CONTENTS

SYNOPSIS

OVERVIEW

Description
a brief overview of the tool

Some Simple Examples

Format Of The Input File

Patterns
the extended regular expressions used by flex

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

Actions
how to specify what to do when a pattern is matched

The Generated Scanner
details regarding the scanner that flex produces;
how to control the input source

Start Conditions
introducing context into your scanners, and
managing "mini-scanners"

Multiple Input Buffers
how to manipulate multiple input sources; how to
scan from strings instead of files

End-of-file Rules
special rules for matching the end of the input

Miscellaneous Macros
a summary of macros available to the actions

Values Available To The User
a summary of values available to the actions

Interfacing With Yacc
connecting flex scanners together with yacc parsers

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

Performance Considerations
how to make your scanner go as fast as possible

Generating C++ Scanners
the (experimental) facility for generating C++
scanner classes

Incompatibilities With Lex And POSIX
how flex differs from AT&T lex and the POSIX lex
standard

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

Files
files used by flex

Deficiencies / Bugs
known problems with flex

See Also
other documentation, related tools

Author
includes contact information

SOME SIMPLE EXAMPLES

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

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

Here’s another simple example:

%{
int num_lines = 0, num_chars = 0;
%}

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

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

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

A somewhat more complicated example:

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

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

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

%%

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

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

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

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

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

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

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

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

%%

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

yylex();
}

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

definitions
%%
rules
%%
user code

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

Name definitions have the form:

name definition

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

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

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

{DIGIT}+"."{DIGIT}*

is identical to

([0-9])+"."([0-9])*

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

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

pattern action

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

See below for a further description of patterns and actions.

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

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

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

xmatch the character ’x’
.any character (byte) except newline
[xyz] a "character class"; in this case, the pattern
matches either an ’x’, a ’y’, or a ’z’
[abj-oZ] a "character class" with a range in it; matches
an ’a’, a ’b’, any letter from ’j’ through ’o’,
or a ’Z’
[^A-Z]a "negated character class", i.e., any character
but those in the class. In this case, any
character EXCEPT an uppercase letter.
[^A-Z\n] any character EXCEPT an uppercase letter or
a newline
r* zero or more r’s, where r is any regular expression
r+ one or more r’s
r? zero or one r’s (that is, "an optional r")
r{2,5}anywhere from two to five r’s
r{2,} two or more r’s
r{4} exactly 4 r’s
{name}the expansion of the "name" definition
(see above)
"[xyz]\"foo"
the literal string: [xyz]"foo
\X if X is an ’a’, ’b’, ’f’, ’n’, ’r’, ’t’, or ’v’,
then the ANSI-C interpretation of \x.
Otherwise, a literal ’X’ (used to escape
operators such as ’*’)
\0 a NUL character (ASCII code 0)
\123 the character with octal value 123
\x2a the character with hexadecimal value 2a
(r) match an r; parentheses are used to override
precedence (see below)

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

r|s either an r or an s

r/s an r but only if it is followed by an s. The
text matched by s is included when determining
whether this rule is the "longest match",
but is then returned to the input before
the action is executed. So the action only
sees the text matched by r. This type
of pattern is called trailing context".
(There are some combinations of r/s that flex
cannot match correctly; see notes in the
Deficiencies / Bugs section below regarding
"dangerous trailing context".)
^r an r, but only at the beginning of a line (i.e.,
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)
<s1,s2,s3>r
same, but in any of start conditions s1,
s2, or s3
<*>r an r in any start condition, even an exclusive one.

<<EOF>> an end-of-file
<s1,s2><<EOF>>
an end-of-file when in start condition s1 or s2

Note that inside of a character class, all regular expression operators lose their special meaning except escape (’#146;) and the character class operators, ’-’, ’]’, and, at the beginning of the class, ’^’.

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

foo|bar*

is the same as

(foo)|(ba(r*))

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

foo|(bar)*

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

(foo|bar)*

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

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

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

For example, the following character classes are all equivalent:

[[:alnum:]]
[[:alpha:][:digit:]]
[[:alpha:]0-9]
[a-zA-Z0-9]

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

Some notes on patterns:

HOW THE INPUT IS MATCHED

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

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

%%

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

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

The advantage of %array is that you can then modify yytext to your heart’s content, and calls to unput() do not destroy yytext (see below). Furthermore, existing lex programs sometimes access yytext externally using declarations of the form:
extern char yytext[];

This definition is erroneous when used with %pointer, but correct for %array.

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

%%
"zap me"

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

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

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

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

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

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

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

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

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

THE GENERATED SCANNER

int yylex()
{
... various definitions and the actions in here ...
}

(If your environment supports function prototypes, then it will be "int yylex( void )".) This definition may be changed by defining the "YY_DECL" macro. For example, you could use:

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

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

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

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

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

By default (and for purposes of efficiency), the scanner uses 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.

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

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

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

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

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

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

%s example
%%

<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:

<*>.|\nECHO;

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

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

int enter_special;

%x SPECIAL
%%
if ( enter_special )
BEGIN(SPECIAL);

<SPECIAL>blahblahblah
...more rules follow...

To illustrate the uses of start conditions, here is a scanner which provides two different interpretations of a string like "123.456". By default it will treat it as three tokens, the integer "123", a dot (’.’), and the integer "456". But if the string is preceded earlier in the line by the string "expect-floats" it will treat it as a single token, the floating-point number 123.456:

%{
#include <math.h>
%}
%s expect

%%
expect-floats BEGIN(expect);

<expect>[0-9]+"."[0-9]+ {
printf( "found a float, = %f\n",
atof( yytext ) );
}
<expect>\n {
/* that’s the end of the line, so
* we need another "expect-number"
* before we’ll recognize any more
* numbers
*/
BEGIN(INITIAL);
}

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

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

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

%x comment
%%
int line_num = 1;

"/*" BEGIN(comment);

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

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

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

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

"/*" {
comment_caller = INITIAL;
BEGIN(comment);
}

...

<foo>"/*" {
comment_caller = foo;
BEGIN(comment);
}

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

Furthermore, you can access the current start condition using the integer-valued YY_START macro. For example, the above assignments to 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 */
BEGIN(INITIAL);
*string_buf_ptr = ’\0’;
/* return string constant token type and
* value to parser
*/
}

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

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

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

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

*string_buf_ptr++ = result;
}

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

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

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

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

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

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

<SCs>{

where SCs is a list of one or more start conditions. Inside the start condition scope, every rule automatically has the prefix <SCs> applied to it, until a ’}’ which matches the initial ’{’. So, for example,

<ESC>{
"\\n" return ’\n’;
"\\r" return ’\r’;
"\\f" return ’\f’;
"\\0" return ’\0’;
}

is equivalent to:

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

Start condition scopes may be nested.

Three routines are available for manipulating stacks of start conditions:

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

YY_BUFFER_STATE yy_create_buffer( FILE *file, int size )

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

void yy_switch_to_buffer( YY_BUFFER_STATE new_buffer )

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

void yy_delete_buffer( YY_BUFFER_STATE buffer )

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

void yy_flush_buffer( YY_BUFFER_STATE buffer )

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

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

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

Here is an example of using these features for writing a scanner which expands include files (the <<EOF>> feature is discussed below):

/* the "incl" state is used for picking up the name
* of an include file
*/
%x incl

%{
#define MAX_INCLUDE_DEPTH 10
YY_BUFFER_STATE include_stack[MAX_INCLUDE_DEPTH];
int include_stack_ptr = 0;
%}

%%
include BEGIN(incl);

[a-z]+ ECHO;
[^a-z\n]*\n? ECHO;

<incl>[ \t]* /* eat the whitespace */
<incl>[^ \t\n]+ { /* got the include file name */
if ( include_stack_ptr >= MAX_INCLUDE_DEPTH )
{
fprintf( stderr, "Includes nested too deeply" );
exit( 1 );
}

include_stack[include_stack_ptr++] =
YY_CURRENT_BUFFER;

yyin = fopen( yytext, "r" );

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

yy_switch_to_buffer(
yy_create_buffer( yyin, YY_BUF_SIZE ) );

BEGIN(INITIAL);
}

<<EOF>> {
if ( --include_stack_ptr < 0 )
{
yyterminate();
}

else
{
yy_delete_buffer( YY_CURRENT_BUFFER );
yy_switch_to_buffer(
include_stack[include_stack_ptr] );
}
}

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

END-OF-FILE RULES

<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();
}

#define YY_USER_ACTION ++ctr[yy_act]

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

int ctr[YY_NUM_RULES];

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

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

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

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

INTERFACING WITH YACC

%{
#include "y.tab.h"
%}

%%

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

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 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

REJECT
%option yylineno
arbitrary trailing context

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

’^’ beginning-of-line operator
yymore()

with the first three all being quite expensive and the last two being quite cheap. Note also that unput() is implemented as a routine call that potentially does quite a bit of work, while yyless() is a quite-cheap macro; so if just putting back some excess text 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;
foobarreturn TOK_KEYWORD;

the file looks like:

State #6 is non-accepting -
associated rule line numbers:
2 3
out-transitions: [ o ]
jam-transitions: EOF [ \001-n p-\177 ]

State #8 is non-accepting -
associated rule line numbers:
3
out-transitions: [ a ]
jam-transitions: EOF [ \001-‘ b-\177 ]

State #9 is non-accepting -
associated rule line numbers:
3
out-transitions: [ r ]
jam-transitions: EOF [ \001-q s-\177 ]

Compressed tables always back up.

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

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

The final comment reminds us that there’s no point going to all the 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]*
<comment>"*"+[^*/\n]*
<comment>\n ++line_num;
<comment>"*"+"/" BEGIN(INITIAL);

This could be sped up by writing it as:

%x comment
%%
int line_num = 1;

"/*" BEGIN(comment);

<comment>[^*\n]*
<comment>[^*\n]*\n ++line_num;
<comment>"*"+[^*/\n]*
<comment>"*"+[^*/\n]*\n ++line_num;
<comment>"*"+"/" BEGIN(INITIAL);

Now instead of each newline requiring the processing of another action, recognizing the newlines is "distributed" over the other rules to keep the matched text as long as possible. Note that adding rules does not slow down the scanner! The speed of the scanner is independent of the number of rules or (modulo the considerations given at the beginning of this section) how complicated the rules are with regard to operators such as ’*’ and ’|’.

A final example in speeding up a scanner: 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.

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:

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’;

\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.

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:

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 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();

[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.

DEFICIENCIES / BUGS

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 current (generally huge) token.

Due to both buffering of input and read-ahead, you cannot intermix calls to <stdio.h> routines, such as, for example, getchar(), with flex rules and expect it to work. Call input() instead.

The total table entries listed by the -v flag excludes the number of table entries needed to determine what rule has been matched. The number of entries is equal to the number of DFA states if the scanner does not use REJECT, and somewhat greater than the number of states if it does.

REJECT cannot be used with the -f or -F options.