FLEX(1)FLEX(1)NAMEflexdoc - fast lexical analyzer generator
SYNOPSIS
flex [-bcdfinpstvFILT8 -C[efmF] -Sskeleton] [filename ...]
DESCRIPTION
flex is a tool for generating scanners: programs which recognized lexi‐
cal patterns in text. flex reads the given input files, or its stan‐
dard 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 corre‐
sponding C code.
SOME SIMPLE EXAMPLES
First some simple examples to get the flavor of how one uses flex. The
following flex input specifies a scanner which whenever it encounters
the string "username" will replace it with the user's login name:
%%
username printf( "%s", getlogin() );
By default, any text not matched by a flex scanner is copied to the
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 new‐
line (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.
The details of this example will be explained in the following sec‐
tions.
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 defini‐
tions 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 def‐
inition is taken to begin at the first non-white-space character fol‐
lowing 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 fol‐
lowed by zero-or-more letters-or-digits. A subsequent reference to
{DIGIT}+"."{DIGIT}*
is identical to
([0-9])+"."([0-9])*
and matches one-or-more digits followed by a '.' followed by zero-or-
more digits.
The rules section of the flex input contains a series of rules of the
form:
pattern action
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 scan‐
ner. 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 exe‐
cuted 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 fea‐
tures).
In the definitions section, an unindented comment (i.e., a line begin‐
ning with "/*") is also copied verbatim to the output up to the next
"*/". Also, any line in the definitions section beginning with '#' is
ignored, though this style of comment is deprecated and may go away in
the future.
PATTERNS
The patterns in the input are written using an extended set of regular
expressions. These are:
x match the character 'x'
. any character 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 '*')
\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
s is not part of the matched text. This type
of pattern is called as "trailing context".
^r an r, but only at the beginning of a line
r$ an r, but only at the end of a line. Equivalent
to "r/\n".
<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
<<EOF>> an end-of-file
<s1,s2><<EOF>>
an end-of-file when in start condition s1 or s2
The regular expressions listed above are grouped according to prece‐
dence, 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)*
Some notes on patterns:
- A negated character class such as the example "[^A-Z]" above
will match a newline unless "\n" (or an equivalent escape
sequence) is one of the characters explicitly present in the
negated character class (e.g., "[^A-Z\n]"). This is unlike how
many other regular expression tools treat negated character
classes, but unfortunately the inconsistency is historically
entrenched. Matching newlines means that a pattern like [^"]*
can match an entire input (overflowing the scanner's input buf‐
fer) unless there's another quote in the input.
- A rule can have at most one instance of trailing context (the
'/' operator or the '$' operator). The start condition, '^',
and "<<EOF>>" patterns can only occur at the beginning of a pat‐
tern, and, as well as with '/' and '$', cannot be grouped inside
parentheses. A '^' which does not occur at the beginning of a
rule or a '$' which does not occur at the end of a rule loses
its special properties and is treated as a normal character.
The following are illegal:
foo/bar$
<sc1>foo<sc2>bar
Note that the first of these, can be written "foo/bar\n".
The following will result in '$' or '^' being treated as a nor‐
mal character:
foo|(bar$)
foo|^bar
If what's wanted is a "foo" or a bar-followed-by-a-newline, the
following could be used (the special '|' action is explained
below):
foo |
bar$ /* action goes here */
A similar trick will work for matching a foo or a bar-at-the-
beginning-of-a-line.
HOW THE INPUT IS MATCHED
When the generated scanner is run, it analyzes its input looking for
strings which match any of its patterns. If it finds more than one
match, it takes the one matching the most text (for trailing context
rules, this includes the length of the trailing part, even though it
will then be returned to the input). If it finds two or more matches
of the same length, the rule listed first in the flex input file is
chosen.
Once the match is determined, the text corresponding to the match
(called the token) is made available in the global character pointer
yytext, and its length in the global integer yyleng. The action corre‐
sponding 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 char‐
acter 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.
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 "zap me" from its input:
%%
"zap me"
(It will copy all other characters in the input to the output since
they will be matched by the default rule.)
Here is a program which compresses multiple blanks and tabs down to a
single blank, and throws away whitespace found at the end of a line:
%%
[ \t]+ putchar( ' ' );
[ \t]+$ /* ignore this token */
If the action contains a '{', then the action spans till the balancing
'}' is found, and the action may cross multiple lines. flex knows
about C strings and comments and won't be fooled by braces found within
them, but also allows actions to begin with %{ and will consider the
action to be all the text up to the next %} (regardless of ordinary
braces inside the action).
An action consisting solely of a vertical bar ('|') means "same as the
action for the next rule." See below for an illustration.
Actions can include arbitrary C code, including return statements to
return a value to whatever routine called yylex(). Each time yylex()
is called it continues processing tokens from where it last left off
until it either reaches the end of the file or executes a return. Once
it reaches an end-of-file, however, then any subsequent call to yylex()
will simply immediately return, unless yyrestart() is first called (see
below).
Actions are not allowed to modify yytext or yyleng.
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 scan‐
ner in the corresponding start condition (see below).
- REJECT directs the scanner to proceed on to the "second best"
rule which matched the input (or a prefix of the input). The
rule is chosen as described above in "How the Input is Matched",
and yytext and yyleng set up appropriately. It may either be
one which matched as much text as the originally chosen rule but
came later in the flex input file, or one which matched less
text. For example, the following will both count the words in
the input and call the routine special() whenever "frob" is
seen:
int word_count = 0;
%%
frob special(); REJECT;
[^ \t\n]+ ++word_count;
Without the REJECT, any "frob"'s in the input would not be
counted as words, since the scanner normally executes only one
action per token. Multiple REJECT's are allowed, each one find‐
ing the next best choice to the currently active rule. For
example, when the following scanner scans the token "abcd", it
will write "abcdabcaba" to the output:
%%
a |
ab |
abc |
abcd ECHO; REJECT;
.|\n /* eat up any unmatched character */
(The first three rules share the fourth's action since they use
the special '|' action.) REJECT is a particularly expensive
feature in terms 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 -f or -F
options (see below).
Note also that unlike the other special actions, REJECT is a
branch; code immediately following it in the action will not be
executed.
- yymore() tells the scanner that the next time it matches a rule,
the corresponding token should be appended onto the current
value of yytext rather than replacing it. For example, given
the input "mega-kludge" the following will write "mega-mega-
kludge" to the output:
%%
mega- ECHO; yymore();
kludge ECHO;
First "mega-" is matched and echoed to the output. Then
"kludge" is matched, but the previous "mega-" is still hanging
around at the beginning of yytext so the ECHO for the "kludge"
rule will actually write "mega-kludge". The presence of
yymore() in the scanner's action entails a minor performance
penalty in the scanner's matching speed.
- yyless(n) returns all but the first n characters of the current
token back to the input stream, where they will be rescanned
when the scanner looks for the next match. yytext and yyleng
are adjusted appropriately (e.g., yyleng will now be equal to n
). For example, on the input "foobar" the following will write
out "foobarbar":
%%
foobar ECHO; yyless(3);
[a-z]+ ECHO;
An argument of 0 to yyless will cause the entire current input
string to be scanned again. Unless you've changed how the scan‐
ner will subsequently process its input (using BEGIN, for exam‐
ple), this will result in an endless loop.
- 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;
unput( ')' );
for ( i = yyleng - 1; i >= 0; --i )
unput( yytext[i] );
unput( '(' );
}
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.
- input() reads the next character from the input stream. For
example, the following is one way to eat up C comments:
%%
"/*" {
register int c;
for ( ; ; )
{
while ( (c = input()) != '*' &&
c != EOF )
; /* eat up text of comment */
if ( c == '*' )
{
while ( (c = input()) == '*' )
;
if ( c == '/' )
break; /* found the end */
}
if ( c == EOF )
{
error( "EOF in comment" );
break;
}
}
}
(Note that if the scanner is compiled using C++, then input() is
instead referred to as yyinput(), in order to avoid a name clash
with the C++ stream by the name of input.)
- yyterminate() can be used in lieu of a return statement in an
action. It terminates the scanner and returns a 0 to the scan‐
ner's caller, indicating "all done". Subsequent calls to the
scanner will immediately return unless preceded by a call to
yyrestart() (see below). By default, yyterminate() is also
called when an end-of-file is encountered. It is a macro and
may be redefined.
THE GENERATED SCANNER
The output of flex is the file lex.yy.c, which contains the scanning
routine yylex(), a number of tables used by it for matching tokens, and
a number of auxiliary routines and macros. By default, yylex() is
declared as follows:
int yylex()
{
... various definitions and the actions in here ...
}
(If your environment supports function prototypes, then it will be "int
yylex( void )".) This definition may be changed by redefining the
"YY_DECL" macro. For example, you could use:
#undef YY_DECL
#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. In the former case, when called
again the scanner will immediately return unless yyrestart() is called
to point yyin at the new input file. ( yyrestart() takes one argument,
a FILE * pointer.) In the latter case (i.e., when an action executes a
return), the scanner may then be called again and it will resume scan‐
ning 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 redefining 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 vari‐
able 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 redefinition of YY_INPUT (in the definitions section of the
input file):
%{
#undef YY_INPUT
#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.
You also can add in things like keeping track of the input line number
this way; but don't expect your scanner to go very fast.
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 call‐
er.
The default yywrap() always returns 1. Presently, to redefine it you
must first "#undef yywrap", as it is currently implemented as a macro.
As indicated by the hedging in the previous sentence, it may be changed
to a true function in the near future.
The scanner writes its ECHO output to the yyout global (default, std‐
out), 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 "<sc>" will only be active when the
scanner is in the start condition named "sc". For example,
<STRING>[^"]* { /* eat up the string body ... */
...
}
will be active only when the scanner is in the "STRING" start condi‐
tion, and
<INITIAL,STRING,QUOTE>\. { /* handle an escape ... */
...
}
will be active only when the current start condition is either "INI‐
TIAL", "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 inclu‐
sive, 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 connec‐
tion between the two. The set of rules:
%s example
%%
<example>foo /* do something */
is equivalent to
%x example
%%
<INITIAL,example>foo /* do something */
The default rule (to ECHO any unmatched character) remains active in
start conditions.
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 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);
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);
One can then implement a "stack" of start conditions using an array of
integers. (It is likely that such stacks will become a full-fledged
flex feature in the future.) Note, though, that start conditions do
not have their own name-space; %s's and %x's declare names in the same
fashion as #define's.
MULTIPLE INPUT BUFFERS
Some scanners (such as those which support "include" files) require
reading from several input streams. As flex scanners do a large amount
of buffering, one cannot control where the next input will be read from
by simply writing a YY_INPUT which is sensitive to the scanning con‐
text. YY_INPUT is only called when the scanner reaches the end of its
buffer, which may be a long time after scanning a statement such as an
"include" which requires switching the input source.
To negotiate these sorts of problems, flex provides a mechanism for
creating and switching between multiple input buffers. An input buffer
is created by using:
YY_BUFFER_STATE yy_create_buffer( FILE *file, int size )
which takes a FILE pointer and a size and creates a buffer associated
with the given file and large enough to hold size characters (when in
doubt, use YY_BUF_SIZE for the size). It returns a YY_BUFFER_STATE
handle, which may then be passed to other routines:
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
sets things up for continued scanning, instead of opening a new file
and pointing yyin at it.
void yy_delete_buffer( YY_BUFFER_STATE buffer )
is used to reclaim the storage associated with a buffer.
yy_new_buffer() is an alias for yy_create_buffer(), provided for com‐
patibility with the C++ use of new and delete for creating and destroy‐
ing 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_switch_to_buffer(
include_stack[include_stack_ptr] );
}
END-OF-FILE RULES
The special rule "<<EOF>>" indicates actions which are to be taken when
an end-of-file is encountered and yywrap() returns non-zero (i.e.,
indicates no further files to process). The action must finish by
doing one of four things:
- the special YY_NEW_FILE action, if yyin has been pointed at a
new file to process;
- a return statement;
- the special yyterminate() action;
- or, switching to a new buffer using yy_switch_to_buffer() as
shown in the example above.
<<EOF>> rules may not be used with other patterns; they may only be
qualified with a list of start conditions. If an unqualified <<EOF>>
rule is given, it applies to all start conditions which do not already
have <<EOF>> actions. To specify an <<EOF>> rule for only the initial
start condition, use
<INITIAL><<EOF>>
These rules are useful for catching things like unclosed comments. An
example:
%x quote
%%
...other rules for dealing with quotes...
<quote><<EOF>> {
error( "unterminated quote" );
yyterminate();
}
<<EOF>> {
if ( *++filelist )
{
yyin = fopen( *filelist, "r" );
YY_NEW_FILE;
}
else
yyterminate();
}
MISCELLANEOUS MACROS
The macro YY_USER_ACTION can be redefined 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.
The macro YY_USER_INIT may be redefined to provide an action which is
always executed before the first scan (and before the scanner's inter‐
nal initializations are done). For example, it could be used to call a
routine to read in a data table or open a logging file.
In the generated scanner, the actions are all gathered in one large
switch statement and separated using YY_BREAK, which may be redefined.
By default, it is simply a "break", to separate each rule's action from
the following rule's. Redefining YY_BREAK allows, for example, C++
users to #define YY_BREAK to do nothing (while being very careful that
every rule ends with a "break" or a "return"!) to avoid suffering from
unreachable statement warnings where because a rule's action ends with
"return", the YY_BREAK is inaccessible.
INTERFACING WITH YACC
One of the main uses of flex is as a companion to the yacc parser-gen‐
erator. yacc parsers expect to call a routine named yylex() to find
the next input token. The routine is supposed to return the type of
the next token as well as putting any associated value in the global
yylval. To use flex with yacc, one specifies the -d option to yacc to
instruct it to generate the file y.tab.h containing definitions of all
the %tokens appearing in the yacc input. This file is then included in
the flex scanner. For example, if one of the tokens is "TOK_NUMBER",
part of the scanner might look like:
%{
#include "y.tab.h"
%}
%%
[0-9]+ yylval = atoi( yytext ); return TOK_NUMBER;
TRANSLATION TABLE
In the name of POSIX compliance, flex supports a translation table for
mapping input characters into groups. The table is specified in the
first section, and its format looks like:
%t
1 abcd
2 ABCDEFGHIJKLMNOPQRSTUVWXYZ
52 0123456789
6 \t\ \n
%t
This example specifies that the characters 'a', 'b', 'c', and 'd' are
to all be lumped into group #1, upper-case letters in group #2, digits
in group #52, tabs, blanks, and newlines into group #6, and no other
characters will appear in the patterns. The group numbers are actually
disregarded by flex; %t serves, though, to lump characters together.
Given the above table, for example, the pattern "a(AA)*5" is equivalent
to "d(ZQ)*0". They both say, "match any character in group #1, fol‐
lowed by zero-or-more pairs of characters from group #2, followed by a
character from group #52." Thus %t provides a crude way for introduc‐
ing equivalence classes into the scanner specification.
Note that the -i option (see below) coupled with the equivalence
classes which flex automatically generates take care of virtually all
the instances when one might consider using %t. But what the hell,
it's there if you want it.
OPTIONS
flex has the following options:
-b Generate backtracking information to lex.backtrack. This is a
list of scanner states which require backtracking and the input
characters on which they do so. By adding rules one can remove
backtracking states. If all backtracking states are eliminated
and -f or -F 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 compli‐
ance.
NOTE: in previous releases of flex -c specified table-compres‐
sion options. This functionality is now given by the -C flag.
To ease the the impact of this change, when flex encounters -c,
it currently issues a warning message and assumes that -C was
desired instead. In the future this "promotion" of -c to -C
will go away in the name of full POSIX compliance (unless the
POSIX meaning is removed first).
-d makes the generated scanner run in debug mode. Whenever a pat‐
tern is recognized and the global yy_flex_debug is non-zero
(which is the default), the scanner will write to stderr a line
of the form:
--accepting rule at line 53 ("the matched text")
The line number refers to the location of the rule in the file
defining the scanner (i.e., the file that was fed to flex).
Messages are also generated when the scanner backtracks, 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 (take your pick) full table or fast scanner. No table
compression is done. The result is large but fast. This option
is equivalent to -Cf (see below).
-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).
-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 loss of performance in the resulting scanner. Note that
the use of REJECT and variable trailing context (see the BUGS
section in flex(1)) 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 statis‐
tics 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, which is useful for figuring out
where you stand with respect to patches and new releases, and
the next two lines give the date when the scanner was created
and a summary of the flags which were in effect.
-F specifies that the fast scanner table representation should be
used. This representation is about as fast as the full table
representation (-f), and for some sets of patterns will be con‐
siderably smaller (and for others, larger). In general, if the
pattern set contains both "keywords" and a catch-all, "identi‐
fier" rule, such as in the set:
"case" return TOK_CASE;
"switch" return TOK_SWITCH;
...
"default" return TOK_DEFAULT;
[a-z]+ return TOK_ID;
then you're better off using the full table representation. If
only the "identifier" rule is present and you then use a hash
table or some such to detect the keywords, you're better off
using -F.
This option is equivalent to -CF (see below).
-I instructs flex to generate an interactive scanner. Normally,
scanners generated by flex always look ahead one character
before deciding that a rule has been matched. At the cost of
some scanning overhead, flex will generate a scanner which only
looks ahead when needed. Such scanners are called interactive
because if you want to write a scanner for an interactive system
such as a command shell, you will probably want the user's input
to be terminated with a newline, and without -I the user will
have to type a character in addition to the newline in order to
have the newline recognized. This leads to dreadful interactive
performance.
If all this seems to confusing, here's the general rule: if a
human will be typing in input to your scanner, use -I, otherwise
don't; if you don't care about squeezing the utmost performance
from your scanner and you don't want to make any assumptions
about the input to your scanner, use -I.
Note, -I cannot be used in conjunction with full or fast tables,
i.e., the -f, -F, -Cf, or -CF flags.
-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 the original flex input file, and not to the fairly
meaningless line numbers of lex.yy.c. (Unfortunately flex does
not presently generate the necessary directives to "retarget"
the line numbers for those parts of lex.yy.c which it generated.
So if there is an error in the generated code, a meaningless
line number is reported.)
-T makes flex run in trace mode. It will generate a lot of mes‐
sages to stdout concerning the form of the input and the resul‐
tant non-deterministic and deterministic finite automata. This
option is mostly for use in maintaining flex.
-8 instructs flex to generate an 8-bit scanner, i.e., one which can
recognize 8-bit characters. On some sites, flex is installed
with this option as the default. On others, the default is
7-bit characters. To see which is the case, check the verbose
(-v) output for "equivalence classes created". If the denomina‐
tor of the number shown is 128, then by default flex is generat‐
ing 7-bit characters. If it is 256, then the default is 8-bit
characters and the -8 flag is not required (but may be a good
idea to keep the scanner specification portable). Feeding a
7-bit scanner 8-bit characters will result in infinite loops,
bus errors, or other such fireworks, so when in doubt, use the
flag. Note that if equivalence classes are used, 8-bit scanners
take only slightly more table space than 7-bit scanners (128
bytes, to be exact); if equivalence classes are not used, how‐
ever, then the tables may grow up to twice their 7-bit size.
-C[efmF]
controls the degree of table compression.
-Ce directs flex to construct equivalence classes, i.e., sets of
characters which have identical lexical properties (for example,
if the only appearance of digits in the flex input is in the
character class "[0-9]" then the digits '0', '1', ..., '9' will
all be put in the same equivalence class). Equivalence classes
usually give dramatic reductions in the final table/object file
sizes (typically a factor of 2-5) and are pretty cheap perfor‐
mance-wise (one array look-up per character scanned).
-Cf specifies that the full scanner tables should be generated -
flex should not compress the tables by taking advantages of sim‐
ilar transition functions for different states.
-CF specifies that the alternate fast scanner representation
(described above under the -F flag) should be used.
-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 com‐
pressed tables, but they have a moderate performance impact (one
or two "if" tests and one array look-up per character scanned).
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 ta‐
ble is not being compressed. Otherwise the options may be
freely mixed.
The default setting is -Cem, which specifies that flex should
generate equivalence classes and meta-equivalence classes. This
setting provides the highest degree of table compression. You
can trade off faster-executing scanners at the cost of larger
tables with the following generally being true:
slowest & smallest
-Cem
-Cm
-Ce
-C
-C{f,F}e
-C{f,F}
fastest & largest
Note that scanners with the smallest tables are usually gener‐
ated 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 pro‐
duction scanners.
-C options are not cumulative; whenever the flag is encountered,
the previous -C settings are forgotten.
-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.
PERFORMANCE CONSIDERATIONS
The main design goal of flex is that it generate high-performance scan‐
ners. It has been optimized for dealing well with large sets of rules.
Aside from the effects of table compression on scanner speed outlined
above, there are a number of options/actions which degrade performance.
These are, from most expensive to least:
REJECT
pattern sets that require backtracking
arbitrary trailing context
'^' beginning-of-line operator
yymore()
with the first three all being quite expensive and the last two being
quite cheap.
REJECT should be avoided at all costs when performance is important.
It is a particularly expensive option.
Getting rid of backtracking 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.backtrack 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 backtrack.
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 backtrack 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 hap‐
pened, 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 a 'b', the scan‐
ner will have to back up to accept "foo". Similarly, the comment for
State #9 concerns when "fooba" has been scanned.
The final comment reminds us that there's no point going to all the
trouble of removing backtracking from the rules unless we're using -f
or -F, since there's no performance gain doing so with compressed scan‐
ners.
The way to remove the backtracking 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 backtracking 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.
Backtracking 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
backtracking (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 backtracking).
Variable trailing context (where both the leading and trailing parts do
not have a fixed length) entails almost the same performance loss as
REJECT (i.e., substantial). So when possible a rule like:
%%
mouse|rat/(cat|dog) run();
is better written:
%%
mouse/cat|dog run();
rat/cat|dog run();
or as
%%
mouse|rat/cat run();
mouse|rat/dog run();
Note that here the special '|' action does not provide any savings, and
can even make things worse (see BUGS in flex(1)).
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 backtracking
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 back‐
tracking when it has scanned a token like "auto" and then the next
character is something other than a newline or a letter. Previously it
would then just match the "auto" rule and be done, but now it has no
"auto" rule, only a "auto\n" rule. To eliminate the possibility of
backtracking, 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.
INCOMPATIBILITIES WITH LEX AND POSIX
flex is a rewrite of the Unix lex tool (the two implementations do not
share any code, though), with some extensions and incompatibilities,
both of which are of concern to those who wish to write scanners
acceptable to either implementation. At present, the POSIX lex draft
is very close to the original lex implementation, so some of these
incompatibilities are also in conflict with the POSIX draft. But the
intent is that except as noted below, flex as it presently stands will
ultimately be POSIX conformant (i.e., that those areas of conflict with
the POSIX draft will be resolved in flex's favor). Please bear in mind
that all the comments which follow are with regard to the POSIX draft
standard of Summer 1989, and not the final document (or subsequent
drafts); they are included so flex users can be aware of the standard‐
ization issues and those areas where flex may in the near future
undergo changes incompatible with its current definition.
flex is fully compatible with lex with the following exceptions:
- The undocumented lex scanner internal variable yylineno is not
supported. It is difficult to support this option efficiently,
since it requires examining every character scanned and reexam‐
ining the characters when the scanner backs up. Things get more
complicated when the end of buffer or file is reached or a NUL
is scanned (since the scan must then be restarted with the
proper line number count), or the user uses the yyless(),
unput(), or REJECT actions, or the multiple input buffer func‐
tions.
The fix is to add rules which, upon seeing a newline, increment
yylineno. This is usually an easy process, though it can be a
drag if some of the patterns can match multiple newlines along
with other characters.
yylineno is not part of the POSIX draft.
- 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 redefining the YY_INPUT macro.
The flex restriction that input() cannot be redefined is in
accordance with the POSIX draft, but YY_INPUT has not yet been
accepted into the draft (and probably won't; it looks like the
draft will simply not specify any way of controlling the scan‐
ner's input other than by making an initial assignment to yyin).
- flex scanners do not use stdio for input. Because of this, when
writing an interactive scanner one must explicitly call fflush()
on the stream associated with the terminal after writing out a
prompt. With lex such writes are automatically flushed since
lex scanners use getchar() for their input. Also, when writing
interactive scanners with flex, the -I flag must be used.
- flex scanners are not as reentrant as lex scanners. In particu‐
lar, if you have an interactive scanner and an interrupt handler
which long-jumps out of the scanner, and the scanner is subse‐
quently 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 );
- output() is not supported. Output from the ECHO macro is done
to the file-pointer yyout (default stdout).
The POSIX draft mentions that an output() routine exists but
currently gives no details as to what it does.
- lex does not support exclusive start conditions (%x), though
they are in the current POSIX draft.
- When definitions are expanded, flex encloses them in parenthe‐
ses. With lex, the following:
NAME [A-Z][A-Z0-9]*
%%
foo{NAME}? printf( "Found it\n" );
%%
will not match the string "foo" because when the macro is
expanded the rule is equivalent to "foo[A-Z][A-Z0-9]*?" and the
precedence is such that the '?' is associated with "[A-Z0-9]*".
With flex, the rule will be expanded to "foo([A-Z][A-Z0-9]*)?"
and so the string "foo" will match. Note that because of this,
the ^, $, <s>, /, and <<EOF>> operators cannot be used in a flex
definition.
The POSIX draft interpretation is the same as flex's.
- To specify a character class which matches anything but a left
bracket (']'), in lex one can use "[^]]" but with flex one must
use "[^\]]". The latter works with lex, too.
- The lex %r (generate a Ratfor scanner) option is not supported.
It is not part of the POSIX draft.
- If you are providing your own yywrap() routine, you must include
a "#undef yywrap" in the definitions section (section 1). Note
that the "#undef" will have to be enclosed in %{}'s.
The POSIX draft specifies that yywrap() is a function and this
is very unlikely to change; so flex users are warned that
yywrap() is likely to be changed to a function in the near
future.
- After a call to unput(), yytext and yyleng are undefined until
the next token is matched. This is not the case with lex or the
present POSIX draft.
- The precedence of the {} (numeric range) operator is different.
lex interprets "abc{1,3}" as "match one, two, or three occur‐
rences of 'abc'", whereas flex interprets it as "match 'ab' fol‐
lowed by one, two, or three occurrences of 'c'". The latter is
in agreement with the current POSIX draft.
- The precedence of the ^ operator is different. lex interprets
"^foo|bar" as "match either 'foo' at the beginning of a line, or
'bar' anywhere", whereas flex interprets it as "match either
'foo' or 'bar' if they come at the beginning of a line". The
latter is in agreement with the current POSIX draft.
- To refer to yytext outside of the scanner source file, the cor‐
rect definition with flex is "extern char *yytext" rather than
"extern char yytext[]". This is contrary to the current POSIX
draft but a point on which flex will not be changing, as the
array representation entails a serious performance penalty. It
is hoped that the POSIX draft will be emended to support the
flex variety of declaration (as this is a fairly painless change
to require of lex users).
- yyin is initialized by lex to be stdin; flex, on the other hand,
initializes yyin to NULL and then assigns it to stdin the first
time the scanner is called, providing yyin has not already been
assigned to a non-NULL value. The difference is subtle, but the
net effect is that with flex scanners, yyin does not have a
valid value until the scanner has been called.
- 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.
The following flex features are not included in lex or the POSIX draft
standard:
yyterminate()
<<EOF>>
YY_DECL
#line directives
%{}'s around actions
yyrestart()
comments beginning with '#' (deprecated)
multiple actions on a line
This last feature refers to the fact that with flex you can put multi‐
ple 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
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). Make an explicit
reference to the action in your flex input file. (Note that previously
flex supported a %used/%unused mechanism for dealing with this problem;
this feature is still supported but now deprecated, and will go away
soon unless the author hears from people who can argue compellingly
that they need it.)
flex scanner jammed - a scanner compiled with -s has encountered an
input string which wasn't matched by any of its rules.
flex input buffer overflowed - a scanner rule matched a string long
enough to overflow the scanner's internal input buffer (16K bytes by
default - controlled by YY_BUF_SIZE in "flex.skel". Note that to rede‐
fine this macro, you must first #undefine it).
scanner requires -8 flag - Your scanner specification includes recog‐
nizing 8-bit characters and you did not specify the -8 flag (and your
site has not installed flex with -8 as the default).
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 scan‐
ner, use:
yyrestart( yyin );
too many %t classes! - You managed to put every single character into
its own %t class. flex requires that at least one of the classes share
characters.
DEFICIENCIES / BUGS
See flex(1).
SEE ALSOflex(1), lex(1), yacc(1), sed(1), awk(1x).
M. E. Lesk and E. Schmidt, LEX - Lexical Analyzer Generator
AUTHOR
Vern Paxson, with the help of many ideas and much inspiration from Van
Jacobson. Original version by Jef Poskanzer. The fast table represen‐
tation 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 Casey Leedom, benson@odi.com, Keith Bostic, Frederic Brehm,
Nick Christopher, Jason Coughlin, Scott David Daniels, Leo Eskin, Chris
Faylor, Eric Goldman, Eric Hughes, Jeffrey R. Jones, Kevin B. Kenny,
Ronald Lamprecht, Greg Lee, Craig Leres, Mohamed el Lozy, Jim Meyering,
Marc Nozell, Esmond Pitt, Jef Poskanzer, Jim Roskind, Dave Tallman,
Frank Whaley, Ken Yap, and those whose names have slipped my marginal
mail-archiving skills but whose contributions are appreciated all the
same.
Thanks to Keith Bostic, John Gilmore, Craig Leres, Bob Mulcahy, 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 Ove Ewerlid for the
basics of support for NUL's; and to Eric Hughes for the basics of sup‐
port for multiple buffers.
Work is being done on extending flex to generate scanners in which the
state machine is directly represented in C code rather than tables.
These scanners may well be substantially faster than those generated
using -f or -F. If you are working in this area and are interested in
comparing notes and seeing whether redundant work can be avoided, con‐
tact Ove Ewerlid (ewerlid@mizar.DoCS.UU.SE).
This work was primarily done when I was at 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 Paxson
Computer Science Department
4126 Upson Hall
Cornell University
Ithaca, NY 14853-7501
vern@cs.cornell.edu
decvax!cornell!vern
Version 2.3 26 May 1990 FLEX(1)
