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flex, lex - fast lexical analyzer generator


flex [-bcdfhilnpstvwBFILTV78+? -C[aefFmr] -ooutput -Pprefix -Sskeleton] [--help --version]
[filename ...]


This manual describes flex, a tool for generating programs that perform pattern-matching
on text. The manual includes both tutorial and reference sections:

a brief overview of the tool

Some Simple Examples

Format Of The Input File

the extended regular expressions used by flex

How The Input Is Matched
the rules for determining what has been matched

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

flex command-line options, and the "%option"

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

those error messages produced by flex (or scanners
it generates) whose meanings might not be apparent

files used by flex

Deficiencies / Bugs
known problems with flex

See Also
other documentation, related tools

includes contact information


flex is a tool for generating scanners: programs which recognize lexical patterns in text.
flex reads the given input files, or its standard input if no file names are given, for a
description of a scanner to generate. The description is in the form of pairs of regular
expressions and C code, called rules. flex generates as output a C source file, lex.yy.c,
which defines a routine yylex(). This file is compiled and linked with the -ll library to
produce an executable. When the executable is run, it analyzes its input for occurrences
of the regular expressions. Whenever it finds one, it executes the corresponding C code.


First some simple examples to get the flavor of how one uses flex. The following flex
input specifies a scanner which whenever it encounters the string "username" will replace
it with the user's login name:

username printf( "%s", getlogin() );

By default, any text not matched by a flex scanner is copied to the output, so the net
effect of this scanner is to copy its input file to its output with each occurrence of
"username" expanded. In this input, there is just one rule. "username" is the pattern
and the "printf" is the action. The "%%" marks the beginning of the rules.

Here's another simple example:

int num_lines = 0, num_chars = 0;

\n ++num_lines; ++num_chars;
. ++num_chars;

printf( "# of lines = %d, # of chars = %d\n",
num_lines, num_chars );

This scanner counts the number of characters and the number of lines in its input (it
produces no output other than the final report on the counts). The first line declares
two globals, "num_lines" and "num_chars", which are accessible both inside yylex() and in
the main() routine declared after the second "%%". There are two rules, one which matches
a newline ("\n") and increments both the line count and the character count, and one which
matches any character other than a newline (indicated by the "." regular expression).

A somewhat more complicated example:

/* scanner for a toy Pascal-like language */

/* need this for the call to atof() below */
#include <math.h>

DIGIT [0-9]
ID [a-z][a-z0-9]*


{DIGIT}+ {
printf( "An integer: %s (%d)\n", yytext,
atoi( yytext ) );

{DIGIT}+"."{DIGIT}* {
printf( "A float: %s (%g)\n", yytext,
atof( yytext ) );

if|then|begin|end|procedure|function {
printf( "A keyword: %s\n", yytext );

{ID} printf( "An identifier: %s\n", yytext );

"+"|"-"|"*"|"/" printf( "An operator: %s\n", yytext );

"{"[^}\n]*"}" /* eat up one-line comments */

[ \t\n]+ /* eat up whitespace */

. printf( "Unrecognized character: %s\n", yytext );


main( argc, argv )
int argc;
char **argv;
++argv, --argc; /* skip over program name */
if ( argc > 0 )
yyin = fopen( argv[0], "r" );
yyin = stdin;


This is the beginnings of a simple scanner for a language like Pascal. It identifies
different types of tokens and reports on what it has seen.

The details of this example will be explained in the following sections.


The flex input file consists of three sections, separated by a line with just %% in it:

user code

The definitions section contains declarations of simple name definitions to simplify the
scanner specification, and declarations of start conditions, which are explained in a
later section.

Name definitions have the form:

name definition

The "name" is a word beginning with a letter or an underscore ('_') followed by zero or
more letters, digits, '_', or '-' (dash). The definition is taken to begin at the first
non-white-space character following the name and continuing to the end of the line. The
definition can subsequently be referred to using "{name}", which will expand to
"(definition)". For example,

DIGIT [0-9]
ID [a-z][a-z0-9]*

defines "DIGIT" to be a regular expression which matches a single digit, and "ID" to be a
regular expression which matches a letter followed by zero-or-more letters-or-digits. A
subsequent reference to


is identical to


and matches one-or-more digits followed by a '.' followed by zero-or-more digits.

The rules section of the flex input contains a series of rules of the form:

pattern action

where the pattern must be unindented and the action must begin on the same line.

See below for a further description of patterns and actions.

Finally, the user code section is simply copied to lex.yy.c verbatim. It is used for
companion routines which call or are called by the scanner. The presence of this section
is optional; if it is missing, the second %% in the input file may be skipped, too.

In the definitions and rules sections, any indented text or text enclosed in %{ and %} is
copied verbatim to the output (with the %{}'s removed). The %{}'s must appear unindented
on lines by themselves.

In the rules section, any indented or %{} text appearing before the first rule may be used
to declare variables which are local to the scanning routine and (after the declarations)
code which is to be executed whenever the scanning routine is entered. Other indented or
%{} text in the rule section is still copied to the output, but its meaning is not well-
defined and it may well cause compile-time errors (this feature is present for POSIX
compliance; see below for other such features).

In the definitions section (but not in the rules section), an unindented comment (i.e., a
line beginning with "/*") is also copied verbatim to the output up to the next "*/".


The patterns in the input are written using an extended set of regular expressions. These

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)
the literal string: [xyz]"foo
\X if X is an 'a', 'b', 'f', 'n', 'r', 't', or 'v',
then the ANSI-C interpretation of \x.
Otherwise, a literal 'X' (used to escape
operators such as '*')
\0 a NUL character (ASCII code 0)
\123 the character with octal value 123
\x2a the character with hexadecimal value 2a
(r) match an r; parentheses are used to override
precedence (see below)

rs the regular expression r followed by the
regular expression s; called "concatenation"

r|s either an r or an s

r/s an r but only if it is followed by an s. The
text matched by s is included when determining
whether this rule is the "longest match",
but is then returned to the input before
the action is executed. So the action only
sees the text matched by r. This type
of pattern is called trailing context".
(There are some combinations of r/s that flex
cannot match correctly; see notes in the
Deficiencies / Bugs section below regarding
"dangerous trailing context".)
^r an r, but only at the beginning of a line (i.e.,
when 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)
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
an end-of-file when in start condition s1 or s2

Note that inside of a character class, all regular expression operators lose their special
meaning except escape ('\') and the character class operators, '-', ']', and, at the
beginning of the class, '^'.

The regular expressions listed above are grouped according to precedence, from highest
precedence at the top to lowest at the bottom. Those grouped together have equal
precedence. For example,


is the same as


since the '*' operator has higher precedence than concatenation, and concatenation higher
than alternation ('|'). This pattern therefore matches either the string "foo" or the
string "ba" followed by zero-or-more r's. To match "foo" or zero-or-more "bar"'s, use:


and to match zero-or-more "foo"'s-or-"bar"'s:


In addition to characters and ranges of characters, character classes can also contain
character class expressions. These are expressions enclosed inside [: and :] delimiters
(which themselves must appear between the '[' and ']' of the character class; other
elements may occur inside the character class, too). The valid expressions are:

[:alnum:] [:alpha:] [:blank:]
[:cntrl:] [:digit:] [:graph:]
[:lower:] [:print:] [:punct:]
[:space:] [:upper:] [:xdigit:]

These expressions all designate a set of characters equivalent to the corresponding
standard C isXXX function. For example, [:alnum:] designates those characters for which
isalnum() returns true - i.e., any alphabetic or numeric. Some systems don't provide
isblank(), so flex defines [:blank:] as a blank or a tab.

For example, the following character classes are all equivalent:


If your scanner is case-insensitive (the -i flag), then [:upper:] and [:lower:] are
equivalent to [:alpha:].

Some notes on patterns:

- A negated character class such as the example "[^A-Z]" above will match a newline
unless "\n" (or an equivalent escape sequence) is one of the characters explicitly
present in the negated character class (e.g., "[^A-Z\n]"). This is unlike how many
other regular expression tools treat negated character classes, but unfortunately
the inconsistency is historically entrenched. Matching newlines means that a
pattern like [^"]* can match the entire input unless there's another quote in the

- A rule can have at most one instance of trailing context (the '/' operator or the
'$' operator). The start condition, '^', and "<<EOF>>" patterns can only occur at
the beginning of a pattern, and, as well as with '/' and '$', cannot be grouped
inside parentheses. A '^' which does not occur at the beginning of a rule or a '$'
which does not occur at the end of a rule loses its special properties and is
treated as a normal character.

The following are illegal:


Note that the first of these, can be written "foo/bar\n".

The following will result in '$' or '^' being treated as a normal character:


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.


When the generated scanner is run, it analyzes its input looking for strings which match
any of its patterns. If it finds more than one match, it takes the one matching the most
text (for trailing context rules, this includes the length of the trailing part, even
though it will then be returned to the input). If it finds two or more matches of the
same length, the rule listed first in the flex input file is chosen.

Once the match is determined, the text corresponding to the match (called the token) is
made available in the global character pointer yytext, and its length in the global
integer yyleng. The action corresponding to the matched pattern is then executed (a more
detailed description of actions follows), and then the remaining input is scanned for
another match.

If no match is found, then the default rule is executed: the next character in the input
is considered matched and copied to the standard output. Thus, the simplest legal flex
input is:


which generates a scanner that simply copies its input (one character at a time) to its

Note that yytext can be defined in two different ways: either as a character pointer or as
a character array. You can control which definition flex uses by including one of the
special directives %pointer or %array in the first (definitions) section of your flex
input. The default is %pointer, unless you use the -l lex compatibility option, in which
case yytext will be an array. The advantage of using %pointer is substantially faster
scanning and no buffer overflow when matching very large tokens (unless you run out of
dynamic memory). The disadvantage is that you are restricted in how your actions can
modify yytext (see the next section), and calls to the unput() function destroys the
present contents of yytext, which can be a considerable porting headache when moving
between different lex versions.

The advantage of %array is that you can then modify yytext to your heart's content, and
calls to unput() do not destroy yytext (see below). Furthermore, existing lex programs
sometimes access yytext externally using declarations of the form:
extern char yytext[];
This definition is erroneous when used with %pointer, but correct for %array.

%array defines yytext to be an array of YYLMAX characters, which defaults to a fairly
large value. You can change the size by simply #define'ing YYLMAX to a different value in
the first section of your flex input. As mentioned above, with %pointer yytext grows
dynamically to accommodate large tokens. While this means your %pointer scanner can
accommodate very large tokens (such as matching entire blocks of comments), bear in mind
that each time the scanner must resize yytext it also must rescan the entire token from
the beginning, so matching such tokens can prove slow. yytext presently does not
dynamically grow if a call to unput() results in too much text being pushed back; instead,
a run-time error results.

Also note that you cannot use %array with C++ scanner classes (the c++ option; see below).


Each pattern in a rule has a corresponding action, which can be any arbitrary C statement.
The pattern ends at the first non-escaped whitespace character; the remainder of the line
is its action. If the action is empty, then when the pattern is matched the input token
is simply discarded. For example, here is the specification for a program which deletes
all occurrences of "zap me" from its input:

"zap me"

(It will copy all other characters in the input to the output since they will be matched
by the default rule.)

Here is a program which compresses multiple blanks and tabs down to a single blank, and
throws away whitespace found at the end of a line:

[ \t]+ putchar( ' ' );
[ \t]+$ /* ignore this token */

If the action contains a '{', then the action spans till the balancing '}' is found, and
the action may cross multiple lines. flex knows about C strings and comments and won't be
fooled by braces found within them, but also allows actions to begin with %{ and will
consider the action to be all the text up to the next %} (regardless of ordinary braces
inside the action).

An action consisting solely of a vertical bar ('|') means "same as the action for the next
rule." See below for an illustration.

Actions can include arbitrary C code, including return statements to return a value to
whatever routine called yylex(). Each time yylex() is called it continues processing
tokens from where it last left off until it either reaches the end of the file or executes
a return.

Actions are free to modify yytext except for lengthening it (adding characters to its
end--these will overwrite later characters in the input stream). This however does not
apply when using %array (see above); in that case, yytext may be freely modified in any

Actions are free to modify yyleng except they should not do so if the action also includes
use of yymore() (see below).

There are a number of special directives which can be included within an action:

- ECHO copies yytext to the scanner's output.

- BEGIN followed by the name of a start condition places the scanner in the
corresponding start condition (see below).

- REJECT directs the scanner to proceed on to the "second best" rule which matched
the input (or a prefix of the input). The rule is chosen as described above in
"How the Input is Matched", and yytext and yyleng set up appropriately. It may
either be one which matched as much text as the originally chosen rule but came
later in the flex input file, or one which matched less text. For example, the
following will both count the words in the input and call the routine special()
whenever "frob" is seen:

int word_count = 0;

frob special(); REJECT;
[^ \t\n]+ ++word_count;

Without the REJECT, any "frob"'s in the input would not be counted as words, since
the scanner normally executes only one action per token. Multiple REJECT's are
allowed, each one finding the next best choice to the currently active rule. For
example, when the following scanner scans the token "abcd", it will write
"abcdabcaba" to the output:

a |
ab |
abc |
.|\n /* eat up any unmatched character */

(The first three rules share the fourth's action since they use the special '|'
action.) REJECT is a particularly expensive feature in terms of scanner
performance; if it is used in any of the scanner's actions it will slow down all of
the scanner's matching. Furthermore, REJECT cannot be used with the -Cf or -CF
options (see below).

Note also that unlike the other special actions, REJECT is a branch; code
immediately following it in the action will not be executed.

- yymore() tells the scanner that the next time it matches a rule, the corresponding
token should be appended onto the current value of yytext rather than replacing it.
For example, given the input "mega-kludge" the following will write "mega-mega-
kludge" to the output:

mega- ECHO; yymore();
kludge ECHO;

First "mega-" is matched and echoed to the output. Then "kludge" is matched, but
the previous "mega-" is still hanging around at the beginning of yytext so the ECHO
for the "kludge" rule will actually write "mega-kludge".

Two notes regarding use of yymore(). First, yymore() depends on the value of yyleng
correctly reflecting the size of the current token, so you must not modify yyleng if you
are using yymore(). Second, the presence of yymore() in the scanner's action entails a
minor performance penalty in the scanner's matching speed.

- yyless(n) returns all but the first n characters of the current token back to the
input stream, where they will be rescanned when the scanner looks for the next
match. yytext and yyleng are adjusted appropriately (e.g., yyleng will now be
equal to n ). For example, on the input "foobar" the following will write out

foobar ECHO; yyless(3);
[a-z]+ ECHO;

An argument of 0 to yyless will cause the entire current input string to be scanned
again. Unless you've changed how the scanner will subsequently process its input
(using BEGIN, for example), this will result in an endless loop.

Note that yyless is a macro and can only be used in the flex input file, not from other
source files.

- unput(c) puts the character c back onto the input stream. It will be the next
character scanned. The following action will take the current token and cause it
to be rescanned enclosed in parentheses.

int i;
/* Copy yytext because unput() trashes yytext */
char *yycopy = strdup( yytext );
unput( ')' );
for ( i = yyleng - 1; i >= 0; --i )
unput( yycopy[i] );
unput( '(' );
free( yycopy );

Note that since each unput() puts the given character back at the beginning of the
input stream, pushing back strings must be done back-to-front.

An important potential problem when using unput() is that if you are using %pointer (the
default), a call to unput() destroys the contents of yytext, starting with its rightmost
character and devouring one character to the left with each call. If you need the value
of yytext preserved after a call to unput() (as in the above example), you must either
first copy it elsewhere, or build your scanner using %array instead (see How The Input Is

Finally, note that you cannot put back EOF to attempt to mark the input stream with an

- input() reads the next character from the input stream. For example, the following
is one way to eat up C comments:

"/*" {
int c;

for ( ; ; )
while ( (c = input()) != '*' &&
c != EOF )
; /* eat up text of comment */

if ( c == '*' )
while ( (c = input()) == '*' )
if ( c == '/' )
break; /* found the end */

if ( c == EOF )
error( "EOF in comment" );

(Note that if the scanner is compiled using C++, then input() is instead referred
to as yyinput(), in order to avoid a name clash with the C++ stream by the name of

- 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

- 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 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

#define YY_DECL float lexscan( a, b ) float a, b;

to give the scanning routine the name lexscan, returning a float, and taking two floats as
arguments. Note that if you give arguments to the scanning routine using a K&R-style/non-
prototyped function declaration, you must terminate the definition with a semi-colon (;).

Whenever yylex() is called, it scans tokens from the global input file yyin (which
defaults to stdin). It continues until it either reaches an end-of-file (at which point
it returns the value 0) or one of its actions executes a return statement.

If the scanner reaches an end-of-file, subsequent calls are undefined unless either yyin
is pointed at a new input file (in which case scanning continues from that file), or
yyrestart() is called. yyrestart() takes one argument, a FILE * pointer (which can be
nil, if you've set up YY_INPUT to scan from a source other than yyin), and initializes
yyin for scanning from that file. Essentially there is no difference between just
assigning yyin to a new input file or using yyrestart() to do so; the latter is available
for compatibility with previous versions of flex, and because it can be used to switch
input files in the middle of scanning. It can also be used to throw away the current
input buffer, by calling it with an argument of yyin; but better is to use YY_FLUSH_BUFFER
(see above). Note that yyrestart() does not reset the start condition to INITIAL (see
Start Conditions, below).

If yylex() stops scanning due to executing a return statement in one of the actions, the
scanner may then be called again and it will resume scanning where it left off.

By default (and for purposes of efficiency), the scanner uses block-reads rather than
simple getc() calls to read characters from yyin. The nature of how it gets its input can
be controlled by defining the YY_INPUT macro. YY_INPUT's calling sequence is
"YY_INPUT(buf,result,max_size)". Its action is to place up to max_size characters in the
character array buf and return in the integer variable result either the number of
characters read or the constant YY_NULL (0 on Unix systems) to indicate EOF. The default
YY_INPUT reads from the global file-pointer "yyin".

A sample definition of YY_INPUT (in the definitions section of the input file):

#define YY_INPUT(buf,result,max_size) \
{ \
int c = getchar(); \
result = (c == EOF) ? YY_NULL : (buf[0] = c, 1); \

This definition will change the input processing to occur one character at a time.

When the scanner receives an end-of-file indication from YY_INPUT, it then checks the
yywrap() function. If yywrap() returns false (zero), then it is assumed that the function
has gone ahead and set up yyin to point to another input file, and scanning continues. If
it returns true (non-zero), then the scanner terminates, returning 0 to its caller. Note
that in either case, the start condition remains unchanged; it does not revert to INITIAL.

If you do not supply your own version of yywrap(), then you must either use %option
noyywrap (in which case the scanner behaves as though yywrap() returned 1), or you must
link with -ll to obtain the default version of the routine, which always returns 1.

Three routines are available for scanning from in-memory buffers rather than files:
yy_scan_string(), yy_scan_bytes(), and yy_scan_buffer(). See the discussion of them below
in the section Multiple Input Buffers.

The scanner writes its ECHO output to the yyout global (default, stdout), which may be
redefined by the user simply by assigning it to some other FILE pointer.


flex provides a mechanism for conditionally activating rules. Any rule whose pattern is
prefixed with "<sc>" will only be active when the scanner is in the start condition named
"sc". For example,

<STRING>[^"]* { /* eat up the string body ... */

will be active only when the scanner is in the "STRING" start condition, and

<INITIAL,STRING,QUOTE>\. { /* handle an escape ... */

will be active only when the current start condition is either "INITIAL", "STRING", or

Start conditions are declared in the definitions (first) section of the input using
unindented lines beginning with either %s or %x followed by a list of names. The former
declares inclusive start conditions, the latter exclusive start conditions. A start
condition is activated using the BEGIN action. Until the next BEGIN action is executed,
rules with the given start condition will be active and rules with other start conditions
will be inactive. If the start condition is inclusive, then rules with no start
conditions at all will also be active. If it is exclusive, then only rules qualified with
the start condition will be active. A set of rules contingent on the same exclusive start
condition describe a scanner which is independent of any of the other rules in the flex
input. Because of this, exclusive start conditions make it easy to specify "mini-
scanners" which scan portions of the input that are syntactically different from the rest
(e.g., comments).

If the distinction between inclusive and exclusive start conditions is still a little
vague, here's a simple example illustrating the connection between the two. The set of

%s example

<example>foo do_something();

bar something_else();

is equivalent to

%x example

<example>foo do_something();

<INITIAL,example>bar something_else();

Without the <INITIAL,example> qualifier, the bar pattern in the second example wouldn't be
active (i.e., couldn't match) when in start condition example. If we just used <example>
to qualify bar, though, then it would only be active in example and not in INITIAL, while
in the first example it's active in both, because in the first example the example start
condition is an inclusive (%s) start condition.

Also note that the special start-condition specifier <*> matches every start condition.
Thus, the above example could also have been written;

%x example

<example>foo do_something();

<*>bar something_else();

The default rule (to ECHO any unmatched character) remains active in start conditions. It
is equivalent to:

<*>.|\n ECHO;

BEGIN(0) returns to the original state where only the rules with no start conditions are
active. This state can also be referred to as the start-condition "INITIAL", so
BEGIN(INITIAL) is equivalent to BEGIN(0). (The parentheses around the start condition
name are not required but are considered good style.)

BEGIN actions can also be given as indented code at the beginning of the rules section.
For example, the following will cause the scanner to enter the "SPECIAL" start condition
whenever yylex() is called and the global variable enter_special is true:

int enter_special;

if ( enter_special )

...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

[0-9]+ {
printf( "found an integer, = %d\n",
atoi( yytext ) );

"." printf( "found a dot\n" );

Here is a scanner which recognizes (and discards) C comments while maintaining a count of
the current input line.

%x comment
int line_num = 1;

"/*" BEGIN(comment);

<comment>[^*\n]* /* eat anything that's not a '*' */
<comment>"*"+[^*/\n]* /* eat up '*'s not followed by '/'s */
<comment>\n ++line_num;
<comment>"*"+"/" BEGIN(INITIAL);

This scanner goes to a bit of trouble to match as much text as possible with each rule.
In general, when attempting to write a high-speed scanner try to match as much possible in
each rule, as it's a big win.

Note that start-conditions names are really integer values and can be stored as such.
Thus, the above could be extended in the following fashion:

%x comment foo
int line_num = 1;
int comment_caller;

"/*" {
comment_caller = INITIAL;


<foo>"/*" {
comment_caller = foo;

<comment>[^*\n]* /* eat anything that's not a '*' */
<comment>"*"+[^*/\n]* /* eat up '*'s not followed by '/'s */
<comment>\n ++line_num;
<comment>"*"+"/" BEGIN(comment_caller);

Furthermore, you can access the current start condition using the integer-valued YY_START
macro. For example, the above assignments to comment_caller could instead be written

comment_caller = YY_START;

Flex provides YYSTATE as an alias for YY_START (since that is what's used by AT&T lex).

Note that start conditions do not have their own name-space; %s's and %x's declare names
in the same fashion as #define's.

Finally, here's an example of how to match C-style quoted strings using exclusive start
conditions, including expanded escape sequences (but not including checking for a string
that's too long):

%x str

char string_buf[MAX_STR_CONST];
char *string_buf_ptr;

\" string_buf_ptr = string_buf; BEGIN(str);

<str>\" { /* saw closing quote - all done */
*string_buf_ptr = '\0';
/* return string constant token type and
* value to parser

<str>\n {
/* error - unterminated string constant */
/* generate error message */

<str>\\[0-7]{1,3} {
/* octal escape sequence */
int result;

(void) sscanf( yytext + 1, "%o", &result );

if ( result > 0xff )
/* error, constant is out-of-bounds */

*string_buf_ptr++ = result;

<str>\\[0-9]+ {
/* generate error - bad escape sequence; something
* like '\48' or '\0777777'

<str>\\n *string_buf_ptr++ = '\n';
<str>\\t *string_buf_ptr++ = '\t';
<str>\\r *string_buf_ptr++ = '\r';
<str>\\b *string_buf_ptr++ = '\b';
<str>\\f *string_buf_ptr++ = '\f';

<str>\\(.|\n) *string_buf_ptr++ = yytext[1];

<str>[^\\\n\"]+ {
char *yptr = yytext;

while ( *yptr )
*string_buf_ptr++ = *yptr++;

Often, such as in some of the examples above, you wind up writing a whole bunch of rules
all preceded by the same start condition(s). Flex makes this a little easier and cleaner
by introducing a notion of start condition scope. A start condition scope is begun with:


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,

"\\n" return '\n';
"\\r" return '\r';
"\\f" return '\f';
"\\0" return '\0';

is equivalent to:

<ESC>"\\n" return '\n';
<ESC>"\\r" return '\r';
<ESC>"\\f" return '\f';
<ESC>"\\0" return '\0';

Start condition scopes may be nested.

Three routines are available for manipulating stacks of start conditions:

void yy_push_state(int new_state)
pushes the current start condition onto the top of the start condition stack and
switches to new_state as though you had used BEGIN new_state (recall that start
condition names are also integers).

void yy_pop_state()
pops the top of the stack and switches to it via BEGIN.

int yy_top_state()
returns the top of the stack without altering the stack's contents.

The start condition stack grows dynamically and so has no built-in size limitation. If
memory is exhausted, program execution aborts.

To use start condition stacks, your scanner must include a %option stack directive (see
Options below).


Some scanners (such as those which support "include" files) require reading from several
input streams. As flex scanners do a large amount of buffering, one cannot control where
the next input will be read from by simply writing a YY_INPUT which is sensitive to the
scanning context. YY_INPUT is only called when the scanner reaches the end of its buffer,
which may be a long time after scanning a statement such as an "include" which requires
switching the input source.

To negotiate these sorts of problems, flex provides a mechanism for creating and switching
between multiple input buffers. An input buffer is created by using:

YY_BUFFER_STATE yy_create_buffer( FILE *file, int size )

which takes a FILE pointer and a size and creates a buffer associated with the given file
and large enough to hold size characters (when in doubt, use YY_BUF_SIZE for the size).
It returns a YY_BUFFER_STATE handle, which may then be passed to other routines (see
below). The YY_BUFFER_STATE type is a pointer to an opaque struct yy_buffer_state
structure, so you may safely initialize YY_BUFFER_STATE variables to ((YY_BUFFER_STATE) 0)
if you wish, and also refer to the opaque structure in order to correctly declare input
buffers in source files other than that of your scanner. Note that the FILE pointer in
the call to yy_create_buffer is only used as the value of yyin seen by YY_INPUT; if you
redefine YY_INPUT so it no longer uses yyin, then you can safely pass a nil FILE pointer
to yy_create_buffer. You select a particular buffer to scan from using:

void yy_switch_to_buffer( YY_BUFFER_STATE new_buffer )

switches the scanner's input buffer so subsequent tokens will come from new_buffer. Note
that yy_switch_to_buffer() may be used by yywrap() to set things up for continued
scanning, instead of opening a new file and pointing yyin at it. Note also that switching
input sources via either yy_switch_to_buffer() or yywrap() does not change the start

void yy_delete_buffer( YY_BUFFER_STATE buffer )

is used to reclaim the storage associated with a buffer. ( buffer can be nil, in which
case the routine does nothing.) You can also clear the current contents of a buffer

void yy_flush_buffer( YY_BUFFER_STATE buffer )

This function discards the buffer's contents, so the next time the scanner attempts to
match a token from the buffer, it will first fill the buffer anew using YY_INPUT.

yy_new_buffer() is an alias for yy_create_buffer(), provided for compatibility with the
C++ use of new and delete for creating and destroying dynamic objects.

Finally, the YY_CURRENT_BUFFER macro returns a YY_BUFFER_STATE handle to the current

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

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++] =

yyin = fopen( yytext, "r" );

if ( ! yyin )
error( ... );

yy_create_buffer( yyin, YY_BUF_SIZE ) );


<<EOF>> {
if ( --include_stack_ptr < 0 )

yy_delete_buffer( YY_CURRENT_BUFFER );
include_stack[include_stack_ptr] );

Three routines are available for setting up input buffers for scanning in-memory strings
instead of files. All of them create a new input buffer for scanning the string, and
return a corresponding YY_BUFFER_STATE handle (which you should delete with
yy_delete_buffer() when done with it). They also switch to the new buffer using
yy_switch_to_buffer(), so the next call to yylex() will start scanning the string.

yy_scan_string(const char *str)
scans a NUL-terminated string.

yy_scan_bytes(const char *bytes, int len)
scans len bytes (including possibly NUL's) starting at location bytes.

Note that both of these functions create and scan a copy of the string or bytes. (This
may be desirable, since yylex() modifies the contents of the buffer it is scanning.) You
can avoid the copy by using:

yy_scan_buffer(char *base, yy_size_t size)
which scans in place the buffer starting at base, consisting of size bytes, the
last two bytes of which must be YY_END_OF_BUFFER_CHAR (ASCII NUL). These last two
bytes are not scanned; thus, scanning consists of base[0] through base[size-2],

If you fail to set up base in this manner (i.e., forget the final two
YY_END_OF_BUFFER_CHAR bytes), then yy_scan_buffer() returns a nil pointer instead
of creating a new input buffer.

The type yy_size_t is an integral type to which you can cast an integer expression
reflecting the size of the buffer.


The special rule "<<EOF>>" indicates actions which are to be taken when an end-of-file is
encountered and yywrap() returns non-zero (i.e., indicates no further files to process).
The action must finish by doing one of four things:

- assigning yyin to a new input file (in previous versions of flex, after doing the
assignment you had to call the special action YY_NEW_FILE; this is no longer

- 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

<<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


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" );
<<EOF>> {
if ( *++filelist )
yyin = fopen( *filelist, "r" );


The macro YY_USER_ACTION can be defined to provide an action which is always executed
prior to the matched rule's action. For example, it could be #define'd to call a routine
to convert yytext to lower-case. When YY_USER_ACTION is invoked, the variable yy_act
gives the number of the matched rule (rules are numbered starting with 1). Suppose you
want to profile how often each of your rules is matched. The following would do the

#define YY_USER_ACTION ++ctr[yy_act]

where ctr is an array to hold the counts for the different rules. Note that the macro
YY_NUM_RULES gives the total number of rules (including the default rule, even if you use
-s), so a correct declaration for ctr is:

int ctr[YY_NUM_RULES];

The macro YY_USER_INIT may be defined to provide an action which is always executed before
the first scan (and before the scanner's internal initializations are done). For example,
it could be used to call a routine to read in a data table or open a logging file.

The macro yy_set_interactive(is_interactive) can be used to control whether the current
buffer is considered interactive. An interactive buffer is processed more slowly, but
must be used when the scanner's input source is indeed interactive to avoid problems due
to waiting to fill buffers (see the discussion of the -I flag below). A non-zero value in
the macro invocation marks the buffer as interactive, a zero value as non-interactive.
Note that use of this macro overrides %option interactive , %option always-interactive or
%option never-interactive (see Options below). yy_set_interactive() must be invoked prior
to beginning to scan the buffer that is (or is not) to be considered interactive.

The macro yy_set_bol(at_bol) can be used to control whether the current buffer's scanning
context for the next token match is done as though at the beginning of a line. A non-zero
macro argument makes rules anchored with

The macro YY_AT_BOL() returns true if the next token scanned from the current buffer will
have '^' rules active, false otherwise.

In the generated scanner, the actions are all gathered in one large switch statement and
separated using YY_BREAK, which may be redefined. By default, it is simply a "break", to
separate each rule's action from the following rule's. Redefining YY_BREAK allows, for
example, C++ users to #define YY_BREAK to do nothing (while being very careful that every
rule ends with a "break" or a "return"!) to avoid suffering from unreachable statement
warnings where because a rule's action ends with "return", the YY_BREAK is inaccessible.


This section summarizes the various values available to the user in the rule actions.

- char *yytext holds the text of the current token. It may be modified but not
lengthened (you cannot append characters to the end).

If the special directive %array appears in the first section of the scanner
description, then yytext is instead declared char yytext[YYLMAX], where YYLMAX is a
macro definition that you can redefine in the first section if you don't like the
default value (generally 8KB). Using %array results in somewhat slower scanners,
but the value of yytext becomes immune to calls to input() and unput(), which
potentially destroy its value when yytext is a character pointer. The opposite of
%array is %pointer, which is the default.

You cannot use %array when generating C++ scanner classes (the -+ flag).

- int yyleng holds the length of the current token.

- FILE *yyin is the file which by default flex reads from. It may be redefined but
doing so only makes sense before scanning begins or after an EOF has been
encountered. Changing it in the midst of scanning will have unexpected results
since flex buffers its input; use yyrestart() instead. Once scanning terminates
because an end-of-file has been seen, you can assign yyin at the new input file and
then call the scanner again to continue scanning.

- void yyrestart( FILE *new_file ) may be called to point yyin at the new input file.
The switch-over to the new file is immediate (any previously buffered-up input is
lost). Note that calling yyrestart() with yyin as an argument thus throws away the
current input buffer and continues scanning the same input file.

- FILE *yyout is the file to which ECHO actions are done. It can be reassigned by
the user.

- YY_CURRENT_BUFFER returns a YY_BUFFER_STATE handle to the current buffer.

- YY_START returns an integer value corresponding to the current start condition.
You can subsequently use this value with BEGIN to return to that start condition.


One of the main uses of flex is as a companion to the yacc parser-generator. yacc parsers
expect to call a routine named yylex() to find the next input token. The routine is
supposed to return the type of the next token as well as putting any associated value in
the global yylval. To use flex with yacc, one specifies the -d option to yacc to instruct
it to generate the file y.tab.h containing definitions of all the %tokens appearing in the
yacc input. This file is then included in the flex scanner. For example, if one of the
tokens is "TOK_NUMBER", part of the scanner might look like:

#include "y.tab.h"


[0-9]+ yylval = atoi( yytext ); return TOK_NUMBER;


flex has the following options:

-b, --backup
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, --debug
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
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, --full
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, --help
generates a "help" summary of flex's options to stdout and then exits. -? and
--help are synonyms for -h.

-i, --case-insensitive
instructs flex to generate a case-insensitive scanner. The case of letters given
in the flex input patterns will be ignored, and tokens in the input will be matched
regardless of case. The matched text given in yytext will have the preserved case
(i.e., it will not be folded).

-l, --lex-compat
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, --perf-report
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 performance penalty;
use of yymore(), the ^ operator, and the -I flag entail minor performance

-s, --no-default
causes the default rule (that unmatched scanner input is echoed to stdout) to be
suppressed. If the scanner encounters input that does not match any of its rules,
it aborts with an error. This option is useful for finding holes in a scanner's
rule set.

-t, --stdout
instructs flex to write the scanner it generates to standard output instead of

-v, --verbose
specifies that flex should write to stderr a summary of statistics regarding the
scanner it generates. 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, --nowarn
suppresses warning messages.

-B, --batch
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 interactively, 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, --fast
specifies that the fast scanner table representation should be used (and stdio
bypassed). This representation is about as fast as the full table representation
(-f), and for some sets of 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 representation. If only the
"identifier" rule is present 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, --interactive
instructs flex to generate an interactive scanner. An interactive scanner is one
that only looks ahead to decide what token has been matched if it absolutely must.
It turns out that always looking one extra character ahead, even if the scanner has
already seen enough text to disambiguate the current token, is a bit faster than
only looking ahead when necessary. But scanners that always look ahead give
dreadful interactive performance; for example, when a user types a newline, it is
not recognized as a newline token until they enter another token, which often means
typing in another whole line.

Flex scanners default to interactive unless 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.

Note that if isatty() returns false for the scanner input, flex will revert to
batch mode, even if -I was specified. To force interactive mode no matter what,
use %option always-interactive (see Options below).

You can force a scanner to not be interactive by using -B (see above).

-L, --noline
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, --trace
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, --version
prints the version number to stdout and exits. --version is a synonym for -V.

-7, --7bit
instructs flex to generate a 7-bit scanner, i.e., one which can only recognize
7-bit characters in its input. The advantage of using -7 is that the scanner's
tables can be up to half the size of those generated using the -8 option (see
below). The disadvantage is that such scanners often hang or crash if their input
contains an 8-bit character.

Note, however, that unless you generate your scanner 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 which case flex
defaults to generating 7-bit scanners unless your site was always configured to
generate 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 compression options, but also using
equivalence classes as discussed see below), flex still defaults to generating an
8-bit scanner, since usually with these compression options full 8-bit tables are
not much more expensive than 7-bit tables.

-8, --8bit
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 generating 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.

-+, --c++
specifies that you want flex to generate a C++ scanner class. See the section on
Generating C++ Scanners below for details.

controls the degree of table compression and, more generally, trade-offs between
small scanners and fast scanners.

-Ca, --align ("align") instructs flex to trade off larger tables in the generated
scanner for faster performance because the elements of the tables are better
aligned for memory access and computation. On some RISC architectures, fetching
and manipulating longwords 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, --ecs directs flex to construct equivalence classes, i.e., sets of characters
which have identical lexical properties (for example, if the only appearance of
digits in the flex input is in the character class "[0-9]" then the digits '0',
'1', ..., '9' will all be put in the same equivalence class). Equivalence classes
usually give dramatic reductions in the final table/object file sizes (typically a
factor of 2-5) and are pretty cheap performance-wise (one array look-up per
character scanned).

-Cf specifies that the full scanner tables should be generated - flex should not
compress the tables by taking advantages of similar transition functions for
different states.

-CF specifies that the alternative fast scanner representation (described above
under the -F flag) should be used. This option cannot be used with -+.

-Cm, --meta-ecs directs flex to construct meta-equivalence classes, which are sets
of equivalence classes (or characters, if equivalence classes are not being used)
that are commonly used together. Meta-equivalence classes are often a big win when
using compressed tables, but they have a moderate performance impact (one or two
"if" tests and one array look-up per character scanned).

-Cr, --read 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
system, 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-equivalence classes if the table is not being compressed. Otherwise the
options may be freely mixed, and are cumulative.

The default setting is -Cem, which specifies that flex should 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
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

-Cfe is often a good compromise between speed and size for production scanners.

-ooutput, --outputfile=FILE
directs flex to write the scanner to the file output instead of lex.yy.c. If you
combine -o with the -t option, then the scanner is written to stdout but its #line
directives (see the -L option above) refer to the file output.

-Pprefix, --prefix=STRING
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:


(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.

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 -ll no longer provides one for
you by default.

-Sskeleton_file, --skel=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.

-X, --posix-compat
maximal compatibility with POSIX lex.

track line count in yylineno.

name of C++ class.

create a C header file in addition to the scanner.

write tables to FILE.

#define macro defn (default defn is '1').

-R, --reentrant
generate a reentrant C scanner

scanner for bison pure parser.

include yylloc support.

initialize yyin/yyout to stdin/stdout.

--noansi-definitions old-style function definitions.

empty parameter list in prototypes.

do not include <unistd.h>.

do not generate a particular FUNCTION.

flex also provides a mechanism for controlling options within the scanner specification
itself, rather than from the flex command-line. This is done by including %option
directives in the first section of the scanner specification. 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 preceded by the word "no" (with no
intervening whitespace) to negate their meaning. A number are equivalent to flex flags or
their negation:

7bit -7 option
8bit -8 option
align -Ca option
backup -b option
batch -B option
c++ -+ option

caseful or
case-sensitive opposite of -i (default)

case-insensitive or
caseless -i option

debug -d option
default opposite of -s option
ecs -Ce option
fast -F option
full -f option
interactive -I option
lex-compat -l option
meta-ecs -Cm option
perf-report -p option
read -Cr option
stdout -t option
verbose -v option
warn opposite of -w option
(use "%option nowarn" for -w)

array equivalent to "%array"
pointer equivalent to "%pointer" (default)

Some %option's provide features otherwise not available:

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).

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).

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.

directs flex to generate a scanner that maintains the number of the current line
read from its input in the global variable yylineno. This option is implied by
%option lex-compat.

yywrap if unset (i.e., %option noyywrap), makes the scanner not call yywrap() upon an end-
of-file, but simply assume that there are no more files to scan (until the user
points yyin at a new file and calls yylex() again).

flex scans 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 unsetting them to indicate it actually is not used (e.g., %option

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::LexerError()) if called. See Generating C++ Scanners, below, for additional

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

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).


The main design goal of flex is that it generate high-performance scanners. It has been
optimized for dealing well with large sets of rules. Aside from the effects on scanner
speed of the table compression -C options outlined above, there are a number of
options/actions which degrade performance. These are, from most expensive to least:

%option yylineno
arbitrary trailing context

pattern sets that require backing up
%option interactive
%option always-interactive

'^' beginning-of-line operator

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:
out-transitions: [ a ]
jam-transitions: EOF [ \001-` b-\177 ]

State #9 is non-accepting -
associated rule line numbers:
out-transitions: [ r ]
jam-transitions: EOF [ \001-q s-\177 ]

Compressed tables always back up.

The first few lines tell us that there's a scanner state in which it can make a transition
on an 'o' but not on any other character, and that in that state the currently scanned
text does not match any rule. The state occurs when trying to match the rules found at
lines 2 and 3 in the input file. If the scanner is in that state and then reads something
other than an 'o', it will have to back up to find a rule which is matched. With a bit of
headscratching one can see that this must be the state it's in when it has seen "fo".
When this has happened, if anything other than another 'o' is seen, the scanner will have
to back up to simply match the 'f' (by the default rule).

The comment regarding State #8 indicates there's a problem when "foob" has been scanned.
Indeed, on any character other than an 'a', the scanner will have to back up to accept
"foo". Similarly, the comment for State #9 concerns when "fooba" has been scanned and an
'r' does not follow.

The final comment reminds us that there's no point going to all the trouble of removing
backing up from the rules unless we're using -Cf or -CF, since there's no performance gain
doing so with compressed scanners.

The way to remove the backing up is to add "error" rules:

foo return TOK_KEYWORD;
foobar return TOK_KEYWORD;

fooba |
foob |
fo {
/* false alarm, not really a keyword */
return TOK_ID;

Eliminating backing up among a list of keywords can also be done using a "catch-all" rule:

foo return TOK_KEYWORD;
foobar return TOK_KEYWORD;

[a-z]+ return TOK_ID;

This is usually the best solution when appropriate.

Backing up messages tend to cascade. With a complicated set of rules it's not uncommon to
get hundreds of messages. If one can decipher them, though, it often only takes a dozen
or so rules to eliminate the backing up (though it's easy to make a mistake and have an
error rule 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.

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 performance (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 ++line_num;
<comment>"*"+"/" BEGIN(INITIAL);

This could be sped up by writing it as:

%x comment
int line_num = 1;

"/*" BEGIN(comment);

<comment>[^*\n]*\n ++line_num;
<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 keywords, one per line and with no other extraneous characters,
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 ...
volatile\n |
while\n /* it's a keyword */

[a-z]+\n |
.|\n /* it's not a keyword */

One has to be careful here, as we have now reintroduced backing up into the scanner. In
particular, while we know that there will never be any characters in the input stream
other than letters or newlines, flex can't figure this out, and it will plan for possibly
needing to back up when it has scanned a token like "auto" and then the next character is
something other than a newline or a letter. Previously it would then just match the
"auto" rule and be done, but now it has no "auto" rule, only an "auto\n" rule. To
eliminate the possibility of backing up, we could either 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 newline:

asm\n |
auto\n |
break\n |
... etc ...
volatile\n |
while\n /* it's a keyword */

[a-z]+\n |
[a-z]+ |
.|\n /* it's not a keyword */

Compiled with -Cf, this is about as fast as one can get a flex scanner to go for this
particular problem.

A final note: flex is slow when matching NUL's, particularly when a token contains
multiple NUL's. It's best to write rules which match short amounts of text if it's
anticipated that the text will often include NUL's.

Another final note regarding performance: as mentioned above in the section How the Input
is Matched, dynamically resizing yytext to accommodate huge tokens is a slow process
because it presently requires that the (huge) token be rescanned from the beginning. Thus
if performance is 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.


flex provides two different ways to generate scanners for use with C++. The first way is
to simply compile a scanner generated by flex using a C++ compiler instead of a C
compiler. 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
executable ends in a '+', such as flex++. When using this option, flex defaults to
generating the scanner to the file lex.yy.cc instead of lex.yy.c. The generated scanner
includes the header file 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 section above). Note that you must build the scanner using
%option debug to include debugging information in it.

int debug() const
returns the current setting of the debugging flag.

Also provided are member functions equivalent to yy_switch_to_buffer(), yy_create_buffer()
(though the first argument is an std::istream* object pointer and not a FILE*),
yy_flush_buffer(), yy_delete_buffer(), and yyrestart() (again, the first argument is a
std::istream* object pointer).

The second class defined in FlexLexer.h is yyFlexLexer, which is derived from FlexLexer.
It defines the following additional member functions:

yyFlexLexer( std::istream* arg_yyin = 0, std::ostream* arg_yyout = 0 )
constructs a yyFlexLexer object using the given streams for input and output. If
not specified, the streams default to cin and cout, respectively.

virtual int yylex()
performs the same role 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

virtual void switch_streams(std::istream* new_in = 0,
std::ostream* new_out = 0) reassigns yyin to new_in (if non-nil) and yyout to
new_out (ditto), deleting the previous input buffer if yyin is reassigned.

int yylex( std::istream* new_in, std::ostream* new_out = 0 )
first switches the input streams via switch_streams( new_in, new_out ) and then
returns the value of yylex().

In addition, yyFlexLexer defines the following protected virtual 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

virtual void LexerError( const char* msg )
reports a fatal error message. The default version of this function writes the
message to the stream cerr and exits.

Note that a yyFlexLexer object contains its entire scanning state. Thus you can use such
objects to create reentrant scanners. You can instantiate multiple instances of the same
yyFlexLexer class, and you can also combine multiple 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')

else if(c == '*')
if((c = yyinput()) == '/')

{number} cout << "number " << YYText() << '\n';

\n mylineno++;

{name} cout << "name " << YYText() << '\n';

{string} cout << "string " << YYText() << '\n';


int main( int /* argc */, char** /* argv */ )
FlexLexer* lexer = new yyFlexLexer;
while(lexer->yylex() != 0)
return 0;
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 <FlexLexer.h> in your other sources once per lexer class, first renaming
yyFlexLexer as follows:

#undef yyFlexLexer
#define yyFlexLexer xxFlexLexer
#include <FlexLexer.h>

#undef yyFlexLexer
#define yyFlexLexer zzFlexLexer
#include <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.


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

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 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 redefined is in accordance with the
POSIX specification, which simply does not specify any way of controlling 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

- 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 interactive scanner.

Also note that flex C++ scanner classes are reentrant, so if using C++ is an option
for you, you should use them instead. See "Generating C++ Scanners" above for

- 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

- When definitions are expanded, flex encloses them in parentheses. With lex, the

NAME [A-Z][A-Z0-9]*
foo{NAME}? printf( "Found it\n" );

will not match the string "foo" because when the macro is expanded the rule is
equivalent to "foo[A-Z][A-Z0-9]*?" and the precedence is such that the '?' is
associated with "[A-Z0-9]*". With flex, the rule will be expanded to "foo([A-Z][A-
Z0-9]*)?" and so the string "foo" will match.

Note that if the definition begins with ^ or ends with $ then it is not expanded
with parentheses, to allow these operators to appear in definitions without losing
their special meanings. But the <s>, /, and <<EOF>> operators cannot be used in a
flex definition.

Using -l results in the lex behavior of no parentheses around the 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 specification.

- After a call to unput(), yytext is undefined until the next token is matched,
unless the scanner was built using %array. This is not the case with lex or the
POSIX specification. The -l option does away with this incompatibility.

- The precedence of the {} (numeric range) operator is different. lex interprets
"abc{1,3}" as "match one, two, or three occurrences of 'abc'", whereas flex
interprets it as "match 'ab' followed by one, two, or three occurrences of 'c'".
The latter is in agreement with the POSIX specification.

- The precedence of the ^ operator is different. lex interprets "^foo|bar" as "match
either 'foo' at the beginning of a line, or 'bar' anywhere", whereas flex
interprets it as "match either 'foo' or 'bar' if they come at the beginning of a
line". The latter is in agreement with the POSIX specification.

- The special table-size declarations such as %a supported by lex are not required by
flex scanners; flex ignores them.

- The name FLEX_SCANNER is #define'd so scanners may be written for use with either
flex or lex. Scanners also include YY_FLEX_MAJOR_VERSION and YY_FLEX_MINOR_VERSION
indicating which version of flex generated the scanner (for example, for the 2.5
release, these defines would be 2 and 5 respectively).

The following flex features are not included in lex or the POSIX specification:

C++ scanners
start condition scopes
start condition stacks
interactive/non-interactive scanners
yy_scan_string() and friends
#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 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.


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 YYLMAX 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 scanner specification includes
recognizing the 8-bit character '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 scanner uses REJECT - the scanner was
working on matching an extremely large token and needed to expand the input buffer. This
doesn't work with scanners that use REJECT.

fatal flex scanner internal error--end of buffer missed - This can occur in a scanner
which is reentered after a long-jump has jumped out (or over) the scanner's activation
frame. Before reentering the scanner, use:

yyrestart( yyin );

or, as noted above, switch to using the C++ scanner class.

too many start conditions in <> construct! - you listed more start conditions in a <>
construct than exist (so you must have listed at least one of them twice).

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