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FLEX
!!!FLEX
NAME
SYNOPSIS
OVERVIEW
DESCRIPTION
SOME SIMPLE EXAMPLES
FORMAT OF THE INPUT FILE
PATTERNS
HOW THE INPUT IS MATCHED
ACTIONS
THE GENERATED SCANNER
START CONDITIONS
MULTIPLE INPUT BUFFERS
END-OF-FILE RULES
MISCELLANEOUS MACROS
VALUES AVAILABLE TO THE USER
INTERFACING WITH YACC
OPTIONS
PERFORMANCE CONSIDERATIONS
GENERATING C++ SCANNERS
INCOMPATIBILITIES WITH LEX AND POSIX
DIAGNOSTICS
FILES
DEFICIENCIES / BUGS
SEE ALSO
AUTHOR
----
!!NAME


flex - fast lexical analyzer generator
!!SYNOPSIS


__flex [[-bcdfhilnpstvwBFILTV78+? -C[[aefFmr] -ooutput
-Pprefix -Sskeleton] [[--help --version]__ ''[[filename
...]''
!!OVERVIEW


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


Description
a brief overview of the tool
Some Simple Examples
Format Of The Input File
Patterns
the extended regular expressions used by flex
How The Input Is Matched
the rules for determining what has been matched
Actions
how to specify what to do when a pattern is matched
The Generated Scanner
details regarding the scanner that flex produces;
how to control the input source
Start Conditions
introducing context into your scanners, and
managing
!!DESCRIPTION


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


First some simple examples to get the flavor of how one uses
''flex.'' The following ''flex'' input specifies a
scanner which whenever it encounters the string
''


%%
username    printf(
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 ''pattern'' and the ''action.'' The ''


Here's another simple example:


int num_lines = 0, num_chars = 0;
%%
n      ++num_lines; ++num_chars;
.       ++num_chars;
%%
main()
{
yylex();
printf(
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, yylex()__ and in the __main()__ routine declared after the second __


A somewhat more complicated example:


/* scanner for a toy Pascal-like language */
%{
/* need this for the call to atof() below */
#include
This is the beginnings of a simple scanner for a language like Pascal. It identifies different types of ''tokens'' and reports on what it has seen.


The details of this example will be explained in the
following sections.
!!FORMAT OF THE INPUT FILE


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


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


Name definitions have the form:


name definition
The


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


{DIGIT}+
is identical to


([[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).


In the definitions section (but not in the rules section),
an unindented comment (i.e., a line beginning with
!!PATTERNS


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


x          match the character 'x'
.          any character (byte) except newline
[[xyz]      a
Note that inside of a character class, all regular expression operators lose their special meaning except escape ('') and the character class operators, '-', ']', and, at the beginning of the class, '^'.


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


foo|bar*
is the same as


(foo)|(ba(r*))
since the '*' operator has higher precedence than concatenation, and concatenation higher than alternation ('|'). This pattern therefore matches ''either'' the string ''or'' the string ''


foo|(bar)*
and to match zero-or-more


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


[[:alnum:] [[:alpha:] [[:blank:]
[[:cntrl:] [[:digit:] [[:graph:]
[[:lower:] [[:print:] [[:punct:]
[[:space:] [[:upper:] [[:xdigit:]
These expressions all designate a set of characters equivalent to the corresponding standard C __isXXX__ function. For example, __[[:alnum:]__ designates those characters for which __isalnum()__ returns true - i.e., any alphabetic or numeric. Some systems don't provide __isblank(),__ so flex defines __[[:blank:]__ as a blank or a tab.


For example, the following character classes are all
equivalent:


[[[[:alnum:]]
[[[[:alpha:][[:digit:]]
[[[[:alpha:][[0-9]]
[[a-zA-Z0-9]
If your scanner is case-insensitive (the __-i__ flag), then __[[:upper:]__ and __[[:lower:]__ are equivalent to __[[:alpha:].__


Some notes on patterns:


-


A negated character class such as the example
will match a newline'' unless
''


-


A rule can have at most one instance of trailing context
(the '/' operator or the '$' operator). The start condition,
'^', and


The following are illegal:


foo/bar$
Note that the first of these, can be written


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


foo|(bar$)
foo|^bar
If what's wanted is a


foo      |
bar$     /* action goes here */
A similar trick will work for matching a foo or a bar-at-the-beginning-of-a-line.
!!HOW THE INPUT IS MATCHED


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


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


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


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


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


The advantage of __%array__ is that you can then modify
__yytext__ to your heart's content, and calls to
__unput()__ do not destroy __yytext__ (see below).
Furthermore, existing ''lex'' programs sometimes access
__yytext__ externally using declarations of the
form:


    extern char yytext[[];
This definition is erroneous when used with __%pointer,__ but correct for __%array.__


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


Also note that you cannot use __%array__ with C++ scanner
classes (the __c++__ option; see below).
!!ACTIONS


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


%%
(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


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


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


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


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


-


__ECHO__ copies yytext to the scanner's
output.


-


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


-


__REJECT__ directs the scanner to proceed on to the
__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 ''


int word_count = 0;
%%
frob        special(); REJECT;
[[^ tn]+   ++word_count;
Without the __REJECT,__ any __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 __


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


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


-


__yymore()__ tells the scanner that the next time it
matches a rule, the corresponding token should be
''appended'' onto the current value of __yytext__
rather than replacing it. For example, given the input
__


%%
mega-    ECHO; yymore();
kludge   ECHO;
First yytext__ so the __ECHO__ for the __


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


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


Finally, note that you cannot put back __EOF__ to attempt
to mark the input stream with an end-of-file.


-


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


%%
(Note that if the scanner is compiled using __C++,__ then __input()__ is instead referred to as __yyinput(),__ in order to avoid a name clash with the __C++__ stream by the name of ''input.)''


-


__YY_FLUSH_BUFFER__ flushes the scanner's internal buffer
so that the next time the scanner attempts to match a token,
it will first refill the buffer using __YY_INPUT__ (see
The Generated Scanner, below). This action is a special case
of the more general __yy_flush_buffer()__ function,
described below in the section Multiple Input
Buffers.


-


__yyterminate()__ can be used in lieu of a return
statement in an action. It terminates the scanner and
returns a 0 to the scanner's caller, indicating
__yyterminate()__ is also called
when an end-of-file is encountered. It is a macro and may be
redefined.
!!THE GENERATED SCANNER


The output of ''flex'' is the file __lex.yy.c,__ which
contains the scanning routine __yylex(),__ a number of
tables used by it for matching tokens, and a number of
auxiliary routines and macros. By default, __yylex()__ is
declared as follows:


int yylex()
{
... various definitions and the actions in here ...
}
(If your environment supports function prototypes, then it will be


#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''


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
__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
''


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


%{
#define YY_INPUT(buf,result,max_size) \
{ \
int c = getchar(); \
result = (c == EOF) ? YY_NULL : (buf[[0] = c, 1); \
}
%}
This definition will change the input processing to occur one character at a time.


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


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


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


The scanner writes its __ECHO__ output to the
''yyout'' global (default, stdout), which may be
redefined by the user simply by assigning it to some other
__FILE__ pointer.
!!START CONDITIONS


''flex'' provides a mechanism for conditionally
activating rules. Any rule whose pattern is prefixed with
''


will be active only when the scanner is in the


will be active only when the current start condition is either


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
''


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
%%
is equivalent to


%x example
%%
Without the ____ 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 ____ to qualify ''bar,'' though, then it would only be active in __example__ and not in __INITIAL,__ while in the first example it's active in both, because in the first example the __example__ startion condition is an ''inclusive'' __(%s)__ start condition.


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


%x example
%%
The default rule (to __ECHO__ any unmatched character) remains active in start conditions. It is equivalent to:


__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 __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
__yylex()__
is called and the global variable ''enter_special'' is
true:


int enter_special;
%x SPECIAL
%%
if ( enter_special )
BEGIN(SPECIAL);
To illustrate the uses of start conditions, here is a scanner which provides two different interpretations of a string like


%{
#include
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;
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;
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__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;
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 '''' applied to it, until a '''}''' which matches the initial '''{'.'' So, for example,


is equivalent to:


Start condition scopes may be nested.


Three routines are available for manipulating stacks of
start conditions:


__void yy_push_state(int new_state)__


pushes the current start condition onto the top of the start
condition stack and switches to ''new_state'' as though
you had used __BEGIN new_state__ (recall that start
condition names are also integers).


__void yy_pop_state()__


pops the top of the stack and switches to it via
__BEGIN.__


__int yy_top_state()__


returns the top of the stack without altering the stack's
contents.


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


To use start condition stacks, your scanner must include a
__%option stack__ directive (see Options
below).
!!MULTIPLE INPUT BUFFERS


Some scanners (such as those which support
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
__


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
____ feature is discussed
below):


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


__yy_scan_string(const char *str)__


scans a NUL-terminated string.


__yy_scan_bytes(const char *bytes, int
len)__


scans ''len'' bytes (including possibly NUL's) starting
at location ''bytes.''


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


__yy_scan_buffer(char *base, yy_size_t
size)__


which scans in place the buffer starting at ''base,''
consisting of ''size'' bytes, the last two bytes of which
''must'' be __YY_END_OF_BUFFER_CHAR__ (ASCII NUL).
These last two bytes are not scanned; thus, scanning
consists of __base[[0]__ through __base[[size-2],__
inclusive.


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


The type __yy_size_t__ is an integral type to which you
can cast an integer expression reflecting the size of the
buffer.
!!END-OF-FILE RULES


The special rule


-


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


-


executing a ''return'' statement;


-


executing the special __yyterminate()__
action;


-


or, switching to a new buffer using
__yy_switch_to_buffer()__ as shown in the example
above.


all'' start conditions which do
not already have
''


These rules are useful for catching things like unclosed comments. An example:


%x quote
%%
...other rules for dealing with quotes...
!!MISCELLANEOUS MACROS


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


#define YY_USER_ACTION ++ctr[[yy_act]
where ''ctr'' is an array to hold the counts for the different rules. Note that the macro __YY_NUM_RULES__ gives the total number of rules (including the default rule, even if you use __-s),__ so a correct declaration for ''ctr'' is:


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


The macro __yy_set_interactive(is_interactive)__ can be
used to control whether the current buffer is considered
''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 always-interactive__ or
__%option never-interactive__ (see Options below).
__yy_set_interactive()__ must be invoked prior to
beginning to scan the buffer that is (or is not) to be
considered interactive.


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


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


In the generated scanner, the actions are all gathered in
one large switch statement and separated using
__YY_BREAK,__ which may be redefined. By default, it is
simply a
__YY_BREAK__
allows, for example, C++ users to #define YY_BREAK to do
nothing (while being very careful that every rule ends with
a
__YY_BREAK__ is inaccessible.
!!VALUES AVAILABLE TO THE USER


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


-


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


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


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


-


__int yyleng__ holds the length of the current
token.


-


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


-


__void yyrestart( FILE *new_file )__ may be called to
point ''yyin'' at the new input file. The switch-over to
the new file is immediate (any previously buffered-up input
is lost). Note that calling __yyrestart()__ with
''yyin'' as an argument thus throws away the current
input buffer and continues 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.
!!INTERFACING WITH YACC


One of the main uses of ''flex'' is as a companion to the
''yacc'' parser-generator. ''yacc'' parsers expect to
call a routine named __yylex()__ to find the next input
token. The routine is supposed to return the type of the
next token as well as putting any associated value in the
global __yylval.__ To use ''flex'' with ''yacc,''
one specifies the __-d__ option to ''yacc'' to
instruct it to generate the file __y.tab.h__ containing
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
''


%{
#include
!!OPTIONS


''flex'' has the following options:


__-b__


Generate backing-up information to ''lex.backup.'' This
is a list of scanner states which require backing up and the
input characters on which they do so. By adding rules one
can remove backing-up states. If ''all'' backing-up
states are eliminated and __-Cf__ or __-CF__ is used,
the generated scanner will run faster (see the __-p__
flag). Only users who wish to squeeze every last cycle out
of their scanners need worry about this option. (See the
section on Performance Considerations below.)


__-c__


is a do-nothing, deprecated option included for POSIX
compliance.


__-d__


makes the generated scanner run in ''debug'' mode.
Whenever a pattern is recognized and the global
__yy_flex_debug__ is non-zero (which is the default), the
scanner will write to ''stderr'' a line of the
form:


--accepting rule at line 53 (
The 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__


specifies ''fast scanner.'' No table compression is done
and stdio is bypassed. The result is large but fast. This
option is equivalent to __-Cfr__ (see
below).


__-h__


generates a flex's''
options to ''stdout'' and then exits. __-?__ and
__--help__ are synonyms for __-h.__


__-i__


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


__-l__


turns on maximum compatibility with the original AT
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
__YY_FLEX_LEX_COMPAT__
being #define'd in the generated scanner.


__-n__


is another do-nothing, deprecated option included only for
POSIX compliance.


__-p__


generates a performance report to stderr. The report
consists of comments regarding features of the ''flex''
input file which will cause a serious loss of performance in
the resulting scanner. If you give the flag twice, you will
also get comments regarding features that lead to minor
performance losses.


Note that the use of __REJECT, %option yylineno,__ and
variable trailing context (see the Deficiencies / Bugs
section below) entails a substantial performance penalty;
use of ''yymore(),'' the __^__ operator, and the
__-I__ flag entail minor performance
penalties.


__-s__


causes the ''default rule'' (that unmatched scanner input
is echoed to ''stdout)'' to be suppressed. If the scanner
encounters input that does not match any of its rules, it
aborts with an error. This option is useful for finding
holes in a scanner's rule set.


__-t__


instructs ''flex'' to write the scanner it generates to
standard output instead of __lex.yy.c.__


__-v__


specifies that ''flex'' should write to ''stderr'' a
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__


suppresses warning messages.


__-B__


instructs ''flex'' to generate a ''batch'' scanner,
the opposite of ''interactive'' scanners generated by
__-I__ (see below). In general, you use __-B__ when
you are ''certain'' that your scanner will never be used
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__


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
__


then you're better off using the full table representation. If only the -F.__


This option is equivalent to __-CFr__ (see below). It
cannot be used with __-+.__


__-I__


instructs ''flex'' to generate an ''interactive''
scanner. An interactive scanner is one that only looks ahead
to decide what token has been matched if it 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.


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


__-L__


instructs ''flex'' not to generate __#line__
directives. Without this option, ''flex'' peppers the
generated scanner with #line directives so error messages in
the actions will be correctly located with respect to either
the original ''flex'' input file (if the errors are due
to code in the input file), or __lex.yy.c__ (if the
errors are ''flex's'' fault -- you should report these
sorts of errors to the email address given
below).


__-T__


makes ''flex'' run in ''trace'' mode. It will generate
a lot of messages to ''stderr'' concerning the form of
the input and the resultant non-deterministic and
deterministic finite automata. This option is mostly for use
in maintaining ''flex.''


__-V__


prints the version number to ''stdout'' and exits.
__--version__ is a synonym for __-V.__


__-7__


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


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.


__-+__


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


__-C[[aefFmr]__


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


__-Ca__ (
__


__-Ce__ directs ''flex'' to construct ''equivalence
classes,'' i.e., sets of characters which have identical
lexical properties (for example, if the only appearance of
digits in the ''flex'' input is in the character class
''


__-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 alternate fast scanner
representation (described above under the __-F__ flag)
should be used. This option cannot be used with
__-+.__


__-Cm__ directs ''flex'' to construct
''meta-equivalence classes,'' which are sets of
equivalence classes (or characters, if equivalence classes
are not being used) that are commonly used together.
Meta-equivalence classes are often a big win when using
compressed tables, but they have a moderate performance
impact (one or two
''


__-Cr__ causes the generated scanner to ''bypass'' use
of the standard I/O library (stdio) for input. Instead of
calling __fread()__ or __getc(),__ the scanner will
use the __read()__ system call, resulting in a
performance gain which varies from system to 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
Note that scanners with the smallest tables are usually generated and compiled the quickest, so during development you will usually want to use the default, maximal compression.


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


__-ooutput__


directs flex to write the scanner to the file __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__


changes the default ''yy'' prefix used by ''flex'' for
all globally-visible variable and function names to instead
be ''prefix.'' For example, __-Pfoo__ changes the name
of __yytext__ to __footext.__ It also changes the name
of the default output file from __lex.yy.c__ to
__lex.foo.c.__ Here are all of the names
affected:


yy_create_buffer
yy_delete_buffer
yy_flex_debug
yy_init_buffer
yy_flush_buffer
yy_load_buffer_state
yy_switch_to_buffer
yyin
yyleng
yylex
yylineno
yyout
yyrestart
yytext
yywrap
(If you are using a C++ scanner, then only __yywrap__ and __yyFlexLexer__ are affected.) Within your scanner itself, you can still refer to the global variables and functions using either version of their name; but externally, they have the modified name.


This option lets you easily link together multiple
''flex'' programs into the same executable. Note, though,
that using this option also renames __yywrap(),__ so you
now ''must'' either provide your own
(appropriately-named) version of the routine for your
scanner, or use __%option noyywrap,__ as linking with
__-lfl__ no longer provides one for you by
default.


__-Sskeleton_file__


overrides the default skeleton file from which ''flex''
constructs its scanners. You'll never need this option
unless you are doing ''flex'' maintenance or
development.


''flex'' also provides a mechanism for controlling
options within the scanner specification itself, rather than
from the flex command-line. This is done by including
__%option__ directives in the first section of the
scanner 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


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
Some __%option's__ provide features otherwise not available:


__always-interactive__


instructs flex to generate a scanner which always considers
its input
isatty()__ in an attempt
to determine whether the scanner's input source is
interactive and thus should be read a character at a time.
When this option is used, however, then no such call is
made.


__main__


directs flex to provide a default __main()__ program for
the scanner, which simply calls __yylex().__ This option
implies __noyywrap__ (see below).


__never-interactive__


instructs flex to generate a scanner which never considers
its input
isatty()).__ This is the opposite of
__always-interactive.__


__stack__


enables the use of start condition stacks (see Start
Conditions above).


__stdinit__


if set (i.e., __%option stdinit)__ initializes
''yyin'' and ''yyout'' to ''stdin'' and
''stdout,'' instead of the default of ''nil.'' Some
existing ''lex'' programs depend on this behavior, even
though it is not compliant with ANSI C, which does not
require ''stdin'' and ''stdout'' to be compile-time
constant. In a reentrant scanner, however, this is not a
problem since initialization is performed in
''yylex_init'' at runtime.


__yylineno__


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


__yywrap__


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


''flex'' scans 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
noyymore).__


Three options take string-delimited values, offset with
'=':


%option outfile=
is equivalent to __-oABC,__ and


%option prefix=
is equivalent to __-PXYZ.__ Finally,


%option yyclass=
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
(though __yy_push_state()__ and friends won't appear anyway unless you use __%option stack).__
!!PERFORMANCE CONSIDERATIONS


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


REJECT
%option yylineno
arbitrary trailing context
pattern sets that require backing up
%array
%option interactive
%option always-interactive
'^' beginning-of-line operator
yymore()
with the first three all being quite expensive and the last two being quite cheap. Note also that __unput()__ is implemented as a routine call that potentially does quite a bit of work, while __yyless()__ is a quite-cheap macro; so if just putting back some excess text you scanned, use __yyless().__


__REJECT__ should be avoided at all costs when
performance is important. It is a particularly expensive
option.


Getting rid of backing up is messy and often may be an
enormous amount of work for a complicated scanner. In
principal, one begins by using the __-b__ flag to
generate a ''lex.backup'' file. For example, on the
input


%%
foo        return TOK_KEYWORD;
foobar     return TOK_KEYWORD;
the file looks like:


State #6 is non-accepting -
associated rule line numbers:
2       3
out-transitions: [[ o ]
jam-transitions: EOF [[ 001-n  p-177 ]
State #8 is non-accepting -
associated rule line numbers:
3
out-transitions: [[ a ]
jam-transitions: EOF [[ 001-`  b-177 ]
State #9 is non-accepting -
associated rule line numbers:
3
out-transitions: [[ r ]
jam-transitions: EOF [[ 001-q  s-177 ]
Compressed tables always back up.
The first few lines tell us that there's a scanner state in which it can make a transition on an 'o' but not on any other character, and that in that state the currently scanned text does not match any rule. The state occurs when trying to match the rules found at lines 2 and 3 in the input file. If the scanner is in that state and then reads something other than an 'o', it will have to back up to find a rule which is matched. With a bit of headscratching one can see that this must be the state it's in when it has seen


The comment regarding State #8 indicates there's a problem
when


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


%%
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


%%
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;
This could be sped up by writing it as:


%x comment
%%
int line_num = 1;
Now instead of each newline requiring the processing of another action, recognizing the newlines is 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:


%%
asmn    |
auton   |
breakn  |
... etc ...
volatilen |
whilen  /* 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 ''


%%
asmn    |
auton   |
breakn  |
... etc ...
volatilen |
whilen  /* 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
__
!!GENERATING C++ SCANNERS


''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 __istream*__ object pointer and not
a __FILE*), yy_flush_buffer(), yy_delete_buffer(),__ and
__yyrestart()__ (again, the first argument is a
__istream*__ object pointer).


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


__yyFlexLexer( istream* arg_yyin = 0, ostream* arg_yyout =
0 )__


constructs a __yyFlexLexer__ object using the given
streams for input and output. If not specified, the streams
default to __cin__ and __cout,__
respectively.


__virtual int yylex()__


performs the same role is __yylex()__ does for ordinary
flex scanners: it scans the input stream, consuming tokens,
until a rule's action returns a value. If you derive a
subclass __S__ from __yyFlexLexer__ and want to access
the member functions and variables of __S__ inside
__yylex(),__ then you need to use __%option
yyclass=__ to inform ''flex'' that you
will be using that subclass instead of __yyFlexLexer.__
In this case, rather than generating
__yyFlexLexer::yylex(),__ ''flex'' generates
__S::yylex()__ (and also generates a dummy
__yyFlexLexer::yylex()__ that calls
__yyFlexLexer::!LexerError()__ if called).


__virtual void switch_streams(istream* new_in =
0,__


__ostream* new_out = 0)__ reassigns __yyin__ to
__new_in__ (if non-nil) and __yyout__ to
__new_out__ (ditto), deleting the previous input buffer
if __yyin__ is reassigned.


__int yylex( istream* new_in, ostream* new_out = 0
)__


first switches the input streams via __switch_streams(
new_in, new_out )__ and then returns the value of
__yylex().__


In addition, __yyFlexLexer__ defines the following
protected virtual functions which you can redefine in
derived classes to tailor the scanner:


__virtual int !LexerInput( char* buf, int max_size
)__


reads up to __max_size__ characters into __buf__ and
returns the number of characters read. To indicate
end-of-input, return 0 characters. Note that
__-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
__


__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
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 ____ in your other sources once per lexer class, first renaming __yyFlexLexer__ as follows:


#undef yyFlexLexer
#define yyFlexLexer xxFlexLexer
#include
if, for example, you used __%option prefix=__ for one of your scanners and __%option prefix=__ for the other.


IMPORTANT: the present form of the scanning class is
''experimental'' and may change considerably between
major releases.
!!INCOMPATIBILITIES WITH LEX AND POSIX


''flex'' is a rewrite of the AT''lex''
tool (the two implementations do not share any code,
though), with some extensions and incompatibilities, both of
which are of concern to those who wish to write scanners
acceptable to either implementation. Flex is fully compliant
with the POSIX ''lex'' specification, except that when
using __%pointer__ (the default), a call to
__unput()__ destroys the contents of __yytext,__ which
is counter to the POSIX specification.


In this section we discuss all of the known areas of
incompatibility between flex, AT


''flex's'' __-l__ option turns on maximum
compatibility with the original AT__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 POSIX.


-


''flex'' scanners are not as reentrant as ''lex''
scanners. In particular, if you have an interactive scanner
and an interrupt handler which long-jumps out of the
scanner, and the scanner is subsequently called again, you
may get the following message:


fatal flex scanner internal error--end of buffer missed
To reenter the scanner, first use


yyrestart( yyin );
Note that this call will throw away any buffered input; usually this isn't a problem with an 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
''


-


__output()__ is not supported. Output from the
__ECHO__ macro is done to the file-pointer ''yyout''
(default ''stdout).''


__output()__ is not part of the POSIX
specification.


-


''lex'' does not support exclusive start conditions (%x),
though they are in the POSIX specification.


-


When definitions are expanded, ''flex'' encloses them in
parentheses. With lex, the following:


NAME    [[A-Z][[A-Z0-9]*
%%
foo{NAME}?      printf(
will not match the string flex,'' the rule will be expanded to ''


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
____ and ____
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
''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
''flex'' interprets it as
''


-


The precedence of the __^__ operator is different.
''lex'' interprets
''flex'' interprets it as
''


-


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
%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()
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.
!!DIAGNOSTICS


''warning, rule cannot be matched'' indicates that the
given rule cannot be matched because it follows other rules
that will always match the same text as it. For example, in
the following
''


[[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 an scanner which is reentered after
a long-jump has jumped out (or over) the scanner's
activation frame. Before reentering the scanner,
use:


yyrestart( yyin );
or, as noted above, switch to using the C++ scanner class.


''too many start conditions in ''
you listed more start conditions in a
''
!!FILES


__-lfl__


library with which scanners must be linked.


''lex.yy.c''


generated scanner (called ''lexyy.c'' on some
systems).


''lex.yy.cc''


generated C++ scanner class, when using
__-+.__


''''


header file defining the C++ scanner base class,
__!FlexLexer,__ and its derived class,
__yyFlexLexer.__


''flex.skl''


skeleton scanner. This file is only used when building flex,
not when flex executes.


''lex.backup''


backing-up information for __-b__ flag (called
''lex.bck'' on some systems).
!!DEFICIENCIES / BUGS


Some trailing context patterns cannot be properly matched
and generate warning messages (


For some trailing context rules, parts which are actually
fixed-length are not recognized as such, leading to the
abovementioned performance loss. In particular, parts using
'|' or {n} (such as


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
getchar(),__ with ''flex'' rules and expect
it to work. Call __input()__ instead.


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


__REJECT__ cannot be used with the __-f__ or __-F__
options.


The ''flex'' internal algorithms need
documentation.
!!SEE ALSO


lex(1), yacc(1), sed(1), awk(1).


John Levine, Tony Mason, and Doug Brown, ''Lex
'' O'Reilly and Associates. Be sure to get the 2nd
edition.


M. E. Lesk and E. Schmidt, ''LEX - Lexical Analyzer
Generator''


Alfred Aho, Ravi Sethi and Jeffrey Ullman, ''Compilers:
Principles, Techniques and Tools,'' Addison-Wesley (1986).
Describes the pattern-matching techniques used by
''flex'' (deterministic finite automata).
!!AUTHOR


Vern Paxson, with the help of many ideas and much
inspiration from Van Jacobson. Original version by Jef
Poskanzer. The fast table representation is a partial
implementation of a design done by Van Jacobson. The
implementation was done by Kevin Gong and Vern
Paxson.


Thanks to the many ''flex'' beta-testers, feedbackers,
and contributors, especially Francois Pinard, Casey Leedom,
Robert Abramovitz, Stan Adermann, Terry Allen, David
Barker-Plummer, John Basrai, Neal Becker, Nelson H.F. Beebe,
benson@odi.com, Karl Berry, Peter A. Bigot, Simon Blanchard,
Keith Bostic, Frederic Brehm, Ian Brockbank, Kin Cho, Nick
Christopher, Brian Clapper, J.T. Conklin, Jason Coughlin,
Bill Cox, Nick Cropper, Dave Curtis, Scott David Daniels,
Chris G. Demetriou, Theo Deraadt, Mike Donahue, Chuck
Doucette, Tom Epperly, Leo Eskin, Chris Faylor, Chris
Flatters, Jon Forrest, Jeffrey Friedl, Joe Gayda, Kaveh R.
Ghazi, Wolfgang Glunz, Eric Goldman, Christopher M. Gould,
Ulrich Grepel, Peer Griebel, Jan Hajic, Charles Hemphill,
NORO Hideo, Jarkko Hietaniemi, Scott Hofmann, Jeff Honig,
Dana Hudes, Eric Hughes, John Interrante, Ceriel Jacobs,
Michal Jaegermann, Sakari Jalovaara, Jeffrey R. Jones, Henry
Juengst, Klaus Kaempf, Jonathan I. Kamens, Terrence O Kane,
Amir Katz, ken@ken.hilco.com, Kevin B. Kenny, Steve Kirsch,
Winfried Koenig, Marq Kole, Ronald Lamprecht, Greg Lee,
Rohan Lenard, Craig Leres, John Levine, Steve Liddle, David
Loffredo, Mike Long, Mohamed el Lozy, Brian Madsen, Malte,
Joe Marshall, Bengt Martensson, Chris Metcalf, Luke Mewburn,
Jim Meyering, R. Alexander Milowski, Erik Naggum, G.T.
Nicol, Landon Noll, James Nordby, Marc Nozell, Richard
Ohnemus, Karsten Pahnke, Sven Panne, Roland Pesch, Walter
Pelissero, Gaumond Pierre, Esmond Pitt, Jef Poskanzer, Joe
Rahmeh, Jarmo Raiha, Frederic Raimbault, Pat Rankin, Rick
Richardson, Kevin Rodgers, Kai Uwe Rommel, Jim Roskind,
Alberto Santini, Andreas Scherer, Darrell Schiebel, Raf
Schietekat, Doug Schmidt, Philippe Schnoebelen, Andreas
Schwab, Larry Schwimmer, Alex Siegel, Eckehard Stolz,
Jan-Erik Strvmquist, Mike Stump, Paul Stuart, Dave Tallman,
Ian Lance Taylor, Chris Thewalt, Richard M. Timoney, Jodi
Tsai, Paul Tuinenga, Gary Weik, Frank Whaley, Gerhard
Wilhelms, Kent Williams, Ken Yap, Ron Zellar, Nathan Zelle,
David Zuhn, and those whose names have slipped my marginal
mail-archiving skills but whose contributions are
appreciated all the same.


Thanks to Keith Bostic, Jon Forrest, Noah Friedman, John
Gilmore, Craig Leres, John Levine, Bob Mulcahy, G.T. Nicol,
Francois Pinard, Rich Salz, and Richard Stallman for help
with various distribution headaches.


Thanks to Esmond Pitt and Earle Horton for 8-bit character
support; to Benson Margulies and Fred Burke for C++ support;
to Kent Williams and Tom Epperly for C++ class support; to
Ove Ewerlid for support of NUL's; and to Eric Hughes for
support of multiple buffers.


This work was primarily done when I was with the Real Time
Systems Group at the Lawrence Berkeley Laboratory in
Berkeley, CA. Many thanks to all there for the support I
received.


Send comments to vern@ee.lbl.gov.
----
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