FLEX(1) BSD Reference Manual FLEX(1)NAMEflex - fast lexical analyzer generator
SYNOPSISflex [-78BbcdFfhIiLlnpsTtVvw+?] [-C[aeFfmr]] [--help] [--version]
[-ooutput] [-Pprefix] [-Sskeleton] [filename...]
OVERVIEW
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 in-
put source.
Start Conditions
Introducing context into scanners, and managing "mini-scanners".
Multiple Input Buffers
How to manipulate multiple input sources; how to scan from strings in-
stead 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(1) parsers.
Options
flex command-line options, and the "%option" directive.
Performance Considerations
How to make scanners go as fast as possible.
Generating C++ Scanners
The (experimental) facility for generating C++ scanner classes.
Incompatibilities with Lex and POSIX
How flex differs from AT&T lex and the POSIX lex standard.
Files
Files used by flex.
Diagnostics
Those error messages produced by flex (or scanners it generates) whose
meanings might not be apparent.
See Also
Other documentation, related tools.
Authors
Includes contact information.
Bugs
Known problems with flex.
DESCRIPTIONflex is a tool for generating scanners: programs which recognize lexical
patterns in text. flex reads the given input files, or its standard input
if no file names are given, for a description of a scanner to generate.
The description is in the form of pairs of regular expressions and C
code, called rules. flex generates as output a C source file, lex.yy.c,
which defines a routine yylex(). This file is compiled and linked with
the -lfl library to produce an executable. When the executable is run, it
analyzes its input for occurrences of the regular expressions. Whenever
it finds one, it executes the corresponding C code.
SOME SIMPLE EXAMPLES
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 out-
put, 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 char-
acter 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(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
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 ('_') fol-
lowed by zero or more letters, digits, '_', or '-' (dash). The definition
is taken to begin at the first non-whitespace character following the
name and continuing to the end of the line. The definition can subse-
quently be referred to using "{name}", which will expand to
"(definition)". For example:
DIGIT [0-9]
ID [a-z][a-z0-9]*
This defines "DIGIT" to be a regular expression which matches a single
digit, and "ID" to be a regular expression which matches a letter fol-
lowed by zero-or-more letters-or-digits. A subsequent reference to
{DIGIT}+"."{DIGIT}*
is identical to
([0-9])+"."([0-9])*
and matches one-or-more digits followed by a '.' followed by zero-or-more
digits.
The rules section of the flex input contains a series of rules of the
form:
pattern action
The pattern must be unindented and the action must begin on the same
line.
See below for a further description of patterns and actions.
Finally, the user code section is simply copied to lex.yy.c verbatim. It
is used for companion routines which call or are called by the scanner.
The presence of this section is optional; if it is missing, the second
"%%" in the input file may be skipped too.
In the definitions and rules sections, any indented text or text enclosed
in '%{' and '%}' is copied verbatim to the output (with the %{}'s
removed). The %{}'s must appear unindented on lines by themselves.
In the rules section, any indented or %{} text appearing before the first
rule may be used to declare variables which are local to the scanning
routine and (after the declarations) code which is to be executed whenev-
er 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 "*/".
PATTERNS
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 BUGS section below regarding "dangerous
trailing context".)
^r An 'r', but only at the beginning of a line (i.e., just start-
ing 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.
<s>r An 'r', but only in start condition 's' (see below for
discussion of start conditions).
<s1,s2,s3>r
The same, but in any of start conditions s1, s2, or s3.
<*>r An 'r' in any start condition, even an exclusive one.
<<EOF>> An end-of-file.
<s1,s2><<EOF>>
An end-of-file when in start condition s1 or s2.
Note that inside of a character class, all regular expression operators
lose their special meaning except escape ('\') and the character class
operators, '-', ']', and, at the beginning of the class, '^'.
The regular expressions listed above are grouped according to precedence,
from highest precedence at the top to lowest at the bottom. Those grouped
together have equal precedence. For example,
foo|bar*
is the same as
(foo)|(ba(r*))
since the '*' operator has higher precedence than concatenation, and con-
catenation 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:] desig-
nates those characters for which isalnum(3) returns true - i.e., any al-
phabetic or numeric. Some systems don't provide isblank(3), so flex de-
fines [:blank:] as a blank or a tab.
For example, the following character classes are all equivalent:
[[:alnum:]]
[[:alpha:][:digit:]]
[[:alpha:]0-9]
[a-zA-Z0-9]
If the scanner is case-insensitive (the -i flag), then [:upper:] and
[:lower:] are equivalent to [:alpha:].
Some notes on patterns:
- A negated character class such as the example "[^A-Z]" above will
match a newline unless "\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 incon-
sistency is historically entrenched. Matching newlines means that a
pattern like "[^"]*" can match the entire input unless there's anoth-
er 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 fol-
lowing 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
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 re-
turned 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 charac-
ter 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 char-
acter pointer or as a character array. Which definition flex uses can be
controlled by including one of the special directives "%pointer" or
"%array" in the first (definitions) section of flex input. The default is
"%pointer", unless the -l lex compatibility option is used, in which case
yytext will be an array. The advantage of using "%pointer" is substan-
tially faster scanning and no buffer overflow when matching very large
tokens (unless not enough dynamic memory is available). The disadvantage
is that actions are restricted in how they can modify yytext (see the
next section), and calls to the unput() function destroy the present con-
tents of yytext, which can be a considerable porting headache when moving
between different lex versions.
The advantage of "%array" is that yytext can be modified as much as want-
ed, and calls to unput() do not destroy yytext (see below). Furthermore,
existing lex programs sometimes access yytext externally using declara-
tions 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 de-
faults to a fairly large value. The size can be changed by simply
#define'ing YYLMAX to a different value in the first section of flex in-
put. As mentioned above, with "%pointer" yytext grows dynamically to ac-
commodate large tokens. While this means a "%pointer" scanner can accom-
modate 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 un-
put() results in too much text being pushed back; instead, a run-time er-
ror results.
Also note that "%array" cannot be used with C++ scanner classes (the c++
option; see below).
ACTIONS
Each pattern in a rule has a corresponding action, which can be any arbi-
trary C statement. The pattern ends at the first non-escaped whitespace
character; the remainder of the line is its action. If the action is emp-
ty, 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 re-
turn 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 char-
acters 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 ac-
tion 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 count-
ed 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-mega-kludge" to the
output:
%%
mega- ECHO; yymore();
kludge ECHO;
First "mega-" is matched and echoed to the output. Then "kludge"
is matched, but the previous "mega-" is still hanging around at
the beginning of yytext so the ECHO for the "kludge" rule will
actually write "mega-kludge".
Two notes regarding use of yymore(): First, yymore() depends on
the value of yyleng correctly reflecting the size of the current
token, so yyleng must not be modified when using yymore().
Second, the presence of yymore() in the scanner's action entails
a minor performance penalty in the scanner's matching speed.
yyless(n)
Returns all but the first n characters of the current token back
to the input stream, where they will be rescanned when the
scanner looks for the next match. yytext and yyleng are adjusted
appropriately (e.g., yyleng will now be equal to n). For example,
on the input "foobar" the following will write out "foobarbar":
%%
foobar ECHO; yyless(3);
[a-z]+ ECHO;
An argument of 0 to yyless will cause the entire current input
string to be scanned again. Unless how the scanner will subse-
quently process its input has been changed (using BEGIN, for ex-
ample), this will result in an endless loop.
Note that yyless is a macro and can only be used in the flex in-
put file, not from other source files.
unput(c)
Puts the character c back into the input stream. It will be the
next character scanned. The following action will take the
current token and cause it to be rescanned enclosed in
parentheses.
{
int i;
char *yycopy;
/* Copy yytext because unput() trashes yytext */
if ((yycopy = strdup(yytext)) == NULL)
err(1, NULL);
unput(')');
for (i = yyleng - 1; i >= 0; --i)
unput(yycopy[i]);
unput('(');
free(yycopy);
}
Note that since each unput() puts the given character back at the
beginning of the input stream, pushing back strings must be done
back-to-front.
An important potential problem when using unput() is that if us-
ing "%pointer" (the default), a call to unput() destroys the con-
tents of yytext, starting with its rightmost character and de-
vouring one character to the left with each call. If the value of
yytext should be preserved after a call to unput() (as in the
above example), it must either first be copied elsewhere, or the
scanner must be built using "%array" instead (see HOW THE INPUT
IS MATCHED).
Finally, note that EOF cannot be put back to attempt to mark the
input stream with an end-of-file.
input()
Reads the next character from the input stream. For example, the
following is one way to eat up C comments:
%%
"/*" {
int c;
for (;;) {
while ((c = input()) != '*' && c != EOF)
; /* eat up text of comment */
if (c == '*') {
while ((c = input()) == '*')
;
if (c == '/')
break; /* found the end */
}
if (c == EOF) {
errx(1, "EOF in comment");
break;
}
}
}
(Note that if the scanner is compiled using C++, then input() is
instead referred to as yyinput(), in order to avoid a name clash
with the C++ stream by the name of input.)
YY_FLUSH_BUFFER
Flushes the scanner's internal buffer so that the next time the
scanner attempts to match a token, it will first refill the
buffer using YY_INPUT (see THE GENERATED SCANNER, below). This
action is a special case of the more general yy_flush_buffer()
function, described below in the section MULTIPLE INPUT BUFFERS.
yyterminate()
Can be used in lieu of a return statement in an action. It ter-
minates the scanner and returns a 0 to the scanner's caller, in-
dicating "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
The output of flex is the file lex.yy.c, which contains the scanning rou-
tine yylex(), a number of tables used by it for matching tokens, and a
number of auxiliary routines and macros. By default, yylex() is declared
as follows:
int yylex()
{
... various definitions and the actions in here ...
}
(If the environment supports function prototypes, then it will be "int
yylex(void)".) This definition may be changed by defining the YY_DECL
macro. For example:
#define YY_DECL float lexscan(a, b) float a, b;
would give the scanning routine the name lexscan, returning a float, and
taking two floats as arguments. Note that if arguments are given to the
scanning routine using a K&R-style/non-prototyped function declaration,
the definition must be terminated with a semi-colon (';').
Whenever yylex() is called, it scans tokens from the global input file
yyin (which defaults to stdin). It continues until it either reaches an
end-of-file (at which point it returns the value 0) or one of its actions
executes a return statement.
If the scanner reaches an end-of-file, subsequent calls are undefined un-
less either yyin is pointed at a new input file (in which case scanning
continues from that file), or yyrestart() is called. yyrestart() takes
one argument, a FILE * pointer (which can be nil, if YY_INPUT has been
set up to scan from a source other than yyin), and initializes yyin for
scanning from that file. Essentially there is no difference between just
assigning yyin to a new input file or using yyrestart() to do so; the
latter is available for compatibility with previous versions of flex, and
because it can be used to switch input files in the middle of scanning.
It can also be used to throw away the current input buffer, by calling it
with an argument of yyin; but better is to use YY_FLUSH_BUFFER (see
above). Note that yyrestart() does not reset the start condition to
INITIAL (see START CONDITIONS, below).
If yylex() stops scanning due to executing a return statement in one of
the actions, the scanner may then be called again and it will resume
scanning where it left off.
By default (and for purposes of efficiency), the scanner uses block-reads
rather than simple getc(3) calls to read characters from yyin. The nature
of how it gets its input can be controlled by defining the YY_INPUT mac-
ro. 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 de-
fault 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.
START CONDITIONSflex 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 condi-
tions will be inactive. If the start condition is inclusive, then rules
with no start conditions at all will also be active. If it is exclusive,
then only rules qualified with the start condition will be active. A set
of rules contingent on the same exclusive start condition describe a
scanner which is independent of any of the other rules in the flex input.
Because of this, exclusive start conditions make it easy to specify
"mini-scanners" which scan portions of the input that are syntactically
different from the rest (e.g., comments).
If the distinction between inclusive and exclusive start conditions is
still a little vague, here's a simple example illustrating the connection
between the two. The set of rules:
%s example
%%
<example>foo do_something();
bar something_else();
is equivalent to
%x example
%%
<example>foo do_something();
<INITIAL,example>bar something_else();
Without the <INITIAL,example> qualifier, the "bar" pattern in the second
example wouldn't be active (i.e., couldn't match) when in start condition
"example". If we just used <example> to qualify "bar", though, then it
would only be active in "example" and not in INITIAL, while in the first
example it's active in both, because in the first example the "example"
start condition is an inclusive ('%s') start condition.
Also note that the special start-condition specifier '<*>' matches every
start condition. Thus, the above example could also have been written:
%x example
%%
<example>foo do_something();
<*>bar something_else();
The default rule (to ECHO any unmatched character) remains active in
start conditions. It is equivalent to:
<*>.|\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 pro-
vides two different interpretations of a string like "123.456". By de-
fault 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 main-
taining 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 as possible in each rule, as it's a big win.
Note that start-condition names are really integer values and can be
stored as such. Thus, the above could be extended in the following
fashion:
%x comment foo
%%
int line_num = 1;
int comment_caller;
"/*" {
comment_caller = INITIAL;
BEGIN(comment);
}
...
<foo>"/*" {
comment_caller = foo;
BEGIN(comment);
}
<comment>[^*\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, the current start condition can be accessed by using the
integer-valued YY_START macro. For example, the above assignments to
comment_caller could instead be written
comment_caller = YY_START;
Flex provides YYSTATE as an alias for YY_START (since that is what's used
by AT&T lex).
Note that start conditions do not have their own name-space; %s's and
%x's declare names in the same fashion as #define's.
Finally, here's an example of how to match C-style quoted strings using
exclusive start conditions, including expanded escape sequences (but not
including checking for a string that's too long):
%x str
%%
#define MAX_STR_CONST 1024
char string_buf[MAX_STR_CONST];
char *string_buf_ptr;
\" 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 */
} else
*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, a whole bunch of rules are
all preceded by the same start condition(s). flex makes this a little
easier and cleaner by introducing a notion of start condition scope. A
start condition scope is begun with:
<SCs>{
where "SCs" is a list of one or more start conditions. Inside the start
condition scope, every rule automatically has the prefix <SCs> applied to
it, until a '}' which matches the initial '{'. So, for example,
<ESC>{
"\\n" return '\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 con-
dition stack and switches to new_state as though "BEGIN
new_state" had been used (recall that start condition names are
also integers).
void yy_pop_state()
Pops the top of the stack and switches to it via BEGIN.
int yy_top_state()
Returns the top of the stack without altering the stack's con-
tents.
The start condition stack grows dynamically and so has no built-in size
limitation. If memory is exhausted, program execution aborts.
To use start condition stacks, scanners must include a "%option stack"
directive (see OPTIONS below).
MULTIPLE INPUT BUFFERS
Some scanners (such as those which support "include" files) require read-
ing 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 han-
dle, which may then be passed to other routines (see below). The
YY_BUFFER_STATE type is a pointer to an opaque "struct yy_buffer_state"
structure, so YY_BUFFER_STATE variables may be safely initialized to
"((YY_BUFFER_STATE) 0)" if desired, and the opaque structure can also be
referred to in order to correctly declare input buffers in source files
other than that of scanners. Note that the FILE pointer in the call to
yy_create_buffer() is only used as the value of yyin seen by YY_INPUT; if
YY_INPUT is redefined so that it no longer uses yyin, then a nil FILE
pointer can safely be passed to yy_create_buffer(). To select a particu-
lar buffer to scan:
void yy_switch_to_buffer(YY_BUFFER_STATE new_buffer)
It switches the scanner's input buffer so subsequent tokens will come
from new_buffer. Note that yy_switch_to_buffer() may be used by yywrap()
to set things up for continued scanning, instead of opening a new file
and pointing yyin at it. Note also that switching input sources via ei-
ther yy_switch_to_buffer() or yywrap() does not change the start condi-
tion.
void yy_delete_buffer(YY_BUFFER_STATE buffer)
is used to reclaim the storage associated with a buffer. (buffer can be
nil, in which case the routine does nothing.) To clear the current con-
tents of a buffer:
void yy_flush_buffer(YY_BUFFER_STATE buffer)
This function discards the buffer's contents, so the next time the
scanner attempts to match a token from the buffer, it will first fill the
buffer anew using YY_INPUT.
yy_new_buffer() is an alias for yy_create_buffer(), provided for compati-
bility 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)
errx(1, "Includes nested too deeply");
include_stack[include_stack_ptr++] =
YY_CURRENT_BUFFER;
yyin = fopen(yytext, "r");
if (yyin == NULL)
err(1, NULL);
yy_switch_to_buffer(
yy_create_buffer(yyin, YY_BUF_SIZE));
BEGIN(INITIAL);
}
<<EOF>> {
if (--include_stack_ptr < 0)
yyterminate();
else {
yy_delete_buffer(YY_CURRENT_BUFFER);
yy_switch_to_buffer(
include_stack[include_stack_ptr]);
}
}
Three routines are available for setting up input buffers for scanning
in-memory strings instead of files. All of them create a new input buffer
for scanning the string, and return a corresponding YY_BUFFER_STATE han-
dle (which should be deleted afterwards using yy_delete_buffer()). They
also switch to the new buffer using yy_switch_to_buffer(), so the next
call to yylex() will start scanning the string.
yy_scan_string(const char *str)
Scans a NUL-terminated string.
yy_scan_bytes(const char *bytes, int len)
Scans len bytes (including possibly NUL's) starting at location
bytes.
Note that both of these functions create and scan a copy of the string or
bytes. (This may be desirable, since yylex() modifies the contents of the
buffer it is scanning.) The copy can be avoided by using:
yy_scan_buffer(char *base, yy_size_t size)
Which scans the buffer starting at base, consisting of size
bytes, the last two bytes of which must be YY_END_OF_BUFFER_CHAR
(ASCII NUL). These last two bytes are not scanned; thus, scanning
consists of base[0] through base[size-2], inclusive.
If base is not set up in this manner (i.e., forget the final two
YY_END_OF_BUFFER_CHAR bytes), then yy_scan_buffer() returns a nil
pointer instead of creating a new input buffer.
The type yy_size_t is an integral type which can be cast to an
integer expression reflecting the size of the buffer.
END-OF-FILE RULES
The special rule "<<EOF>>" indicates actions which are to be taken when
an end-of-file is encountered and yywrap() returns non-zero (i.e.,
indicates no further files to process). The action must finish by doing
one of four things:
- Assigning yyin to a new input file (in previous versions of flex,
after doing the assignment, it was necessary to call the special ac-
tion YY_NEW_FILE; this is no longer necessary).
- Executing a return statement.
- Executing the special yyterminate() action.
- Switching to a new buffer using yy_switch_to_buffer() as shown in the
example above.
<<EOF>> rules may not be used with other patterns; they may only be qual-
ified 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 ex-
ample:
%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
The macro YY_USER_ACTION can be defined to provide an action which is al-
ways 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). For example, to pro-
file how often each rule is matched, the following would do the trick:
#define YY_USER_ACTION ++ctr[yy_act]
where ctr is an array to hold the counts for the different rules. Note
that the macro YY_NUM_RULES gives the total number of rules (including
the default rule, even if -s is used), so a correct declaration for ctr
is:
int ctr[YY_NUM_RULES];
The macro YY_USER_INIT may be defined to provide an action which is al-
ways executed before the first scan (and before the scanner's internal
initializations are done). For example, it could be used to call a rou-
tine 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 in-
put source is indeed interactive to avoid problems due to waiting to fill
buffers (see the discussion of the -I flag below). A non-zero value in
the macro invocation marks the buffer as interactive, a zero value as
non-interactive. Note that use of this macro overrides "%option always-
interactive" or "%option never-interactive" (see OPTIONS below).
yy_set_interactive() must be invoked prior to beginning to scan the
buffer that is (or is not) to be considered interactive.
The macro yy_set_bol(at_bol) can be used to control whether the current
buffer's scanning context for the next token match is done as though at
the beginning of a line. A non-zero macro argument makes rules anchored
with '^' 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 rules. 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 unreach-
able statement warnings where because a rule's action ends with "return",
the YY_BREAK is inaccessible.
VALUES AVAILABLE TO THE USER
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 (characters cannot be appended 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 can be
redefined in the first section to change the default value
(generally 8KB). Using "%array" results in somewhat slower
scanners, but the value of yytext becomes immune to calls to in-
put() and unput(), which potentially destroy its value when
yytext is a character pointer. The opposite of "%array" is
"%pointer", which is the default.
"%array" cannot be used 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 rede-
fined, 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 in-
put; use yyrestart() instead. Once scanning terminates because an
end-of-file has been seen, yyin can be assigned as the new input
file and the scanner can be called 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 ar-
gument 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 con-
dition. This value can subsequently be used with BEGIN to return
to that start condition.
INTERFACING WITH YACC
One of the main uses of flex is as a companion to the yacc(1) 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,
which is defined externally, and can be a union or any other complex data
structure. 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;
OPTIONSflex has the following options:
-7 Instructs flex to generate a 7-bit scanner, i.e., one which can
only recognize 7-bit characters in its input. The advantage of
using -7 is that the scanner's tables can be up to half the size
of those generated using the -8 option (see below). The disadvan-
tage is that such scanners often hang or crash if their input
contains an 8-bit character.
Note, however, that unless generating a scanner using the -Cf or
-CF table compression options, use of -7 will save only a small
amount of table space, and make the scanner considerably less
portable. flex's default behavior is to generate an 8-bit scanner
unless -Cf or -CF is specified, in which case flex defaults to
generating 7-bit scanners unless it was configured to generate
8-bit scanners (as will often be the case with non-USA sites). It
is possible tell whether flex generated a 7-bit or an 8-bit
scanner by inspecting the flag summary in the -v output as
described below.
Note that if -Cfe or -CFe are used (the table compression op-
tions, but also using equivalence classes as discussed below),
flex still defaults to generating an 8-bit scanner, since usually
with these compression options full 8-bit tables are not much
more expensive than 7-bit tables.
-8 Instructs flex to generate an 8-bit scanner, i.e., one which can
recognize 8-bit characters. This flag is only needed for scanners
generated using -Cf or -CF, as otherwise flex defaults to gen-
erating an 8-bit scanner anyway.
See the discussion of -7 above for flex's default behavior and
the tradeoffs between 7-bit and 8-bit scanners.
-B Instructs flex to generate a batch scanner, the opposite of
interactive scanners generated by -I (see below). In general, -B
is used when the scanner will never be used interactively, and
you want to squeeze a little more performance out of it. If the
aim is instead to squeeze out a lot more performance, use the -Cf
or -CF options (discussed below), which turn on -B automatically
anyway.
-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[aeFfmr]
Controls the degree of table compression and, more generally,
trade-offs between small scanners and fast scanners.
-Ca Instructs flex to trade off larger tables in the generat-
ed scanner for faster performance because the elements of
the tables are better aligned for memory access and com-
putation. On some RISC architectures, fetching and mani-
pulating longwords is more efficient than with smaller-
sized units such as shortwords. This option can double
the size of the tables used by the scanner.
-Ce Directs flex to construct equivalence classes, i.e., sets
of characters which have identical lexical properties
(for example, if the only appearance of digits in the
flex input is in the character class "[0-9]" then the di-
gits '0', '1', '...', '9' will all be put in the same
equivalence class). Equivalence classes usually give
dramatic reductions in the final table/object file sizes
(typically a factor of 2-5) and are pretty cheap
performance-wise (one array look-up per character
scanned).
-CF Specifies that the alternate fast scanner representation
(described below under the -F option) should be used.
This option cannot be used with -+.
-Cf Specifies that the full scanner tables should be generat-
ed - flex should not compress the tables by taking advan-
tage of similar transition functions for different
states.
-Cm Directs flex to construct meta-equivalence classes, which
are sets of equivalence classes (or characters, if
equivalence classes are not being used) that are commonly
used together. Meta-equivalence classes are often a big
win when using compressed tables, but they have a
moderate performance impact (one or two "if" tests and
one array look-up per character scanned).
-Cr Causes the generated scanner to bypass use of the stan-
dard I/O library (stdio) for input. Instead of calling
fread(3) or getc(3), the scanner will use the read(2)
system call, resulting in a performance gain which varies
from system to system, but in general is probably negli-
gible unless -Cf or -CF are being used. Using -Cr can
cause strange behavior if, for example, reading from yyin
using stdio prior to calling the scanner (because the
scanner will miss whatever text previous reads left in
the stdio input buffer).
-Cr has no effect if YY_INPUT is defined (see THE
GENERATED SCANNER above).
A lone -C specifies that the scanner tables should be compressed
but neither equivalence classes nor meta-equivalence classes
should be used.
The options -Cf or -CF and -Cm do not make sense together - there
is no opportunity for meta-equivalence classes if the table is
not being compressed. Otherwise the options may be freely mixed,
and are cumulative.
The default setting is -Cem which specifies that flex should gen-
erate equivalence classes and meta-equivalence classes. This set-
ting provides the highest degree of table compression. It is pos-
sible to trade off faster-executing scanners at the cost of
larger tables with the following generally being true:
slowest & smallest
-Cem
-Cm
-Ce
-C
-C{f,F}e
-C{f,F}
-C{f,F}a
fastest & largest
Note that scanners with the smallest tables are usually generated
and compiled the quickest, so during development the default is
usually best, maximal compression.
-Cfe is often a good compromise between speed and size for pro-
duction scanners.
-c 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 ("the matched text")
The line number refers to the location of the rule in the file
defining the scanner (i.e., the file that was fed to flex). Mes-
sages are also generated when the scanner backs up, accepts the
default rule, reaches the end of its input buffer (or encounters
a NUL; at this point, the two look the same as far as the
scanner's concerned), or reaches an end-of-file.
-F Specifies that the fast scanner table representation should be
used (and stdio bypassed). This representation is about as fast
as the full table representation (-f), and for some sets of pat-
terns will be considerably smaller (and for others, larger). In
general, if the pattern set contains both "keywords" and a
catch-all, "identifier" rule, such as in the set:
"case" return TOK_CASE;
"switch" return TOK_SWITCH;
...
"default" return TOK_DEFAULT;
[a-z]+ return TOK_ID;
then it's better to use the full table representation. If only
the "identifier" rule is present and a hash table or some such is
used to detect the keywords, it's better to use -F.
This option is equivalent to -CFr (see above). It cannot be used
with -+.
-f Specifies fast scanner. No table compression is done and stdio is
bypassed. The result is large but fast. This option is equivalent
to -Cfr (see above).
-h Generates a help summary of flex's options to stdout and then ex-
its. -? and --help are synonyms for -h.
-I Instructs flex to generate an interactive scanner. An interactive
scanner is one that only looks ahead to decide what token has
been matched if it absolutely must. It turns out that always
looking one extra character ahead, even if the scanner has al-
ready seen enough text to disambiguate the current token, is a
bit faster than only looking ahead when necessary. But scanners
that always look ahead give dreadful interactive performance; for
example, when a user types a newline, it is not recognized as a
newline token until they enter another token, which often means
typing in another whole line.
flex scanners default to interactive unless -Cf or -CF table-
compression options are specified (see above). That's because if
high-performance is most important, one of these options should
be used, so if they weren't, flex assumes it is preferrable to
trade off a bit of run-time performance for intuitive interactive
behavior. Note also that -I cannot be used in conjunction with
-Cf or -CF. Thus, this option is not really needed; it is on by
default for all those cases in which it is allowed.
A scanner can be forced to not be interactive by using -B (see
above).
-i Instructs flex to generate a case-insensitive scanner. The case
of letters given in the flex input patterns will be ignored, and
tokens in the input will be matched regardless of case. The
matched text given in yytext will have the preserved case (i.e.,
it will not be folded).
-L Instructs flex not to generate "#line" directives. Without this
option, flex peppers the generated scanner with #line directives
so error messages in the actions will be correctly located with
respect to either the original flex input file (if the errors are
due to code in the input file), or lex.yy.c (if the errors are
flex's fault - these sorts of errors should be reported to the
email address given below).
-l Turns on maximum compatibility with the original AT&T lex imple-
mentation. Note that this does not mean full compatibility. Use
of this option costs a considerable amount of performance, and it
cannot be used with the -+, -f, -F, -Cf, or -CF options. For de-
tails on the compatibilities it provides, see the section
INCOMPATIBILITIES WITH LEX AND POSIX below. This option also
results in the name YY_FLEX_LEX_COMPAT being #define'd in the
generated scanner.
-n Another do-nothing, deprecated option included only for POSIX
compliance.
-ooutput
Directs flex to write the scanner to the file output instead of
lex.yy.c. If -o is combined with the -t option, then the scanner
is written to stdout but its "#line" directives (see the -L op-
tion above) refer to the file output.
-Pprefix
Changes the default "yy" prefix used by flex for all globally
visible variable and function names to instead be prefix. For ex-
ample, -Pfoo changes the name of yytext to footext. It also
changes the name of the default output file from lex.yy.c to
lex.foo.c. Here are all of the names affected:
yy_create_buffer
yy_delete_buffer
yy_flex_debug
yy_init_buffer
yy_flush_buffer
yy_load_buffer_state
yy_switch_to_buffer
yyin
yyleng
yylex
yylineno
yyout
yyrestart
yytext
yywrap
(If using a C++ scanner, then only yywrap and yyFlexLexer are af-
fected.) Within the scanner itself, it is still possible to refer
to the global variables and functions using either version of
their name; but externally, they have the modified name.
This option allows multiple flex programs to be easily linked to-
gether into the same executable. Note, though, that using this
option also renames yywrap(), so now either an (appropriately
named) version of the routine for the scanner must be supplied,
or "%option noyywrap" must be used, as linking with -lfl no
longer provides one by default.
-p Generates a performance report to stderr. The report consists of
comments regarding features of the flex input file which will
cause a serious loss of performance in the resulting scanner. If
the flag is specified twice, comments regarding features that
lead to minor performance losses will also be reported>
Note that the use of REJECT, "%option yylineno", and variable
trailing context (see the BUGS section below) entails a substan-
tial performance penalty; use of yymore(), the '^' operator, and
the -I flag entail minor performance penalties.
-Sskeleton
Overrides the default skeleton file from which flex constructs
its scanners. This option is needed only for flex maintenance or
development.
-s Causes the default rule (that unmatched scanner input is echoed
to stdout) to be suppressed. If the scanner encounters input that
does not match any of its rules, it aborts with an error. This
option is useful for finding holes in a scanner's rule set.
-T Makes flex run in trace mode. It will generate a lot of messages
to stderr concerning the form of the input and the resultant
non-deterministic and deterministic finite automata. This option
is mostly for use in maintaining flex.
-t Instructs flex to write the scanner it generates to standard out-
put instead of lex.yy.c.
-V Prints the version number to stdout and exits. --version is a
synonym for -V.
-v Specifies that flex should write to stderr a summary of statis-
tics regarding the scanner it generates. Most of the statistics
are meaningless to the casual flex user, but the first line iden-
tifies the version of flex (same as reported by -V), and the next
line the flags used when generating the scanner, including those
that are on by default.
-w Suppresses warning messages.
-+ Specifies that flex should generate a C++ scanner class. See the
section on GENERATING C++ SCANNERS below for details.
flex also provides a mechanism for controlling options within the scanner
specification itself, rather than from the flex command-line. This is
done by including "%option" directives in the first section of the
scanner specification. Multiple options can be specified with a single
"%option" directive, and multiple directives in the first section of the
flex input file.
Most options are given simply as names, optionally preceded by the word
"no" (with no intervening whitespace) to negate their meaning. A number
are equivalent to flex flags or their negation:
7bit -7 option
8bit -8 option
align -Ca option
backup -b option
batch -B option
c++ -+ option
caseful or
case-sensitive opposite of -i (default)
case-insensitive or
caseless -i option
debug -d option
default opposite of -s option
ecs -Ce option
fast -F option
full -f option
interactive -I option
lex-compat -l option
meta-ecs -Cm option
perf-report -p option
read -Cr option
stdout -t option
verbose -v option
warn opposite of -w option
(use "%option nowarn" for -w)
array equivalent to "%array"
pointer equivalent to "%pointer" (default)
Some %option's provide features otherwise not available:
always-interactive
Instructs flex to generate a scanner which always considers its
input "interactive". Normally, on each new input file the scanner
calls isatty() in an attempt to determine whether the scanner's
input source is interactive and thus should be read a character
at a time. When this option is used, however, no such call is
made.
main Directs flex to provide a default main() program for the scanner,
which simply calls yylex(). This option implies "noyywrap" (see
below).
never-interactive
Instructs flex to generate a scanner which never considers its
input "interactive" (again, no call made to isatty()). This is
the opposite of "always-interactive".
stack Enables the use of start condition stacks (see START CONDITIONS
above).
stdinit
If set (i.e., "%option stdinit"), initializes yyin and yyout to
stdin and stdout, instead of the default of "nil". Some existing
lex programs depend on this behavior, even though it is not com-
pliant with ANSI C, which does not require stdin and stdout to be
compile-time constant.
yylineno
Directs flex to generate a scanner that maintains the number of
the current line read from its input in the global variable
yylineno. This option is implied by "%option lex-compat".
yywrap If unset (i.e., "%option noyywrap"), makes the scanner not call
yywrap() upon an end-of-file, but simply assume that there are no
more files to scan (until the user points yyin at a new file and
calls yylex() again).
flex scans rule actions to determine whether the REJECT or yymore()
features are being used. The "reject" and "yymore" options are available
to override its decision as to whether to use the options, either by set-
ting them (e.g., "%option reject") to indicate the feature is indeed
used, or unsetting them to indicate it actually is not used (e.g.,
"%option noyymore").
Three options take string-delimited values, offset with '=':
%option outfile="ABC"
is equivalent to -oABC, and
%option prefix="XYZ"
is equivalent to -PXYZ. Finally,
%option yyclass="foo"
only applies when generating a C++ scanner (-+ option). It informs flex
that "foo" has been derived as a subclass of yyFlexLexer, so flex will
place actions in the member function "foo::yylex()" instead of
"yyFlexLexer::yylex()". It also generates a "yyFlexLexer::yylex()" member
function that emits a run-time error (by invoking
"yyFlexLexer::LexerError()") if called. See GENERATING C++ SCANNERS,
below, for additional information.
A number of options are available for lint(1) purists who want to
suppress the appearance of unneeded routines in the generated scanner.
Each of the following, if unset (e.g., "%option nounput"), results in the
corresponding routine not appearing in the generated scanner:
input, unput
yy_push_state, yy_pop_state, yy_top_state
yy_scan_buffer, yy_scan_bytes, yy_scan_string
(though yy_push_state() and friends won't appear anyway unless "%option
stack" is being used).
PERFORMANCE CONSIDERATIONS
The main design goal of flex is that it generate high-performance
scanners. It has been optimized for dealing well with large sets of
rules. Aside from the effects on scanner speed of the table compression
-C options outlined above, there are a number of options/actions which
degrade performance. These are, from most expensive to least:
REJECT
%option yylineno
arbitrary trailing context
pattern sets that require backing up
%array
%option interactive
%option always-interactive
'^' beginning-of-line operator
yymore()
with the first three all being quite expensive and the last two being
quite cheap. Note also that unput() is implemented as a routine call that
potentially does quite a bit of work, while yyless() is a quite-cheap
macro; so if just putting back some excess text, use yyless().
REJECT should be avoided at all costs when performance is important. It
is a particularly expensive option.
Getting rid of backing up is messy and often may be an enormous amount of
work for a complicated scanner. In principal, one begins by using the -b
flag to generate a lex.backup file. For example, on the input
%%
foo return TOK_KEYWORD;
foobar return TOK_KEYWORD;
the file looks like:
State #6 is non-accepting -
associated rule line numbers:
2 3
out-transitions: [ o ]
jam-transitions: EOF [ \001-n p-\177 ]
State #8 is non-accepting -
associated rule line numbers:
3
out-transitions: [ a ]
jam-transitions: EOF [ \001-` b-\177 ]
State #9 is non-accepting -
associated rule line numbers:
3
out-transitions: [ r ]
jam-transitions: EOF [ \001-q s-\177 ]
Compressed tables always back up.
The first few lines tell us that there's a scanner state in which it can
make a transition on an 'o' but not on any other character, and that in
that state the currently scanned text does not match any rule. The state
occurs when trying to match the rules found at lines 2 and 3 in the input
file. If the scanner is in that state and then reads something other than
an 'o', it will have to back up to find a rule which is matched. With a
bit of headscratching one can see that this must be the state it's in
when it has seen 'fo'. When this has happened, if anything other than
another 'o' is seen, the scanner will have to back up to simply match the
'f' (by the default rule).
The comment regarding State #8 indicates there's a problem when "foob"
has been scanned. Indeed, on any character other than an 'a', the scanner
will have to back up to accept "foo". Similarly, the comment for State #9
concerns when "fooba" has been scanned and an 'r' does not follow.
The final comment reminds us that there's no point going to all the trou-
ble of removing backing up from the rules unless we're using -Cf or -CF,
since there's no performance gain doing so with compressed scanners.
The way to remove the backing up is to add "error" rules:
%%
foo return TOK_KEYWORD;
foobar return TOK_KEYWORD;
fooba |
foob |
fo {
/* false alarm, not really a keyword */
return TOK_ID;
}
Eliminating backing up among a list of keywords can also be done using a
"catch-all" rule:
%%
foo return TOK_KEYWORD;
foobar return TOK_KEYWORD;
[a-z]+ return TOK_ID;
This is usually the best solution when appropriate.
Backing up messages tend to cascade. With a complicated set of rules it's
not uncommon to get hundreds of messages. If one can decipher them,
though, it often only takes a dozen or so rules to eliminate the backing
up (though it's easy to make a mistake and have an error rule accidental-
ly match a valid token; a possible future flex feature will be to au-
tomatically add rules to eliminate backing up).
It's important to keep in mind that the benefits of eliminating backing
up are gained only if every instance of backing up is eliminated. Leaving
just one gains nothing.
Variable trailing context (where both the leading and trailing parts do
not have a fixed length) entails almost the same performance loss as
REJECT (i.e., substantial). So when possible a rule like:
%%
mouse|rat/(cat|dog) run();
is better written:
%%
mouse/cat|dog run();
rat/cat|dog run();
or as
%%
mouse|rat/cat run();
mouse|rat/dog run();
Note that here the special '|' action does not provide any savings, and
can even make things worse (see BUGS below).
Another area where the user can increase a scanner's performance (and one
that's easier to implement) arises from the fact that the longer the to-
kens matched, the faster the scanner will run. This is because with long
tokens the processing of most input characters takes place in the (short)
inner scanning loop, and does not often have to go through the additional
work of setting up the scanning environment (e.g., yytext) for the ac-
tion. Recall the scanner for C comments:
%x comment
%%
int line_num = 1;
"/*" BEGIN(comment);
<comment>[^*\n]*
<comment>"*"+[^*/\n]*
<comment>\n ++line_num;
<comment>"*"+"/" BEGIN(INITIAL);
This could be sped up by writing it as:
%x comment
%%
int line_num = 1;
"/*" BEGIN(comment);
<comment>[^*\n]*
<comment>[^*\n]*\n ++line_num;
<comment>"*"+[^*/\n]*
<comment>"*"+[^*/\n]*\n ++line_num;
<comment>"*"+"/" BEGIN(INITIAL);
Now instead of each newline requiring the processing of another action,
recognizing the newlines is "distributed" over the other rules to keep
the matched text as long as possible. Note that adding rules does not
slow down the scanner! The speed of the scanner is independent of the
number of rules or (modulo the considerations given at the beginning of
this section) how complicated the rules are with regard to operators such
as '*' and '|'.
A final example in speeding up a scanner: scan through a file containing
identifiers and keywords, one per line and with no other extraneous char-
acters, and recognize all the keywords. A natural first approach is:
%%
asm |
auto |
break |
... etc ...
volatile |
while /* it's a keyword */
.|\n /* it's not a keyword */
To eliminate the back-tracking, introduce a catch-all rule:
%%
asm |
auto |
break |
... etc ...
volatile |
while /* it's a keyword */
[a-z]+ |
.|\n /* it's not a keyword */
Now, if it's guaranteed that there's exactly one word per line, then we
can reduce the total number of matches by a half by merging in the recog-
nition of newlines with that of the other tokens:
%%
asm\n |
auto\n |
break\n |
... etc ...
volatile\n |
while\n /* it's a keyword */
[a-z]+\n |
.|\n /* it's not a keyword */
One has to be careful here, as we have now reintroduced backing up into
the scanner. In particular, while we know that there will never be any
characters in the input stream other than letters or newlines, flex can't
figure this out, and it will plan for possibly needing to back up when it
has scanned a token like "auto" and then the next character is something
other than a newline or a letter. Previously it would then just match the
"auto" rule and be done, but now it has no "auto" rule, only an "auto\n"
rule. To eliminate the possibility of backing up, we could either dupli-
cate all rules but without final newlines, or, since we never expect to
encounter such an input and therefore don't how it's classified, we can
introduce one more catch-all rule, this one which doesn't include a new-
line:
%%
asm\n |
auto\n |
break\n |
... etc ...
volatile\n |
while\n /* it's a keyword */
[a-z]+\n |
[a-z]+ |
.|\n /* it's not a keyword */
Compiled with -Cf, this is about as fast as one can get a flex scanner to
go for this particular problem.
A final note: flex is slow when matching NUL's, particularly when a token
contains multiple NUL's. It's best to write rules which match short
amounts of text if it's anticipated that the text will often include
NUL's.
Another final note regarding performance: as mentioned above in the sec-
tion HOW THE INPUT IS MATCHED, dynamically resizing yytext to accommodate
huge tokens is a slow process because it presently requires that the
(huge) token be rescanned from the beginning. Thus if performance is vi-
tal, it is better to attempt to match "large" quantities of text but not
"huge" quantities, where the cutoff between the two is at about 8K
characters/token.
GENERATING C++ SCANNERSflex provides two different ways to generate scanners for use with C++.
The first way is to simply compile a scanner generated by flex using a
C++ compiler instead of a C compiler. This should not generate any compi-
lation errors (please report any found to the email address given in the
AUTHORS section below). C++ code can then be used in rule actions instead
of C code. Note that the default input source for scanners remains yyin,
and default echoing is still done to yyout. Both of these remain FILE *
variables and not C++ streams.
flex can also be used to generate a C++ scanner class, using the -+ op-
tion (or, equivalently, "%option c++"), which is automatically specified
if the name of the flex executable ends in a '+', such as flex++. When
using this option, flex defaults to generating the scanner to the file
lex.yy.cc instead of lex.yy.c. The generated scanner includes the header
file <g++/FlexLexer.h>, which defines the interface to two C++ classes.
The first class, FlexLexer, provides an abstract base class defining the
general scanner class interface. It provides the following member func-
tions:
const char* YYText()
Returns the text of the most recently matched token, the
equivalent of yytext.
int YYLeng()
Returns the length of the most recently matched token, the
equivalent of yyleng.
int lineno() const
Returns the current input line number (see "%option yylineno"),
or 1 if "%option yylineno" was not used.
void set_debug(int flag)
Sets the debugging flag for the scanner, equivalent to assigning
to yy_flex_debug (see the OPTIONS section above). Note that the
scanner must be built using "%option debug" to include debugging
information in it.
int debug() const
Returns the current setting of the debugging flag.
Also provided are member functions equivalent to yy_switch_to_buffer(),
yy_create_buffer() (though the first argument is an std::istream* object
pointer and not a FILE*), yy_flush_buffer(), yy_delete_buffer(), and
yyrestart() (again, the first argument is an std::istream* object
pointer).
The second class defined in <g++/FlexLexer.h> is yyFlexLexer, which is
derived from FlexLexer. It defines the following additional member func-
tions:
yyFlexLexer(std::istream* arg_yyin = 0, std::ostream* arg_yyout = 0)
Constructs a yyFlexLexer object using the given streams for input
and output. If not specified, the streams default to cin and
cout, respectively.
virtual int yylex()
Performs the same role as yylex() does for ordinary flex
scanners: it scans the input stream, consuming tokens, until a
rule's action returns a value. If subclass 'S' is derived from
yyFlexLexer, in order to access the member functions and vari-
ables of 'S' inside yylex(), use "%option yyclass="S"" to inform
flex that the 'S' subclass will be used instead of yyFlexLexer.
In this case, rather than generating "yyFlexLexer::yylex()", flex
generates "S::yylex()" (and also generates a dummy
"yyFlexLexer::yylex()" that calls "yyFlexLexer::LexerError()" if
called).
virtual void switch_streams(std::istream* new_in = 0, std::ostream*
new_out = 0)
Reassigns yyin to new_in (if non-nil) and yyout to new_out
(ditto), deleting the previous input buffer if yyin is reas-
signed.
int yylex(std::istream* new_in, std::ostream* new_out = 0)
First switches the input streams via "switch_streams(new_in,
new_out)" and then returns the value of yylex().
In addition, yyFlexLexer defines the following protected virtual func-
tions which can be redefined in derived classes to tailor the scanner:
virtual int LexerInput(char* buf, int max_size)
Reads up to max_size characters into buf and returns the number
of characters read. To indicate end-of-input, return 0 charac-
ters. Note that "interactive" scanners (see the -B and -I flags)
define the macro YY_INTERACTIVE. If LexerInput() has been rede-
fined, and it's necessary to take different actions depending on
whether or not the scanner might be scanning an interactive input
source, it's possible to test for the presence of this name via
"#ifdef".
virtual void LexerOutput(const char* buf, int size)
Writes out size characters from the buffer buf, which, while
NUL-terminated, may also contain "internal" NUL's if the
scanner's rules can match text with NUL's in them.
virtual void LexerError(const char* msg)
Reports a fatal error message. The default version of this func-
tion writes the message to the stream cerr and exits.
Note that a yyFlexLexer object contains its entire scanning state. Thus
such objects can be used to create reentrant scanners. Multiple instances
of the same yyFlexLexer class can be instantiated, and multiple C++
scanner classes can be combined in the same program using the -P option
discussed above.
Finally, note that the "%array" feature is not available to C++ scanner
classes; "%pointer" must be used (the default).
Here is an example of a simple C++ scanner:
// An example of using the flex C++ scanner class.
%{
#include <errno.h>
int mylineno = 0;
%}
string \"[^\n"]+\"
ws [ \t]+
alpha [A-Za-z]
dig [0-9]
name ({alpha}|{dig}|\$)({alpha}|{dig}|[_.\-/$])*
num1 [-+]?{dig}+\.?([eE][-+]?{dig}+)?
num2 [-+]?{dig}*\.{dig}+([eE][-+]?{dig}+)?
number {num1}|{num2}
%%
{ws} /* skip blanks and tabs */
"/*" {
int c;
while ((c = yyinput()) != 0) {
if(c == '\n')
++mylineno;
else if(c == '*') {
if ((c = yyinput()) == '/')
break;
else
unput(c);
}
}
}
{number} cout << "number " << YYText() << '\n';
\n mylineno++;
{name} cout << "name " << YYText() << '\n';
{string} cout << "string " << YYText() << '\n';
%%
int main(int /* argc */, char** /* argv */)
{
FlexLexer* lexer = new yyFlexLexer;
while(lexer->yylex() != 0)
;
return 0;
}
To create multiple (different) lexer classes, use the -P flag (or the
"prefix=" option) to rename each yyFlexLexer to some other xxFlexLexer.
<g++/FlexLexer.h> can then be included in other sources once per lexer
class, first renaming yyFlexLexer as follows:
#undef yyFlexLexer
#define yyFlexLexer xxFlexLexer
#include <g++/FlexLexer.h>
#undef yyFlexLexer
#define yyFlexLexer zzFlexLexer
#include <g++/FlexLexer.h>
If, for example, "%option prefix="xx"" is used for one scanner and
"%option prefix="zz"" is used for the other.
IMPORTANT: the present form of the scanning class is experimental and may
change considerably between major releases.
INCOMPATIBILITIES WITH LEX AND POSIXflex is a rewrite of the AT&T Unix lex tool (the two implementations do
not share any code, though), with some extensions and incompatibilities,
both of which are of concern to those who wish to write scanners accept-
able to either implementation. flex is fully compliant with the POSIX lex
specification, except that when using "%pointer" (the default), a call to
unput() destroys the contents of yytext, which is counter to the POSIX
specification.
In this section we discuss all of the known areas of incompatibility
between flex, AT&T lex, and the POSIX specification.
flex's -l option turns on maximum compatibility with the original AT&T
lex implementation, at the cost of a major loss in the generated
scanner's performance. We note below which incompatibilities can be over-
come using the -l option.
flex is fully compatible with lex with the following exceptions:
- The undocumented lex scanner internal variable yylineno is not sup-
ported unless -l or "%option yylineno" is used.
yylineno should be maintained on a per-buffer basis, rather than a
per-scanner (single global variable) basis.
yylineno is not part of the POSIX specification.
- The input() routine is not redefinable, though it may be called to
read characters following whatever has been matched by a rule. If in-
put() encounters an end-of-file, the normal yywrap() processing is
done. A "real" end-of-file is returned by input() as EOF.
Input is instead controlled by defining the YY_INPUT macro.
The flex restriction that input() cannot be redefined is in accor-
dance with the POSIX specification, which simply does not specify any
way of controlling the scanner's input other than by making an ini-
tial assignment to yyin.
- The unput() routine is not redefinable. This restriction is in accor-
dance with POSIX.
- flex scanners are not as reentrant as lex scanners. In particular, if
a scanner is interactive and an interrupt handler long-jumps out of
the scanner, and the scanner is subsequently called again, the fol-
lowing error message may be displayed:
fatal flex scanner internal error--end of buffer missed
To reenter the scanner, first use
yyrestart(yyin);
Note that this call will throw away any buffered input; usually this
isn't a problem with an interactive scanner.
Also note that flex C++ scanner classes are reentrant, so if using
C++ is an option , they should be used instead. See GENERATING C++
SCANNERS above for details.
- output() is not supported. Output from the ECHO macro is done to the
file-pointer yyout (default stdout).
output() is not part of the POSIX specification.
- lex does not support exclusive start conditions (%x), though they are
in the POSIX specification.
- When definitions are expanded, flex encloses them in parentheses.
With lex, the following:
NAME [A-Z][A-Z0-9]*
%%
foo{NAME}? printf("Found it\n");
%%
will not match the string "foo" because when the macro is expanded
the rule is equivalent to "foo[A-Z][A-Z0-9]*?" and the precedence is
such that the '?' is associated with "[A-Z0-9]*". With flex, the rule
will be expanded to "foo([A-Z][A-Z0-9]*)?" and so the string "foo"
will match.
Note that if the definition begins with '^' or ends with '$' then it
is not expanded with parentheses, to allow these operators to appear
in definitions without losing their special meanings. But the '<s>',
'/', and <<EOF>> operators cannot be used in a flex definition.
Using -l results in the lex behavior of no parentheses around the de-
finition.
The POSIX specification is that the definition be enclosed in
parentheses.
- Some implementations of lex allow a rule's action to begin on a
separate line, if the rule's pattern has trailing whitespace:
%%
foo|bar<space here>
{ foobar_action(); }
flex does not support this feature.
- The lex '%r' (generate a Ratfor scanner) option is not supported. It
is not part of the POSIX specification.
- After a call to unput(), yytext is undefined until the next token is
matched, unless the scanner was built using "%array". This is not the
case with lex or the POSIX specification. The -l option does away
with this incompatibility.
- The precedence of the '{}' (numeric range) operator is different. lex
interprets "abc{1,3}" as match one, two, or three occurrences of
'abc', whereas flex interprets it as match 'ab' followed by one, two,
or three occurrences of 'c'. The latter is in agreement with the
POSIX specification.
- The precedence of the '^' operator is different. lex interprets
"^foo|bar" as match either 'foo' at the beginning of a line, or 'bar'
anywhere, whereas flex interprets it as match either 'foo' or 'bar'
if they come at the beginning of a line. The latter is in agreement
with the POSIX specification.
- The special table-size declarations such as '%a' supported by lex are
not required by flex scanners; flex ignores them.
- The name FLEX_SCANNER is #define'd so scanners may be written for use
with either flex or lex. Scanners also include YY_FLEX_MAJOR_VERSION
and YY_FLEX_MINOR_VERSION indicating which version of flex generated
the scanner (for example, for the 2.5 release, these defines would be
2 and 5, respectively).
The following flex features are not included in lex or the POSIX specifi-
cation:
C++ scanners
%option
start condition scopes
start condition stacks
interactive/non-interactive scanners
yy_scan_string() and friends
yyterminate()yy_set_interactive()yy_set_bol()YY_AT_BOL()
<<EOF>>
<*>
YY_DECL
YY_START
YY_USER_ACTION
YY_USER_INIT
#line directives
%{}'s around actions
multiple actions on a line
plus almost all of the flex flags. The last feature in the list refers to
the fact that with flex Multiple actions ican be placed on the same line,
separated with semi-colons, while with lex, the following
foo handle_foo(); ++num_foos_seen;
is (rather surprisingly) truncated to
foo handle_foo();
flex does not truncate the action. Actions that are not enclosed in
braces are simply terminated at the end of the line.
FILES
flex.skl Skeleton scanner. This file is only used when building
flex, not when flex executes.
lex.backup Backing-up information for the -b flag (called lex.bck
on some systems).
lex.yy.c Generated scanner (called lexyy.c on some systems).
lex.yy.cc Generated C++ scanner class, when using -+.
<g++/FlexLexer.h> Header file defining the C++ scanner base class,
FlexLexer, and its derived class, yyFlexLexer.
/usr/lib/libl.* flex libraries. The /usr/lib/libfl.* libraries are
links to these. Scanners must be linked using either
-ll or -lfl.
DIAGNOSTICS
warning, rule cannot be matched Indicates that the given rule cannot be
matched because it follows other rules that will always match the same
text as it. For example, in the following "foo" cannot be matched because
it comes after an identifier "catch-all" rule:
[a-z]+ got_identifier();
foo got_foo();
Using REJECT in a scanner suppresses this warning.
warning, -s option given but default rule can be matched Means that it
is possible (perhaps only in a particular start condition) that the de-
fault rule (match any single character) is the only one that will match a
particular input. Since -s was given, presumably this is not intended.
reject_used_but_not_detected undefined
yymore_used_but_not_detected undefined These errors can occur at compile
time. They indicate that the scanner uses REJECT or yymore() but that
flex failed to notice the fact, meaning that flex scanned the first two
sections looking for occurrences of these actions and failed to find any,
but somehow they snuck in (via an #include file, for example). Use
"%option reject" or "%option yymore" to indicate to flex that these
features are really needed.
flex scanner jammed A scanner compiled with -s has encountered an input
string which wasn't matched by any of its rules. This error can also oc-
cur due to internal problems.
token too large, exceeds YYLMAX The scanner uses "%array" and one of its
rules matched a string longer than the YYLMAX constant (8K bytes by
default). The value can be increased by #define'ing YYLMAX in the defini-
tions section of flex input.
scanner requires -8 flag to use the character 'x' The scanner specifica-
tion includes recognizing the 8-bit character 'x' and the -8 flag was not
specified, and defaulted to 7-bit because the -Cf or -CF table compres-
sion options were used. See the discussion of the -7 flag for details.
flex scanner push-back overflow unput() was used to push back so much
text that the scanner's buffer could not hold both the pushed-back text
and the current token in yytext. Ideally the scanner should dynamically
resize the buffer in this case, but at present it does not.
input buffer overflow, can't enlarge buffer because scanner uses
REJECT The scanner was working on matching an extremely large token and
needed to expand the input buffer. This doesn't work with scanners that
use REJECT.
fatal flex scanner internal error--end of buffer missed This can occur
in an scanner which is reentered after a long-jump has jumped out (or
over) the scanner's activation frame. Before reentering the scanner, use:
yyrestart(yyin);
or, as noted above, switch to using the C++ scanner class.
too many start conditions in <> construct! More start conditions than
exist were listed in a <> construct (so at least one of them must have
been listed twice).
SEE ALSOawk(1), lex(1), sed(1), yacc(1)
"Lex - A Lexical Analyzer Generator", /usr/share/doc/psd/16.lex/.
John Levine, Tony Mason, and Doug Brown, Lex & Yacc, O'Reilly and
Associates, 2nd edition.
Alfred Aho, Ravi Sethi, and Jeffrey Ullman, Compilers: Principles,
Techniques and Tools, Addison-Wesley, 1986, Describes the pattern-
matching techniques used by flex (deterministic finite automata).
AUTHORS
Vern Paxson, with the help of many ideas and much inspiration from Van
Jacobson. Original version by Jef Poskanzer. The fast table
representation is a partial implementation of a design done by Van
Jacobson. The implementation was done by Kevin Gong and Vern Paxson.
Thanks to the many flex beta-testers, feedbackers, and contributors,
especially Francois Pinard, Casey Leedom, Robert Abramovitz, Stan Ader-
mann, Terry Allen, David Barker-Plummer, John Basrai, Neal Becker, Nelson
H.F. Beebe, benson@odi.com, Karl Berry, Peter A. Bigot, Simon Blanchard,
Keith Bostic, Frederic Brehm, Ian Brockbank, Kin Cho, Nick Christopher,
Brian Clapper, J.T. Conklin, Jason Coughlin, Bill Cox, Nick Cropper, Dave
Curtis, Scott David Daniels, Chris G. Demetriou, Theo de Raadt, Mike
Donahue, Chuck Doucette, Tom Epperly, Leo Eskin, Chris Faylor, Chris
Flatters, Jon Forrest, Jeffrey Friedl, Joe Gayda, Kaveh R. Ghazi, Wolf-
gang Glunz, Eric Goldman, Christopher M. Gould, Ulrich Grepel, Peer
Griebel, Jan Hajic, Charles Hemphill, NORO Hideo, Jarkko Hietaniemi,
Scott Hofmann, Jeff Honig, Dana Hudes, Eric Hughes, John Interrante,
Ceriel Jacobs, Michal Jaegermann, Sakari Jalovaara, Jeffrey R. Jones,
Henry Juengst, Klaus Kaempf, Jonathan I. Kamens, Terrence O Kane, Amir
Katz, ken@ken.hilco.com, Kevin B. Kenny, Steve Kirsch, Winfried Koenig,
Marq Kole, Ronald Lamprecht, Greg Lee, Rohan Lenard, Craig Leres, John
Levine, Steve Liddle, David Loffredo, Mike Long, Mohamed el Lozy, Brian
Madsen, Malte, Joe Marshall, Bengt Martensson, Chris Metcalf, Luke
Mewburn, Jim Meyering, R. Alexander Milowski, Erik Naggum, G.T. Nicol,
Landon Noll, James Nordby, Marc Nozell, Richard Ohnemus, Karsten Pahnke,
Sven Panne, Roland Pesch, Walter Pelissero, Gaumond Pierre, Esmond Pitt,
Jef Poskanzer, Joe Rahmeh, Jarmo Raiha, Frederic Raimbault, Pat Rankin,
Rick Richardson, Kevin Rodgers, Kai Uwe Rommel, Jim Roskind, Alberto San-
tini, Andreas Scherer, Darrell Schiebel, Raf Schietekat, Doug Schmidt,
Philippe Schnoebelen, Andreas Schwab, Larry Schwimmer, Alex Siegel,
Eckehard Stolz, Jan-Erik Strvmquist, Mike Stump, Paul Stuart, Dave Tall-
man, Ian Lance Taylor, Chris Thewalt, Richard M. Timoney, Jodi Tsai, Paul
Tuinenga, Gary Weik, Frank Whaley, Gerhard Wilhelms, Kent Williams, Ken
Yap, Ron Zellar, Nathan Zelle, David Zuhn, and those whose names have
slipped my marginal mail-archiving skills but whose contributions are ap-
preciated all the same.
Thanks to Keith Bostic, Jon Forrest, Noah Friedman, John Gilmore, Craig
Leres, John Levine, Bob Mulcahy, G.T. Nicol, Francois Pinard, Rich Salz,
and Richard Stallman for help with various distribution headaches.
Thanks to Esmond Pitt and Earle Horton for 8-bit character support; to
Benson Margulies and Fred Burke for C++ support; to Kent Williams and Tom
Epperly for C++ class support; to Ove Ewerlid for support of NUL's; and
to Eric Hughes for support of multiple buffers.
This work was primarily done when I was with the Real Time Systems Group
at the Lawrence Berkeley Laboratory in Berkeley, CA. Many thanks to all
there for the support I received.
Send comments to <vern@ee.lbl.gov>.
BUGS
Some trailing context patterns cannot be properly matched and generate
warning messages (dangerous trailing context). These are patterns where
the ending of the first part of the rule matches the beginning of the
second part, such as "zx*/xy*", where the 'x*' matches the 'x' at the be-
ginning of the trailing context. (Note that the POSIX draft states that
the text matched by such patterns is undefined.)
For some trailing context rules, parts which are actually fixed-length
are not recognized as such, leading to the above mentioned performance
loss. In particular, parts using '|' or '{n}' (such as "foo{3}") are al-
ways considered variable-length.
Combining trailing context with the special '|' action can result in
fixed trailing context being turned into the more expensive variable
trailing context. For example, in the following:
%%
abc |
xyz/def
Use of unput() invalidates yytext and yyleng, unless the "%array" direc-
tive or the -l option has been used.
Pattern-matching of NUL's is substantially slower than matching other
characters.
Dynamic resizing of the input buffer is slow, as it entails rescanning
all the text matched so far by the current (generally huge) token.
Due to both buffering of input and read-ahead, it is not possible to in-
termix calls to <stdio.h> routines, such as, for example, getchar(), with
flex rules and expect it to work. Call input() instead.
The total table entries listed by the -v flag excludes the number of
table entries needed to determine what rule has been matched. The number
of entries is equal to the number of DFA states if the scanner does not
use REJECT, and somewhat greater than the number of states if it does.
REJECT cannot be used with the -f or -F options.
The flex internal algorithms need documentation.
MirOS BSD #10-current April 1, 1995 39
