FLEX(1) FLEX(1)
NAME
flex - fast lexical analyzer generator
SYNOPSIS
flex [-bcdfhilnpstvwBFILTV78+? -C[aefFmr] -ooutput -Ppre-
fix -Sskeleton] [--help --version] [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 input source
Start Conditions
introducing context into your scanners, and
managing "mini-scanners"
Multiple Input Buffers
how to manipulate multiple input sources; how to
scan from strings instead of files
End-of-file Rules
special rules for matching the end of the input
Miscellaneous Macros
a summary of macros available to the actions
Values Available To The User
a summary of values available to the actions
Interfacing With Yacc
connecting flex scanners together with yacc parsers
Options
flex command-line options, and the "%option"
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directive
Performance Considerations
how to make your scanner go as fast as possible
Generating C++ Scanners
the (experimental) facility for generating C++
scanner classes
Incompatibilities With Lex And POSIX
how flex differs from AT&T lex and the POSIX lex
standard
Diagnostics
those error messages produced by flex (or scanners
it generates) whose meanings might not be apparent
Files
files used by flex
Deficiencies / Bugs
known problems with flex
See Also
other documentation, related tools
Author
includes contact information
DESCRIPTION
flex is a tool for generating scanners: programs which
recognized lexical patterns in text. flex reads the given
input files, or its standard input if no file names are
given, for a description of a scanner to generate. The
description is in the form of pairs of regular expressions
and C code, called rules. flex generates as output a C
source file, lex.yy.c, which defines a routine yylex().
This file is compiled and linked with the -lfl library to
produce an executable. When the executable is run, it
analyzes its input for occurrences of the regular expres-
sions. Whenever it finds one, it executes the correspond-
ing 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
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copied to the output, so the net effect of this scanner is
to copy its input file to its output with each occurrence
of "username" expanded. In this input, there is just one
rule. "username" is the pattern and the "printf" is the
action. The "%%" marks the beginning of the rules.
Here's another simple example:
int num_lines = 0, num_chars = 0;
%%
\n ++num_lines; ++num_chars;
. ++num_chars;
%%
main()
{
yylex();
printf( "# of lines = %d, # of chars = %d\n",
num_lines, num_chars );
}
This scanner counts the number of characters and the num-
ber 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() rou-
tine declared after the second "%%". There are two rules,
one which matches a newline ("\n") and increments both the
line count and the character count, and one which matches
any character other than a newline (indicated by the "."
regular expression).
A somewhat more complicated example:
/* scanner for a toy Pascal-like language */
%{
/* need this for the call to atof() below */
#include <math.h>
%}
DIGIT [0-9]
ID [a-z][a-z0-9]*
%%
{DIGIT}+ {
printf( "An integer: %s (%d)\n", yytext,
atoi( yytext ) );
}
{DIGIT}+"."{DIGIT}* {
printf( "A float: %s (%g)\n", yytext,
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atof( yytext ) );
}
if|then|begin|end|procedure|function {
printf( "A keyword: %s\n", yytext );
}
{ID} printf( "An identifier: %s\n", yytext );
"+"|"-"|"*"|"/" printf( "An operator: %s\n", yytext );
"{"[^}\n]*"}" /* eat up one-line comments */
[ \t\n]+ /* eat up whitespace */
. printf( "Unrecognized character: %s\n", yytext );
%%
main( argc, argv )
int argc;
char **argv;
{
++argv, --argc; /* skip over program name */
if ( argc > 0 )
yyin = fopen( argv[0], "r" );
else
yyin = stdin;
yylex();
}
This is the beginnings of a simple scanner for a language
like Pascal. It identifies different types of tokens and
reports on what it has seen.
The details of this example will be explained in the fol-
lowing 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.
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Name definitions have the form:
name definition
The "name" is a word beginning with a letter or an under-
score ('_') followed by zero or more letters, digits, '_',
or '-' (dash). The definition is taken to begin at the
first non-white-space character following the name and
continuing to the end of the line. The definition can
subsequently be referred to using "{name}", which will
expand to "(definition)". For example,
DIGIT [0-9]
ID [a-z][a-z0-9]*
defines "DIGIT" to be a regular expression which matches a
single digit, and "ID" to be a regular expression which
matches a letter followed by zero-or-more letters-or-dig-
its. A subsequent reference to
{DIGIT}+"."{DIGIT}*
is identical to
([0-9])+"."([0-9])*
and matches one-or-more digits followed by a '.' followed
by zero-or-more digits.
The rules section of the flex input contains a series of
rules of the form:
pattern action
where the pattern must be unindented and the action must
begin on the same line.
See below for a further description of patterns and
actions.
Finally, the user code section is simply copied to
lex.yy.c verbatim. It is used for companion routines
which call or are called by the scanner. The presence of
this section is optional; if it is missing, the second %%
in the input file may be skipped, too.
In the definitions and rules sections, any indented text
or text enclosed in %{ and %} is copied verbatim to the
output (with the %{}'s removed). The %{}'s must appear
unindented on lines by themselves.
In the rules section, any indented or %{} text appearing
before the first rule may be used to declare variables
which are local to the scanning routine and (after the
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declarations) code which is to be executed whenever the
scanning routine is entered. Other indented or %{} text
in the rule section is still copied to the output, but its
meaning is not well-defined and it may well cause compile-
time errors (this feature is present for POSIX compliance;
see below for other such features).
In the definitions section (but not in the rules section),
an unindented comment (i.e., a line beginning with "/*")
is also copied verbatim to the output up to the next "*/".
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
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r/s an r but only if it is followed by an s. The
text matched by s is included when determining
whether this rule is the "longest match",
but is then returned to the input before
the action is executed. So the action only
sees the text matched by r. This type
of pattern is called trailing context".
(There are some combinations of r/s that flex
cannot match correctly; see notes in the
Deficiencies / Bugs section below regarding
"dangerous trailing context".)
^r an r, but only at the beginning of a line (i.e.,
which just starting to scan, or right after a
newline has been scanned).
r$ an r, but only at the end of a line (i.e., just
before a newline). Equivalent to "r/\n".
Note that flex's notion of "newline" is exactly
whatever the C compiler used to compile flex
interprets '\n' as; in particular, on some DOS
systems you must either filter out \r's in the
input yourself, or explicitly use r/\r\n for "r$".
<s>r an r, but only in start condition s (see
below for discussion of start conditions)
<s1,s2,s3>r
same, but in any of start conditions s1,
s2, or s3
<*>r an r in any start condition, even an exclusive one.
<<EOF>> an end-of-file
<s1,s2><<EOF>>
an end-of-file when in start condition s1 or s2
Note that inside of a character class, all regular expres-
sion 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 low-
est 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 concate-
nation, and concatenation higher than alternation ('|').
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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, char-
acter classes can also contain character class expres-
sions. 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 equiv-
alent to the corresponding standard C isXXX function. For
example, [:alnum:] designates those characters for which
isalnum() returns true - i.e., any alphabetic or numeric.
Some systems don't provide isblank(), so flex defines
[:blank:] as a blank or a tab.
For example, the following character classes are all
equivalent:
[[:alnum:]]
[[:alpha:][:digit:]]
[[:alpha:]0-9]
[a-zA-Z0-9]
If your scanner is case-insensitive (the -i flag), then
[:upper:] and [:lower:] are equivalent to [:alpha:].
Some notes on patterns:
- A negated character class such as the example "[^A-
Z]" above will match a newline unless "\n" (or an
equivalent escape sequence) is one of the charac-
ters explicitly present in the negated character
class (e.g., "[^A-Z\n]"). This is unlike how many
other regular expression tools treat negated char-
acter classes, but unfortunately the inconsistency
is historically entrenched. Matching newlines
means that a pattern like [^"]* can match the
entire input unless there's another quote in the
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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 prop-
erties and is treated as a normal character.
The following are illegal:
foo/bar$
<sc1>foo<sc2>bar
Note that the first of these, can be written
"foo/bar\n".
The following will result in '$' or '^' being
treated as a normal character:
foo|(bar$)
foo|^bar
If what's wanted is a "foo" or a bar-followed-by-a-
newline, the following could be used (the special
'|' action is explained below):
foo |
bar$ /* action goes here */
A similar trick will work for matching a foo or a
bar-at-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 returned to the input). If it finds two or more
matches of the same length, the rule listed first in the
flex input file is chosen.
Once the match is determined, the text corresponding to
the match (called the token) is made available in the
global character pointer yytext, and its length in the
global integer yyleng. The action corresponding to the
matched pattern is then executed (a more detailed descrip-
tion of actions follows), and then the remaining input is
scanned for another match.
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If no match is found, then the default rule is executed:
the next character in the input is considered matched and
copied to the standard output. Thus, the simplest legal
flex input is:
%%
which generates a scanner that simply copies its input
(one character at a time) to its output.
Note that yytext can be defined in two different ways:
either as a character pointer or as a character array.
You can control which definition flex uses by including
one of the special directives %pointer or %array in the
first (definitions) section of your flex input. The
default is %pointer, unless you use the -l lex compatibil-
ity option, in which case yytext will be an array. The
advantage of using %pointer is substantially faster scan-
ning and no buffer overflow when matching very large
tokens (unless you run out of dynamic memory). The disad-
vantage is that you are restricted in how your actions can
modify yytext (see the next section), and calls to the
unput() function destroys the present contents of yytext,
which can be a considerable porting headache when moving
between different lex versions.
The advantage of %array is that you can then modify yytext
to your heart's content, and calls to unput() do not
destroy yytext (see below). Furthermore, existing lex
programs sometimes access yytext externally using 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 defaults to a fairly large value. You can change
the size by simply #define'ing YYLMAX to a different value
in the first section of your flex input. As mentioned
above, with %pointer yytext grows dynamically to accommo-
date large tokens. While this means your %pointer scanner
can accommodate very large tokens (such as matching entire
blocks of comments), bear in mind that each time the scan-
ner must resize yytext it also must rescan the entire
token from the beginning, so matching such tokens can
prove slow. yytext presently does not dynamically grow if
a call to unput() results in too much text being pushed
back; instead, a run-time error results.
Also note that you cannot use %array with C++ scanner
classes (the c++ option; see below).
ACTIONS
Each pattern in a rule has a corresponding action, which
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can be any arbitrary C statement. The pattern ends at the
first non-escaped whitespace character; the remainder of
the line is its action. If the action is empty, then when
the pattern is matched the input token is simply dis-
carded. For example, here is the specification for a pro-
gram 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 mul-
tiple lines. flex knows about C strings and comments and
won't be fooled by braces found within them, but also
allows actions to begin with %{ and will consider the
action to be all the text up to the next %} (regardless of
ordinary braces inside the action).
An action consisting solely of a vertical bar ('|') means
"same as the action for the next rule." See below for an
illustration.
Actions can include arbitrary C code, including return
statements to return a value to whatever routine called
yylex(). Each time yylex() is called it continues pro-
cessing tokens from where it last left off until it either
reaches the end of the file or executes a return.
Actions are free to modify yytext except for lengthening
it (adding characters to its end--these will overwrite
later characters in the input stream). This however does
not apply when using %array (see above); in that case,
yytext may be freely modified in any way.
Actions are free to modify yyleng except they should not
do so if the action also includes use of yymore() (see
below).
There are a number of special directives which can be
included within an action:
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- ECHO copies yytext to the scanner's output.
- BEGIN followed by the name of a start condition
places the scanner in the corresponding start con-
dition (see below).
- REJECT directs the scanner to proceed on to the
"second best" rule which matched the input (or a
prefix of the input). The rule is chosen as
described above in "How the Input is Matched", and
yytext and yyleng set up appropriately. It may
either be one which matched as much text as the
originally chosen rule but came later in the flex
input file, or one which matched less text. For
example, the following will both count the words in
the input and call the routine special() whenever
"frob" is seen:
int word_count = 0;
%%
frob special(); REJECT;
[^ \t\n]+ ++word_count;
Without the REJECT, any "frob"'s in the input would
not be counted as words, since the scanner normally
executes only one action per token. Multiple
REJECT's are allowed, each one finding the next
best choice to the currently active rule. For
example, when the following scanner scans the token
"abcd", it will write "abcdabcaba" to the output:
%%
a |
ab |
abc |
abcd ECHO; REJECT;
.|\n /* eat up any unmatched character */
(The first three rules share the fourth's action
since they use the special '|' action.) REJECT is
a particularly expensive feature in terms of scan-
ner performance; if it is used in any of the scan-
ner's actions it will slow down all of the scan-
ner'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
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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 you must not modify yyleng
if you are using yymore(). Second, the presence of
yymore() in the scanner's action entails a minor perfor-
mance 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 follow-
ing will write out "foobarbar":
%%
foobar ECHO; yyless(3);
[a-z]+ ECHO;
An argument of 0 to yyless will cause the entire
current input string to be scanned again. Unless
you've changed how the scanner will subsequently
process its input (using BEGIN, for example), this
will result in an endless loop.
Note that yyless is a macro and can only be used in the
flex input file, not from other source files.
- unput(c) puts the character c back onto the input
stream. It will be the next character scanned.
The following action will take the current token
and cause it to be rescanned enclosed in parenthe-
ses.
{
int i;
/* Copy yytext because unput() trashes yytext */
char *yycopy = strdup( yytext );
unput( ')' );
for ( i = yyleng - 1; i >= 0; --i )
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unput( yycopy[i] );
unput( '(' );
free( yycopy );
}
Note that since each unput() puts the given charac-
ter back at the beginning of the input stream,
pushing back strings must be done back-to-front.
An important potential problem when using unput() is that
if you are using %pointer (the default), a call to unput()
destroys the contents of yytext, starting with its right-
most character and devouring one character to the left
with each call. If you need the value of yytext preserved
after a call to unput() (as in the above example), you
must either first copy it elsewhere, or build your scanner
using %array instead (see How The Input Is Matched).
Finally, note that you cannot put back EOF to attempt to
mark the input stream with an end-of-file.
- input() reads the next character from the input
stream. For example, the following is one way to
eat up C comments:
%%
"/*" {
register int c;
for ( ; ; )
{
while ( (c = input()) != '*' &&
c != EOF )
; /* eat up text of comment */
if ( c == '*' )
{
while ( (c = input()) == '*' )
;
if ( c == '/' )
break; /* found the end */
}
if ( c == EOF )
{
error( "EOF in comment" );
break;
}
}
}
(Note that if the scanner is compiled using C++,
then input() is instead referred to as yyinput(),
in order to avoid a name clash with the C++ stream
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by the name of input.)
- YY_FLUSH_BUFFER flushes the scanner's internal
buffer so that the next time the scanner attempts
to match a token, it will first refill the buffer
using YY_INPUT (see The Generated Scanner, below).
This action is a special case of the more general
yy_flush_buffer() function, described below in the
section Multiple Input Buffers.
- yyterminate() can be used in lieu of a return
statement in an action. It terminates the scanner
and returns a 0 to the scanner's caller, indicating
"all done". By default, yyterminate() is also
called when an end-of-file is encountered. It is a
macro and may be redefined.
THE GENERATED SCANNER
The output of flex is the file lex.yy.c, which contains
the scanning routine yylex(), a number of tables used by
it for matching tokens, and a number of auxiliary routines
and macros. By default, yylex() is declared as follows:
int yylex()
{
... various definitions and the actions in here ...
}
(If your environment supports function prototypes, then it
will be "int yylex( void )".) This definition may be
changed by defining the "YY_DECL" macro. For example, you
could use:
#define YY_DECL float lexscan( a, b ) float a, b;
to give the scanning routine the name lexscan, returning a
float, and taking two floats as arguments. Note that if
you give arguments to the scanning routine using a K&R-
style/non-prototyped function declaration, you must termi-
nate the definition with a semi-colon (;).
Whenever yylex() is called, it scans tokens from the
global input file yyin (which defaults to stdin). It con-
tinues until it either reaches an end-of-file (at which
point it returns the value 0) or one of its actions exe-
cutes a return statement.
If the scanner reaches an end-of-file, subsequent calls
are undefined unless either yyin is pointed at a new input
file (in which case scanning continues from that file), or
yyrestart() is called. yyrestart() takes one argument, a
FILE * pointer (which can be nil, if you've set up
YY_INPUT to scan from a source other than yyin), and ini-
tializes yyin for scanning from that file. Essentially
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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 state-
ment in one of the actions, the scanner may then be called
again and it will resume scanning where it left off.
By default (and for purposes of efficiency), the scanner
uses block-reads rather than simple getc() calls to read
characters from yyin. The nature of how it gets its input
can be controlled by defining the YY_INPUT macro.
YY_INPUT's calling sequence is
"YY_INPUT(buf,result,max_size)". Its action is to place
up to max_size characters in the character array buf and
return in the integer variable result either the number of
characters read or the constant YY_NULL (0 on Unix sys-
tems) to indicate EOF. The default YY_INPUT reads from
the global file-pointer "yyin".
A sample definition of YY_INPUT (in the definitions sec-
tion 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 condi-
tion 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.
Version 2.5 April 1995 16
FLEX(1) FLEX(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 sim-
ply by assigning it to some other FILE pointer.
START CONDITIONS
flex provides a mechanism for conditionally activating
rules. Any rule whose pattern is prefixed with "<sc>"
will only be active when the scanner is in the start con-
dition 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 exe-
cuted, rules with the given start condition will be active
and rules with other start conditions will be inactive.
If the start condition is inclusive, then rules with no
start conditions at all will also be active. If it is
exclusive, then only rules qualified with the start condi-
tion 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 exam-
ple illustrating the connection between the two. The set
of rules:
%s example
Version 2.5 April 1995 17
FLEX(1) FLEX(1)
%%
<example>foo do_something();
bar something_else();
is equivalent to
%x example
%%
<example>foo do_something();
<INITIAL,example>bar something_else();
Without the <INITIAL,example> qualifier, the bar pattern
in the second example wouldn't be active (i.e., couldn't
match) when in start condition example. If we just used
<example> to qualify bar, though, then it would only be
active in example and not in INITIAL, while in the first
example it's active in both, because in the first example
the example startion condition is an inclusive (%s) start
condition.
Also note that the special start-condition specifier <*>
matches every start condition. Thus, the above example
could also have been written;
%x example
%%
<example>foo do_something();
<*>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 parenthe-
ses 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 follow-
ing will cause the scanner to enter the "SPECIAL" start
condition whenever yylex() is called and the global vari-
able enter_special is true:
Version 2.5 April 1995 18
FLEX(1) FLEX(1)
int enter_special;
%x SPECIAL
%%
if ( enter_special )
BEGIN(SPECIAL);
<SPECIAL>blahblahblah
...more rules follow...
To illustrate the uses of start conditions, here is a
scanner which provides two different interpretations of a
string like "123.456". By default it will treat it as
three tokens, the integer "123", a dot ('.'), and the
integer "456". But if the string is preceded earlier in
the line by the string "expect-floats" it will treat it as
a single token, the floating-point number 123.456:
%{
#include <math.h>
%}
%s expect
%%
expect-floats BEGIN(expect);
<expect>[0-9]+"."[0-9]+ {
printf( "found a float, = %f\n",
atof( yytext ) );
}
<expect>\n {
/* that's the end of the line, so
* we need another "expect-number"
* before we'll recognize any more
* numbers
*/
BEGIN(INITIAL);
}
[0-9]+ {
printf( "found an integer, = %d\n",
atoi( yytext ) );
}
"." printf( "found a dot\n" );
Here is a scanner which recognizes (and discards) C com-
ments while maintaining a count of the current input line.
%x comment
%%
int line_num = 1;
Version 2.5 April 1995 19
FLEX(1) FLEX(1)
"/*" BEGIN(comment);
<comment>[^*\n]* /* eat anything that's not a '*' */
<comment>"*"+[^*/\n]* /* eat up '*'s not followed by '/'s */
<comment>\n ++line_num;
<comment>"*"+"/" BEGIN(INITIAL);
This scanner goes to a bit of trouble to match as much
text as possible with each rule. In general, when
attempting to write a high-speed scanner try to match as
much possible in each rule, as it's a big win.
Note that start-conditions names are really integer values
and can be stored as such. Thus, the above could be
extended in the following fashion:
%x comment foo
%%
int line_num = 1;
int comment_caller;
"/*" {
comment_caller = INITIAL;
BEGIN(comment);
}
...
<foo>"/*" {
comment_caller = foo;
BEGIN(comment);
}
<comment>[^*\n]* /* eat anything that's not a '*' */
<comment>"*"+[^*/\n]* /* eat up '*'s not followed by '/'s */
<comment>\n ++line_num;
<comment>"*"+"/" BEGIN(comment_caller);
Furthermore, you can access the current start condition
using the integer-valued YY_START macro. For example, the
above assignments to comment_caller could instead be writ-
ten
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
Version 2.5 April 1995 20
FLEX(1) FLEX(1)
expanded escape sequences (but not including checking for
a string that's too long):
%x str
%%
char string_buf[MAX_STR_CONST];
char *string_buf_ptr;
\" string_buf_ptr = string_buf; BEGIN(str);
<str>\" { /* saw closing quote - all done */
BEGIN(INITIAL);
*string_buf_ptr = '\0';
/* return string constant token type and
* value to parser
*/
}
<str>\n {
/* error - unterminated string constant */
/* generate error message */
}
<str>\\[0-7]{1,3} {
/* octal escape sequence */
int result;
(void) sscanf( yytext + 1, "%o", &result );
if ( result > 0xff )
/* error, constant is out-of-bounds */
*string_buf_ptr++ = result;
}
<str>\\[0-9]+ {
/* generate error - bad escape sequence; something
* like '\48' or '\0777777'
*/
}
<str>\\n *string_buf_ptr++ = '\n';
<str>\\t *string_buf_ptr++ = '\t';
<str>\\r *string_buf_ptr++ = '\r';
<str>\\b *string_buf_ptr++ = '\b';
<str>\\f *string_buf_ptr++ = '\f';
<str>\\(.|\n) *string_buf_ptr++ = yytext[1];
<str>[^\\\n\"]+ {
char *yptr = yytext;
Version 2.5 April 1995 21
FLEX(1) FLEX(1)
while ( *yptr )
*string_buf_ptr++ = *yptr++;
}
Often, such as in some of the examples above, you wind up
writing a whole bunch of rules all preceded by the same
start condition(s). Flex makes this a little easier and
cleaner by introducing a notion of start condition scope.
A start condition scope is begun with:
<SCs>{
where SCs is a list of one or more start conditions.
Inside the start condition scope, every rule automatically
has the prefix <SCs> applied to it, until a '}' which
matches the initial '{'. So, for example,
<ESC>{
"\\n" return '\n';
"\\r" return '\r';
"\\f" return '\f';
"\\0" return '\0';
}
is equivalent to:
<ESC>"\\n" return '\n';
<ESC>"\\r" return '\r';
<ESC>"\\f" return '\f';
<ESC>"\\0" return '\0';
Start condition scopes may be nested.
Three routines are available for manipulating stacks of
start conditions:
void yy_push_state(int new_state)
pushes the current start condition onto the top of
the start condition stack and switches to new_state
as though you had used BEGIN new_state (recall that
start condition names are also integers).
void yy_pop_state()
pops the top of the stack and switches to it via
BEGIN.
int yy_top_state()
returns the top of the stack without altering the
stack's contents.
The start condition stack grows dynamically and so has no
built-in size limitation. If memory is exhausted, program
execution aborts.
Version 2.5 April 1995 22
FLEX(1) FLEX(1)
To use start condition stacks, your scanner must include a
%option stack directive (see Options below).
MULTIPLE INPUT BUFFERS
Some scanners (such as those which support "include"
files) require reading from several input streams. As
flex scanners do a large amount of buffering, one cannot
control where the next input will be read from by simply
writing a YY_INPUT which is sensitive to the scanning con-
text. YY_INPUT is only called when the scanner reaches
the end of its buffer, which may be a long time after
scanning a statement such as an "include" which requires
switching the input source.
To negotiate these sorts of problems, flex provides a
mechanism for creating and switching between multiple
input buffers. An input buffer is created by using:
YY_BUFFER_STATE yy_create_buffer( FILE *file, int size )
which takes a FILE pointer and a size and creates a buffer
associated with the given file and large enough to hold
size characters (when in doubt, use YY_BUF_SIZE for the
size). It returns a YY_BUFFER_STATE handle, which may
then be passed to other routines (see below). The
YY_BUFFER_STATE type is a pointer to an opaque struct
yy_buffer_state structure, so you may safely initialize
YY_BUFFER_STATE variables to ((YY_BUFFER_STATE) 0) if you
wish, and also refer to the opaque structure in order to
correctly declare input buffers in source files other than
that of your scanner. Note that the FILE pointer in the
call to yy_create_buffer is only used as the value of yyin
seen by YY_INPUT; if you redefine YY_INPUT so it no longer
uses yyin, then you can safely pass a nil FILE pointer to
yy_create_buffer. You select a particular buffer to scan
from using:
void yy_switch_to_buffer( YY_BUFFER_STATE new_buffer )
switches the scanner's input buffer so subsequent tokens
will come from new_buffer. Note that
yy_switch_to_buffer() may be used by yywrap() to set
things up for continued scanning, instead of opening a new
file and pointing yyin at it. Note also that switching
input sources via either yy_switch_to_buffer() or yywrap()
does not change the start condition.
void yy_delete_buffer( YY_BUFFER_STATE buffer )
is used to reclaim the storage associated with a buffer.
( buffer can be nil, in which case the routine does noth-
ing.) You can also clear the current contents of a buffer
using:
Version 2.5 April 1995 23
FLEX(1) FLEX(1)
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(), pro-
vided for compatibility with the C++ use of new and delete
for creating and destroying dynamic objects.
Finally, the YY_CURRENT_BUFFER macro returns a
YY_BUFFER_STATE handle to the current buffer.
Here is an example of using these features for writing a
scanner which expands include files (the <<EOF>> feature
is discussed below):
/* the "incl" state is used for picking up the name
* of an include file
*/
%x incl
%{
#define MAX_INCLUDE_DEPTH 10
YY_BUFFER_STATE include_stack[MAX_INCLUDE_DEPTH];
int include_stack_ptr = 0;
%}
%%
include BEGIN(incl);
[a-z]+ ECHO;
[^a-z\n]*\n? ECHO;
<incl>[ \t]* /* eat the whitespace */
<incl>[^ \t\n]+ { /* got the include file name */
if ( include_stack_ptr >= MAX_INCLUDE_DEPTH )
{
fprintf( stderr, "Includes nested too deeply" );
exit( 1 );
}
include_stack[include_stack_ptr++] =
YY_CURRENT_BUFFER;
yyin = fopen( yytext, "r" );
if ( ! yyin )
error( ... );
yy_switch_to_buffer(
yy_create_buffer( yyin, YY_BUF_SIZE ) );
BEGIN(INITIAL);
Version 2.5 April 1995 24
FLEX(1) FLEX(1)
}
<<EOF>> {
if ( --include_stack_ptr < 0 )
{
yyterminate();
}
else
{
yy_delete_buffer( YY_CURRENT_BUFFER );
yy_switch_to_buffer(
include_stack[include_stack_ptr] );
}
}
Three routines are available for setting up input buffers
for scanning in-memory strings instead of files. All of
them create a new input buffer for scanning the string,
and return a corresponding YY_BUFFER_STATE handle (which
you should delete with yy_delete_buffer() when done with
it). They also switch to the new buffer using
yy_switch_to_buffer(), so the next call to yylex() will
start scanning the string.
yy_scan_string(const char *str)
scans a NUL-terminated string.
yy_scan_bytes(const char *bytes, int len)
scans len bytes (including possibly NUL's) starting
at location bytes.
Note that both of these functions create and scan a copy
of the string or bytes. (This may be desirable, since
yylex() modifies the contents of the buffer it is scan-
ning.) You can avoid the copy by using:
yy_scan_buffer(char *base, yy_size_t size)
which scans in place the buffer starting at base,
consisting of size bytes, the last two bytes of
which must be YY_END_OF_BUFFER_CHAR (ASCII NUL).
These last two bytes are not scanned; thus, scan-
ning consists of base[0] through base[size-2],
inclusive.
If you fail to set up base in this manner (i.e.,
forget the final two YY_END_OF_BUFFER_CHAR bytes),
then yy_scan_buffer() returns a nil pointer instead
of creating a new input buffer.
The type yy_size_t is an integral type to which you
can cast an integer expression reflecting the size
of the buffer.
Version 2.5 April 1995 25
FLEX(1) FLEX(1)
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 pro-
cess). The action must finish by doing one of four
things:
- assigning yyin to a new input file (in previous
versions of flex, after doing the assignment you
had to call the special action YY_NEW_FILE; this is
no longer necessary);
- executing a return statement;
- executing the special yyterminate() action;
- or, switching to a new buffer using
yy_switch_to_buffer() as shown in the example
above.
<<EOF>> rules may not be used with other patterns; they
may only be qualified with a list of start conditions. If
an unqualified <<EOF>> rule is given, it applies to all
start conditions which do not already have <<EOF>>
actions. To specify an <<EOF>> rule for only the initial
start condition, use
<INITIAL><<EOF>>
These rules are useful for catching things like unclosed
comments. An example:
%x quote
%%
...other rules for dealing with quotes...
<quote><<EOF>> {
error( "unterminated quote" );
yyterminate();
}
<<EOF>> {
if ( *++filelist )
yyin = fopen( *filelist, "r" );
else
yyterminate();
}
MISCELLANEOUS MACROS
The macro YY_USER_ACTION can be defined to provide an
action which is always executed prior to the matched
rule's action. For example, it could be #define'd to call
Version 2.5 April 1995 26
FLEX(1) FLEX(1)
a routine to convert yytext to lower-case. When
YY_USER_ACTION is invoked, the variable yy_act gives the
number of the matched rule (rules are numbered starting
with 1). Suppose you want to profile how often each of
your rules is matched. The following would do the trick:
#define YY_USER_ACTION ++ctr[yy_act]
where ctr is an array to hold the counts for the different
rules. Note that the macro YY_NUM_RULES gives the total
number of rules (including the default rule, even if you
use -s), so a correct declaration for ctr is:
int ctr[YY_NUM_RULES];
The macro YY_USER_INIT may be defined to provide an action
which is always executed before the first scan (and before
the scanner's internal initializations are done). For
example, it could be used to call a routine to read in a
data table or open a logging file.
The macro yy_set_interactive(is_interactive) can be used
to control whether the current buffer is considered inter-
active. An interactive buffer is processed more slowly,
but must be used when the scanner's input source is indeed
interactive to avoid problems due to waiting to fill
buffers (see the discussion of the -I flag below). A 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
The macro YY_AT_BOL() returns true if the next token
scanned from the current buffer will have '^' rules
active, false otherwise.
In the generated scanner, the actions are all gathered in
one large switch statement and separated using YY_BREAK,
which may be redefined. By default, it is simply a
"break", to separate each rule's action from the following
rule's. Redefining YY_BREAK allows, for example, C++
users to #define YY_BREAK to do nothing (while being very
careful that every rule ends with a "break" or a
"return"!) to avoid suffering from unreachable statement
warnings where because a rule's action ends with "return",
Version 2.5 April 1995 27
FLEX(1) FLEX(1)
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 (you cannot
append characters to the end).
If the special directive %array appears in the
first section of the scanner description, then
yytext is instead declared char yytext[YYLMAX],
where YYLMAX is a macro definition that you can
redefine in the first section if you don't like the
default value (generally 8KB). Using %array
results in somewhat slower scanners, but the value
of yytext becomes immune to calls to input() and
unput(), which potentially destroy its value when
yytext is a character pointer. The opposite of
%array is %pointer, which is the default.
You cannot use %array when generating C++ scanner
classes (the -+ flag).
- int yyleng holds the length of the current token.
- FILE *yyin is the file which by default flex reads
from. It may be redefined but doing so only makes
sense before scanning begins or after an EOF has
been encountered. Changing it in the midst of
scanning will have unexpected results since flex
buffers its input; use yyrestart() instead. Once
scanning terminates because an end-of-file has been
seen, you can assign yyin at the new input file and
then call the scanner again to continue scanning.
- void yyrestart( FILE *new_file ) may be called to
point yyin at the new input file. The switch-over
to the new file is immediate (any previously
buffered-up input is lost). Note that calling
yyrestart() with yyin as an argument thus throws
away the current input buffer and continues scan-
ning the same input file.
- FILE *yyout is the file to which ECHO actions are
done. It can be reassigned by the user.
- YY_CURRENT_BUFFER returns a YY_BUFFER_STATE handle
to the current buffer.
- YY_START returns an integer value corresponding to
the current start condition. You can subsequently
use this value with BEGIN to return to that start
Version 2.5 April 1995 28
FLEX(1) FLEX(1)
condition.
INTERFACING WITH YACC
One of the main uses of flex is as a companion to the yacc
parser-generator. yacc parsers expect to call a routine
named yylex() to find the next input token. The routine
is supposed to return the type of the next token as well
as putting any associated value in the global yylval. To
use flex with yacc, one specifies the -d option to yacc to
instruct it to generate the file y.tab.h containing defi-
nitions of all the %tokens appearing in the yacc input.
This file is then included in the flex scanner. For exam-
ple, 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;
OPTIONS
flex has the following options:
-b Generate backing-up information to lex.backup.
This is a list of scanner states which require
backing up and the input characters on which they
do so. By adding rules one can remove backing-up
states. If all backing-up states are eliminated
and -Cf or -CF is used, the generated scanner will
run faster (see the -p flag). Only users who wish
to squeeze every last cycle out of their scanners
need worry about this option. (See the section on
Performance Considerations below.)
-c is a do-nothing, deprecated option included for
POSIX compliance.
-d makes the generated scanner run in debug mode.
Whenever a pattern is recognized and the global
yy_flex_debug is non-zero (which is the default),
the scanner will write to stderr a line of the
form:
--accepting rule at line 53 ("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). Messages are also generated
when the scanner backs up, accepts the default
rule, reaches the end of its input buffer (or
Version 2.5 April 1995 29
FLEX(1) FLEX(1)
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 fast scanner. No table compression is
done and stdio is bypassed. The result is large
but fast. This option is equivalent to -Cfr (see
below).
-h generates a "help" summary of flex's options to
stdout and then exits. -? and --help are synonyms
for -h.
-i instructs flex to generate a case-insensitive scan-
ner. 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 turns on maximum compatibility with the original
AT&T lex implementation. 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 details 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 is another do-nothing, deprecated option included
only for POSIX compliance.
-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 you
give the flag twice, you will also get comments
regarding features that lead to minor performance
losses.
Note that the use of REJECT, %option yylineno, and
variable trailing context (see the Deficiencies /
Bugs section below) entails a substantial perfor-
mance penalty; use of yymore(), the ^ operator, and
the -I flag entail minor performance penalties.
-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.
Version 2.5 April 1995 30
FLEX(1) FLEX(1)
-t instructs flex to write the scanner it generates to
standard output instead of lex.yy.c.
-v specifies that flex should write to stderr a sum-
mary of statistics regarding the scanner it gener-
ates. Most of the statistics are meaningless to
the casual flex user, but the first line identifies
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.
-B instructs flex to generate a batch scanner, the
opposite of interactive scanners generated by -I
(see below). In general, you use -B when you are
certain that your scanner will never be used inter-
actively, and you want to squeeze a little more
performance out of it. If your goal is instead to
squeeze out a lot more performance, you should be
using the -Cf or -CF options (discussed below),
which turn on -B automatically anyway.
-F specifies that the fast scanner table representa-
tion should be used (and stdio bypassed). This
representation is about as fast as the full table
representation (-f), and for some sets of patterns
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 you're better off using the full table repre-
sentation. If only the "identifier" rule is pre-
sent and you then use a hash table or some such to
detect the keywords, you're better off using -F.
This option is equivalent to -CFr (see below). It
cannot be used with -+.
-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 abso-
lutely must. It turns out that always looking one
extra character ahead, even if the scanner has
already seen enough text to disambiguate the cur-
rent token, is a bit faster than only looking ahead
when necessary. But scanners that always look
Version 2.5 April 1995 31
FLEX(1) FLEX(1)
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 you use
the -Cf or -CF table-compression options (see
below). That's because if you're looking for high-
performance you should be using one of these
options, so if you didn't, flex assumes you'd
rather trade off a bit of run-time performance for
intuitive interactive behavior. Note also that you
cannot use -I 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.
You can force a scanner to not be interactive by
using -B (see above).
-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 -- you
should report these sorts of errors to the email
address given below).
-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.
-V prints the version number to stdout and exits.
--version is a synonym for -V.
-7 instructs flex to generate a 7-bit scanner, i.e.,
one which can only recognized 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 disadvantage is that such scanners often hang
or crash if their input contains an 8-bit charac-
ter.
Note, however, that unless you generate your scan-
ner using the -Cf or -CF table compression options,
use of -7 will save only a small amount of table
space, and make your scanner considerably less
portable. Flex's default behavior is to generate
an 8-bit scanner unless you use the -Cf or -CF, in
Version 2.5 April 1995 32
FLEX(1) FLEX(1)
which case flex defaults to generating 7-bit scan-
ners unless your site was always configured to gen-
erate 8-bit scanners (as will often be the case
with non-USA sites). You can tell whether flex
generated a 7-bit or an 8-bit scanner by inspecting
the flag summary in the -v output as described
above.
Note that if you use -Cfe or -CFe (those table com-
pression options, but also using equivalence
classes as discussed see below), flex still
defaults to generating an 8-bit scanner, since usu-
ally 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 generat-
ing 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.
-+ specifies that you want flex to generate a C++
scanner class. See the section on Generating C++
Scanners below for details.
-C[aefFmr]
controls the degree of table compression and, more
generally, trade-offs between small scanners and
fast scanners.
-Ca ("align") instructs flex to trade off larger
tables in the generated scanner for faster perfor-
mance because the elements of the tables are better
aligned for memory access and computation. On some
RISC architectures, fetching and manipulating long-
words is more efficient than with smaller-sized
units such as shortwords. This option can double
the size of the tables used by your scanner.
-Ce directs flex to construct equivalence classes,
i.e., sets of characters which have identical lexi-
cal properties (for example, if the only appearance
of digits in the flex input is in the character
class "[0-9]" then the digits '0', '1', ..., '9'
will all be put in the same equivalence class).
Equivalence classes usually give dramatic reduc-
tions in the final table/object file sizes (typi-
cally a factor of 2-5) and are pretty cheap perfor-
mance-wise (one array look-up per character
Version 2.5 April 1995 33
FLEX(1) FLEX(1)
scanned).
-Cf specifies that the full scanner tables should
be generated - flex should not compress the tables
by taking advantages of similar transition func-
tions for different states.
-CF specifies that the alternate fast scanner rep-
resentation (described above under the -F flag)
should be used. This option cannot be used with
-+.
-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-equiv-
alence classes are often a big win when using com-
pressed tables, but they have a moderate perfor-
mance impact (one or two "if" tests and one array
look-up per character scanned).
-Cr causes the generated scanner to bypass use of
the standard I/O library (stdio) for input.
Instead of calling fread() or getc(), the scanner
will use the read() system call, resulting in a
performance gain which varies from system to sys-
tem, but in general is probably negligible unless
you are also using -Cf or -CF. Using -Cr can cause
strange behavior if, for example, you read from
yyin using stdio prior to calling the scanner
(because the scanner will miss whatever text your
previous reads left in the stdio input buffer).
-Cr has no effect if you define YY_INPUT (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-equiva-
lence 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 generate equivalence classes and meta-
equivalence classes. This setting provides the
highest degree of table compression. You can trade
off faster-executing scanners at the cost of larger
tables with the following generally being true:
slowest & smallest
Version 2.5 April 1995 34
FLEX(1) FLEX(1)
-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 you will usually want to use the
default, maximal compression.
-Cfe is often a good compromise between speed and
size for production scanners.
-ooutput
directs flex to write the scanner to the file out-
put instead of lex.yy.c. If you combine -o with
the -t option, then the scanner is written to std-
out but its #line directives (see the -L option
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 example, -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 you are using a C++ scanner, then only yywrap
and yyFlexLexer are affected.) Within your scanner
itself, you can still refer to the global variables
and functions using either version of their name;
but externally, they have the modified name.
Version 2.5 April 1995 35
FLEX(1) FLEX(1)
This option lets you easily link together multiple
flex programs into the same executable. Note,
though, that using this option also renames
yywrap(), so you now must either provide your own
(appropriately named) version of the routine for
your scanner, or use %option noyywrap, as linking
with -lfl no longer provides one for you by
default.
-Sskeleton_file
overrides the default skeleton file from which flex
constructs its scanners. You'll never need this
option unless you are doing flex maintenance or
development.
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 specifica-
tion. You can specify multiple options with a single
%option directive, and multiple directives in the first
section of your flex input file.
Most options are given simply as names, optionally pre-
ceded 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
Version 2.5 April 1995 36
FLEX(1) FLEX(1)
(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, then 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 compliant 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 scan-
ner not call yywrap() upon an end-of-file, but sim-
ply assume that there are no more files to scan
(until the user points yyin at a new file and calls
yylex() again).
flex scans your rule actions to determine whether you use
the REJECT or yymore() features. The reject and yymore
options are available to override its decision as to
whether you use the options, either by setting them (e.g.,
%option reject) to indicate the feature is indeed used, or
Version 2.5 April 1995 37
FLEX(1) FLEX(1)
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 you have derived foo as a subclass of
yyFlexLexer, so flex will place your 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::Lexer-
Error()) if called. See Generating C++ Scanners, below,
for additional information.
A number of options are available for lint 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 you use %option stack).
PERFORMANCE CONSIDERATIONS
The main design goal of flex is that it generate high-per-
formance scanners. It has been optimized for dealing well
with large sets of rules. Aside from the effects on scan-
ner 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
Version 2.5 April 1995 38
FLEX(1) FLEX(1)
'^' 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 you scanned, 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 head-
scratching one can see that this must be the state it's in
when it has seen "fo". When this has happened, if
Version 2.5 April 1995 39
FLEX(1) FLEX(1)
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 con-
cerns 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 trouble of removing backing up from the rules
unless we're using -Cf or -CF, since there's no perfor-
mance 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 mes-
sages. 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
accidentally match a valid token. A possible future flex
feature will be to automatically add rules to eliminate
backing up).
It's important to keep in mind that you gain the benefits
of eliminating backing up only if you eliminate every
instance of backing up. Leaving just one means you gain
nothing.
Version 2.5 April 1995 40
FLEX(1) FLEX(1)
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 Deficiencies
/ Bugs below).
Another area where the user can increase a scanner's per-
formance (and one that's easier to implement) arises from
the fact that the longer the tokens 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 action. 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]*
Version 2.5 April 1995 41
FLEX(1) FLEX(1)
<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: suppose you want
to scan through a file containing identifiers and key-
words, one per line and with no other extraneous charac-
ters, 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 recognition of newlines with that
of the other tokens:
%%
asm\n |
auto\n |
break\n |
... etc ...
Version 2.5 April 1995 42
FLEX(1) FLEX(1)
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 a "auto\n" rule.
To eliminate the possibility of backing up, we could
either duplicate 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, particu-
larly 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 section How the Input is Matched, dynamically
resizing yytext to accommodate huge tokens is a slow pro-
cess because it presently requires that the (huge) token
be rescanned from the beginning. Thus if performance is
vital, you should 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++ SCANNERS
flex provides two different ways to generate scanners for
use with C++. The first way is to simply compile a
Version 2.5 April 1995 43
FLEX(1) FLEX(1)
scanner generated by flex using a C++ compiler instead of
a C compiler. You should not encounter any compilations
errors (please report any you find to the email address
given in the Author section below). You can then use C++
code in your rule actions instead of C code. Note that
the default input source for your scanner remains yyin,
and default echoing is still done to yyout. Both of these
remain FILE * variables and not C++ streams.
You can also use flex to generate a C++ scanner class,
using the -+ option (or, equivalently, %option c++), which
is automatically specified if the name of the flex exe-
cutable 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 functions:
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 sec-
tion above). Note that you must build the scanner
using %option debug to include debugging informa-
tion 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 istream* object pointer and not a
FILE*), yy_flush_buffer(), yy_delete_buffer(), and
yyrestart() (again, the first argument is a istream*
object pointer).
The second class defined in g++/FlexLexer.h is
yyFlexLexer, which is derived from FlexLexer. It defines
the following additional member functions:
Version 2.5 April 1995 44
FLEX(1) FLEX(1)
yyFlexLexer( istream* arg_yyin = 0, 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 is yylex() does for ordinary
flex scanners: it scans the input stream, consuming
tokens, until a rule's action returns a value. If
you derive a subclass S from yyFlexLexer and want
to access the member functions and variables of S
inside yylex(), then you need to use %option
yyclass="S" to inform flex that you will be using
that subclass 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(istream* new_in = 0,
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 reassigned.
int yylex( istream* new_in, 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 functions which you can redefine 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 characters. Note that
"interactive" scanners (see the -B and -I flags)
define the macro YY_INTERACTIVE. If you redefine
LexerInput() and need to take different actions
depending on whether or not the scanner might be
scanning an interactive input source, you can 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 function writes the message to the stream
Version 2.5 April 1995 45
FLEX(1) FLEX(1)
cerr and exits.
Note that a yyFlexLexer object contains its entire scan-
ning state. Thus you can use such objects to create reen-
trant scanners. You can instantiate multiple instances of
the same yyFlexLexer class, and you can also combine mul-
tiple C++ scanner classes together in the same program
using the -P option discussed above.
Finally, note that the %array feature is not available to
C++ scanner classes; you must use %pointer (the default).
Here is an example of a simple C++ scanner:
// An example of using the flex C++ scanner class.
%{
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';
Version 2.5 April 1995 46
FLEX(1) FLEX(1)
\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;
}
If you want to create multiple (different) lexer classes,
you use the -P flag (or the prefix= option) to rename each
yyFlexLexer to some other xxFlexLexer. You then can
include <g++/FlexLexer.h> in your 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, you used %option prefix="xx" for one of
your scanners and %option prefix="zz" for the other.
IMPORTANT: the present form of the scanning class is
experimental and may change considerably between major
releases.
INCOMPATIBILITIES WITH LEX AND POSIX
flex 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 acceptable 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 overcome using the -l
Version 2.5 April 1995 47
FLEX(1) FLEX(1)
option.
flex is fully compatible with lex with the following
exceptions:
- The undocumented lex scanner internal variable
yylineno is not supported 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 input() 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 rede-
fined is in accordance with the POSIX specifica-
tion, which simply does not specify any way of con-
trolling the scanner's input other than by making
an initial assignment to yyin.
- The unput() routine is not redefinable. This
restriction is in accordance with POSIX.
- flex scanners are not as reentrant as lex scanners.
In particular, if you have an interactive scanner
and an interrupt handler which long-jumps out of
the scanner, and the scanner is subsequently called
again, you may get the following message:
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 inter-
active scanner.
Also note that flex C++ scanner classes are reen-
trant, so if using C++ is an option for you, you
should use them instead. See "Generating C++ Scan-
ners" above for details.
Version 2.5 April 1995 48
FLEX(1) FLEX(1)
- 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 paren-
theses around the definition.
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 speci-
fication.
- 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
Version 2.5 April 1995 49
FLEX(1) FLEX(1)
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 lat-
ter is in agreement with the POSIX specification.
- The special table-size declarations such as %a sup-
ported 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. Scan-
ners 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 respec-
tively).
The following flex features are not included in lex or the
POSIX specification:
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 you can put
Version 2.5 April 1995 50
FLEX(1) FLEX(1)
multiple actions 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.
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 default 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 or
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 you snuck some in (via a #include
file, for example). Use %option reject or %option yymore
to indicate to flex that you really do use these features.
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 occur due to internal
problems.
token too large, exceeds YYLMAX - your scanner uses %array
and one of its rules matched a string longer than the YYL-
MAX constant (8K bytes by default). You can increase the
value by #define'ing YYLMAX in the definitions section of
your flex input.
scanner requires -8 flag to use the character 'x' - Your
Version 2.5 April 1995 51
FLEX(1) FLEX(1)
scanner specification includes recognizing the 8-bit char-
acter 'x' and you did not specify the -8 flag, and your
scanner defaulted to 7-bit because you used the -Cf or -CF
table compression options. See the discussion of the -7
flag for details.
flex scanner push-back overflow - you used unput() 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 scan-
ner 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 activa-
tion 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! - you listed
more start conditions in a <> construct than exist (so you
must have listed at least one of them twice).
FILES
-lfl library with which scanners must be linked.
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.
flex.skl
skeleton scanner. This file is only used when
building flex, not when flex executes.
lex.backup
backing-up information for -b flag (called lex.bck
on some systems).
DEFICIENCIES / BUGS
Some trailing context patterns cannot be properly matched
and generate warning messages ("dangerous trailing
Version 2.5 April 1995 52
FLEX(1) FLEX(1)
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 beginning 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 always 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 directive 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 cur-
rent (generally huge) token.
Due to both buffering of input and read-ahead, you cannot
intermix calls to <stdio.h> routines, such as, for exam-
ple, 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 num-
ber 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.
SEE ALSO
lex(1), yacc(1), sed(1), awk(1).
John Levine, Tony Mason, and Doug Brown, Lex & Yacc,
O'Reilly and Associates. Be sure to get the 2nd edition.
Version 2.5 April 1995 53
FLEX(1) FLEX(1)
M. E. Lesk and E. Schmidt, LEX - Lexical Analyzer Genera-
tor
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).
AUTHOR
Vern Paxson, with the help of many ideas and much inspira-
tion 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 Adermann, 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 Deraadt, Mike Don-
ahue, Chuck Doucette, Tom Epperly, Leo Eskin, Chris Fay-
lor, Chris Flatters, Jon Forrest, Jeffrey Friedl, Joe
Gayda, Kaveh R. Ghazi, Wolfgang 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 Jalo-
vaara, 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 Lof-
fredo, Mike Long, Mohamed el Lozy, Brian Madsen, Malte,
Joe Marshall, Bengt Martensson, Chris Metcalf, Luke Mew-
burn, 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 Rom-
mel, Jim Roskind, Alberto Santini, Andreas Scherer, Dar-
rell Schiebel, Raf Schietekat, Doug Schmidt, Philippe Sch-
noebelen, Andreas Schwab, Larry Schwimmer, Alex Siegel,
Eckehard Stolz, Jan-Erik Strvmquist, Mike Stump, Paul Stu-
art, Dave Tallman, 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 appreciated all the same.
Version 2.5 April 1995 54
FLEX(1) FLEX(1)
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++ sup-
port; to Kent Williams and Tom Epperly for C++ class sup-
port; 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.
Version 2.5 April 1995 55
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