. Essentially
there is no difference between just assigning yyin to a
new input file or using yyrestart() to do so; the latter
is available for compatibility with previous versions of
flex, and because it can be used to switch input files in
the middle of scanning. It can also be used to throw away
the current input buffer, by calling it with an argument
of yyin; but better is to use YY_FLUSH_BUFFER (see above).
Note that yyrestart() does not reset the start condition
to INITIAL (see Start Conditions, below).
If yylex() stops scanning due to executing a return statement
in one of the actions, the scanner may then be called
again and it will resume scanning where it left off.
By default (and for purposes of efficiency), the scanner
uses block-reads rather than simple getc() calls to read
characters from yyin. The nature of how it gets its input
can be controlled by defining the YY_INPUT macro.
YY_INPUT's calling sequence is
"YY_INPUT(buf,result,max_size)". Its action is to place
up to max_size characters in the character array buf and
return in the integer variable result either the number of
characters read or the constant YY_NULL (0 on Unix systems)
to indicate EOF. The default YY_INPUT reads from
the global file-pointer "yyin".
A sample definition of YY_INPUT (in the definitions section
of the input file):
%{
#define YY_INPUT(buf,result,max_size) \
{ \
int c = getchar(); \
result = (c == EOF) ? YY_NULL : (buf[0] = c, 1); \
}
%}
This definition will change the input processing to occur
one character at a time.
When the scanner receives an end-of-file indication from
YY_INPUT, it then checks the yywrap() function. If
yywrap() returns false (zero), then it is assumed that the
function has gone ahead and set up yyin to point to
another input file, and scanning continues. If it returns
true (non-zero), then the scanner terminates, returning 0
to its caller. Note that in either case, the start condition
remains unchanged; it does not revert to INITIAL.
If you do not supply your own version of yywrap(), then
you must either use %option noyywrap (in which case the
scanner behaves as though yywrap() returned 1), or you
must link with -lfl to obtain the default version of the
routine, which always returns 1.
Three routines are available for scanning from in-memory
buffers rather than files: yy_scan_string(),
yy_scan_bytes(), and yy_scan_buffer(). See the discussion
of them below in the section Multiple Input Buffers.
The scanner writes its ECHO output to the yyout global
(default, stdout), which may be redefined by the user simply
by assigning it to some other FILE pointer.
flex provides a mechanism for conditionally activating
rules. Any rule whose pattern is prefixed with "sc" will
only be active when the scanner is in the start condition
named "sc". For example,
STRING[^"]* { /* eat up the string body ... */
...
}
will be active only when the scanner is in the "STRING"
start condition, and
INITIAL,STRING,QUOTE\. { /* handle an escape ... */
...
}
will be active only when the current start condition is
either "INITIAL", "STRING", or "QUOTE".
Start conditions are declared in the definitions (first)
section of the input using unindented lines beginning with
either %s or %x followed by a list of names. The former
declares inclusive start conditions, the latter exclusive
start conditions. A start condition is activated using
the BEGIN action. Until the next BEGIN action is executed,
rules with the given start condition will be active
and rules with other start conditions will be inactive.
If the start condition is inclusive, then rules with no
start conditions at all will also be active. If it is
exclusive, then only rules qualified with the start condition
will be active. A set of rules contingent on the
same exclusive start condition describe a scanner which is
independent of any of the other rules in the flex input.
Because of this, exclusive start conditions make it easy
to specify "mini-scanners" which scan portions of the
input that are syntactically different from the rest
(e.g., comments).
If the distinction between inclusive and exclusive start
conditions is still a little vague, here's a simple example
illustrating the connection between the two. The set
of rules:
%s example
%%
examplefoo do_something();
bar something_else();
is equivalent to
%x example
%%
examplefoo do_something();
INITIAL,examplebar 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
%%
examplefoo 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);
SPECIALblahblahblah
...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 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 ATT lex).
Note that start conditions do not have their own namespace;
%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
INITIALEOF
These rules are useful for catching things like unclosed
comments. An example:
%x quote
%%
...other rules for dealing with quotes...
quoteEOF {
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 inter-
active. 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 nonzero
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
%opti
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