path: root/flex.1

.TH FLEX 1 "24 February 1990" "Version 2.2"
.SH NAME
flex - fast lexical analyzer generator
.SH SYNOPSIS
.B flex
.B [-bcdfinpstvFILT -C[efmF] -Sskeleton]
.I [filename ...]
.SH DESCRIPTION
.I flex
is a tool for generating
.I scanners:
programs which recognized lexical patterns in text.
.I 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
.I rules.  flex
generates as output a C source file,
.B lex.yy.c,
which defines a routine
.B yylex().
This file is compiled and linked with the
.B -ll
library to produce an executable.  When the executable is run,
it analyzes its input for occurrences
of the regular expressions.  Whenever it finds one, it executes
the corresponding C code.
.SH SOME SIMPLE EXAMPLES
.LP
First some simple examples to get the flavor of how one uses
.I flex.
The following
.I flex
input specifies a scanner which whenever it encounters the string
"username" will replace it with the user's login name:
.nf

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

.fi
By default, any text not matched by a
.I 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
.I pattern
and the "printf" is the
.I action.
The "%%" marks the beginning of the rules.
.LP
Here's another simple example:
.nf

        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 );
        }

.fi
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
.B yylex()
and in the
.B main()
routine declared after the second "%%".  There are two rules, one
which matches a newline ("\\n") and increments both the line count and
the character count, and one which matches any character other than
a newline (indicated by the "." regular expression).
.LP
A somewhat more complicated example:
.nf

    /* 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 (%d)\\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();
        }

.fi
This is the beginnings of a simple scanner for a language like
Pascal.  It identifies different types of
.I tokens
and reports on what it has seen.
.LP
The details of this example will be explained in the following
sections.
.SH FORMAT OF THE INPUT FILE
The
.I flex
input file consists of three sections, separated by a line with just
.B %%
in it:
.nf

    definitions
    %%
    rules
    %%
    user code

.fi
The
.I definitions
section contains declarations of simple
.I name
definitions to simplify the scanner specification, and declarations of
.I start conditions,
which are explained in a later section.
.LP
Name definitions have the form:
.nf

    name definition

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

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

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

    {DIGIT}+"."{DIGIT}*

.fi
is identical to
.nf

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

.fi
and matches one-or-more digits followed by a '.' followed
by zero-or-more digits.
.LP
The
.I rules
section of the
.I flex
input contains a series of rules of the form:
.nf

    pattern   action

.fi
where the pattern must be unindented and the action must begin
on the same line.
.LP
See below for a further description of patterns and actions.
.LP
Finally, the user code section is simply copied to
.B 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
.B %%
in the input file may be skipped, too.
.LP
In the definitions and rule sections, any
.I indented
text or text enclosed in
.B %{
and
.B %}
is copied verbatim to the output (with the %{}'s removed).
The %{}'s must appear unindented on lines by themselves.
.LP
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
.I POSIX
compliance; see below for other such features).
.LP
In the definitions section, an unindented comment (i.e., a line
beginning with "/*") is also copied verbatim to the output up
to the next "*/".  Also, any line in the definitions section
beginning with '#' is ignored.
.SH PATTERNS
The patterns in the input are written using an extended set of regular
expressions.  These are:
.nf

    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

.fi
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,
.nf

    foo|bar*

.fi
is the same as
.nf

    (foo)|(ba(r*))

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

    foo|(bar)*

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

    (foo|bar)*

.fi
.LP
Some notes on patterns:
.IP -
A negated character class such as the example "[^A-Z]"
above
.I 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 buffer) unless there's another
quote in the input.
.I -
A rule can have at most one instance of trailing context (the '/' operator
or the '$' operator).  The start condition, '^', and "<<EOF>>" patterns
can only occur at the beginning of a pattern, and, as well as with '/' and '$',
cannot be grouped inside parentheses.  The following are all illegal:
.nf

    foo/bar$
    foo|(bar$)
    foo|^bar
    <sc1>foo<sc2>bar

.fi
(Note that the first of these, though, can be written "foo/bar\\n".)
.SH 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
.I flex
input file is chosen.
.LP
Once the match is determined, the text corresponding to the match
(called the
.I token)
is made available in the global character pointer
.B yytext,
and its length in the global integer
.B yyleng.
The
.I 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.
.LP
If no match is found, then the
.I default rule
is executed: the next character in the input is considered matched and
copied to the standard output.  Thus, the simplest legal
.I flex
input is:
.nf

    %%

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

    %%
    "zap me"

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

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

.fi
.LP
If the action contains a '{', then the action spans till the balancing '}'
is found, and the action may cross multiple lines.
.I flex 
knows about C strings and comments and won't be fooled by braces found
within them, but also allows actions to begin with
.B %{
and will consider the action to be all the text up to the next
.B %}
(regardless of ordinary braces inside the action).
.LP
An action consisting solely of a vertical bar ('|') means "same as
the action for the next rule."  See below for an illustration.
.LP
Actions can include arbitrary C code, including
.B return
statements to return a value to whatever routine called
.B yylex().
Each time
.B 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
.B yylex()
will simply immediately return, unless
.B yyrestart()
is first called (see below).
.LP
Actions are not allowed to modify yytext or yyleng.
.LP
There are a number of special directives which can be included within
an action:
.IP -
.B ECHO
copies yytext to the scanner's output.
.IP -
.B BEGIN
followed by the name of a start condition places the scanner in the
corresponding start condition (see below).
.IP -
.B 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
.B yytext
and
.B yyleng
set up appropriately.
It may either be one which matched as much text
as the originally chosen rule but came later in the
.I 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:
.nf

    %{
    int word_count = 0;
    %}
    %%

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

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

    %%
    a        |
    ab       |
    abc      |
    abcd     ECHO; REJECT;
    .|\\n     /* eat up any unmatched character */

.fi
(The first three rules share the fourth's action since they use
the special '|' action.)
.B REJECT
is a particularly expensive feature in terms scanner performance;
if it is used in
.I any
of the scanner's actions it will slow down
.I all
of the scanner's matching.
.IP -
.B yymore()
tells the scanner that the next time it matches a rule, the corresponding
token should be
.I appended
onto the current value of
.B yytext
rather than replacing it.  For example, given the input "mega-kludge"
the following will write "mega-mega-kludge" to the output:
.nf

    %%
    mega-    ECHO; yymore();
    kludge   ECHO;

.fi
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
.B yytext
so the ECHO for the "kludge" rule will actually write "mega-kludge".
The presence of
.B yymore()
in the scanner's action entails a minor performance penalty in the
scanner's matching speed.
.IP -
.B yyless(n)
returns all but the first
.I n
characters of the current token back to the input stream, where they
will be rescanned when the scanner looks for the next match.
.B yytext
and
.B yyleng
are adjusted appropriately (e.g.,
.B yyleng
will now be equal to
.I n
).  For example, on the input "foobar" the following will write out
"foobarbar":
.nf

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

.fi
An argument of 0 to
.B yyless
will cause the current input string to be scanned again.  Unless you've
changed how the scanner will subsequently process its input (using
.B BEGIN,
for example), this will result in an endless loop.
.IP -
.B unput(c)
puts the character
.I 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.
.nf

    {
    int i;
    unput( ')' );
    for ( i = yyleng - 1; i >= 0; --i )
        unput( yytext[i] );
    unput( '(' );
    }

.fi
Note that since each
.B unput()
puts the given character back at the
.I beginning
of the input stream, pushing back strings must be done back-to-front.
.IP -
.B input()
reads the next character from the input stream.  For example,
the following is one way to eat up C comments:
.nf

    %%
    "/*"        {
                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;
                        }
                    }
                }

.fi
.IP -
.I yyterminate()
can be used in lieu of a return statement in an action.  It terminates
the scanner and returns a 0 to the scanner's caller, indicating "all done".
By default,
.I yyterminate()
is also called when an end-of-file is encountered.  It is a macro and
may be redefined.
.SH THE GENERATED SCANNER
The output of
.I flex
is the file
.B lex.yy.c,
which contains the scanning routine
.B yylex(),
a number of tables used by it for matching tokens, and a number
of auxilliary routines and macros.  By default,
.B yylex()
is declared as follows:
.nf

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

.fi
(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:
.nf

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

.fi
to give it the the scanning routine the name
.I 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 (;).
.LP
Whenever
.B yylex()
is called, it scans tokens from the global input file
.I yyin
(default, 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
.I return
statement.
In the former case, the scanner may not be called again unless
.B void yyrestart( FILE *input_file )
is called, to point
.I yyin
at the new input_file.  In the latter case (i.e., when an action
executes a return), the scanner may then be called again and it
will resume scanning where it left off.
.LP
By default (and for purposes of efficiency), the scanner uses
block-reads rather than simple
.I getc()
calls to read characters from
.I yyin.
The nature of how it gets its input can be controlled by redefining the
.B YY_INPUT
macro.
YY_INPUT's calling sequence is "YY_INPUT(buf,result,max_size)".  Its
action is to place up to
.I max_size
characters in the character array
.I buf
and return in the integer variable
.I 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" (which is by default
.I stdin),
so if you
just want to change the input file, you needn't redefine
YY_INPUT - just point yyin at the input file.
.LP
A sample redefinition of YY_INPUT (in the definitions section of the input
file):
.nf

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

.fi
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.
.LP
When the scanner receives an end-of-file indication from YY_INPUT,
it then checks the
.B yywrap()
function.  If it returns false (zero), then it is assumed that the
function has gone ahead and set up
.I 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.
.LP
The default
.B yywrap()
always returns 1.  Presently, to redefine it you must first
"#undef yywrap", as it is currently implemented as a macro.  As noted
by the hedging in the previous sentence, it may be changed to
a true function in the near future.
.LP
The scanner writes its
.B ECHO
output to the
.I yyout
global (default, stdout), which may be redefined by the user simply
by assigning it to some other FILE*.
.SH START CONDITIONS
.I 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,
.nf

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

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

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

.fi
will be active only when the current start condition is
either "INITIAL", "STRING", or "QUOTE".
.LP
Start conditions
are declared in the definitions (first) section of the input
using unindented lines beginning with either
.B %s
or
.B %x
followed by a list of names.
The former declares
.I inclusive
start conditions, the latter
.I exclusive
start conditions.  A start condition is activated using the
.B BEGIN
action.  Until the next
.B 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
.I inclusive,
then rules with no start conditions at all will also be active.
If it is
.I exclusive,
then
.I only
rules qualified with the start condition will be active.
So a set of rules conditioned on the same exclusive start condition
describe a scanner which is independent of any of the other rules in the
.I 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).
.LP
.B 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
.B BEGIN(INITIAL)
is equivalent to
.B BEGIN(0).
.LP
Here is a scanner which will recognize numbers only if they
are preceded earlier in the line by the string "expect-number":
.nf

    %s expect

    %%
    expect-number        BEGIN(expect);

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

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

    %x comment
    %%
            int line_num = 1;

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

    "/*"         BEGIN(comment);

.fi
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:
.nf

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

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

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

    ...

    <foo>"/*"    {
                 comment_caller = foo;
                 BEGIN(comment);
                 }
.fi
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
.I flex
feature in the future.)  Note, though, that
start conditions do not have their own name-space; %s's and %x's
declare names effectively the same as #define's.
.SH 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 can either
point yyin at a new file to process, in which case the
action
.I must
finish with the special
.I YY_NEW_FILE
action
(this is a branch, so subsequent code in the action won't
be executed), or the action must finish with a
.I return
or
.I yyterminate()
statement.  <<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 only to the
.B INITIAL
start condition, and
.I not
to
.B %s
(or
.B %x)
start conditions.
.LP
These rules are useful for catching things like unclosed comments.
An example:
.nf

    %x quote
    %%
    ...
    <quote><<EOF>>   {
             error( "unterminated quote" );
             yyterminate();
             }
    <<EOF>>          {
             if ( *++filelist )
                     {
                     yyin = fopen( *filelist, "r" );
                     YY_NEW_FILE;
                     }
             else
                yyterminate();
             }

.fi
.SH MISCELLANEOUS MACROS
The macro
.bd
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.
.LP
In the generated scanner, the actions are all gathered in one large
switch statement and separated using
.B YY_BREAK,
which may be redefined.
This 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 a rule's action ends with "return".
.SH INTERFACING WITH YACC
One of the main uses of
.I flex
is as a companion to the
.I yacc
parser-generator.
.I yacc
parsers expect to call the
.B yylex()
routine 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
.B yylval.
To use
.I flex
with
.I yacc,
one specifies the
.B -d
option to
.I yacc
to instruct it to generate the file
.B y.tab.h
containing definitions of all the
.B %tokens
appearing in the
.I yacc
input.  This file is then included in the
.I flex
scanner.  For example, if one of the tokens is "TOK_NUMBER",
part of the scanner might look like:
.nf

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

    %%

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

.fi
.SH TRANSLATION TABLE
In the name of POSIX compliance,
.I flex
supports a
.I translation table
for mapping input characters together into specified sets.
The table is specified in the first section, and its format looks like:
.nf

    %t
    1        abcd
    2        ABCDEFGHIJKLMNOPQRSTUVWXYZ
    52       0123456789
    %t

.fi
This example specifies that the characters 'a', 'b', 'c', and 'd'
are to all be lumped into group #1, the upper-case letters are
to be in group #2, and digits in group #52, and
.I no other characters will appear in the patterns
(note that characters can also be specified in a
.B %t
table using escape sequences).
The group numbers are actually disregarded by
.I flex;
.B %t
serves, though, to lump characters together.  Given the above
table, for example, the pattern "aAA*5" is equivalent to "dZQ*0".
They both say, "match any character in group #1, followed by
a character from group #2, followed by zero-or-more characters
from group #2, followed by a character from group #52."  Thus
.B %t
provides a crude way for introducing equivalence classes into
the scanner specification.  It is the author's belief that the
.B -i
option coupled with the equivalence classes which
.I flex
automatically generates take care of virtually all the instances
when one might consider using
.B %t.
But what the hell, it's there if you want it.
.so options.man
.SH PERFORMANCE CONSIDERATIONS
The main design goal of
.I flex
is that it generate high-performance scanners.  It has been optimized
for dealing well with large sets of rules.  Aside from the effects
outlined above of table compression on scanner speed,
there are a number of options/actions which degrade performance.  These
are, in decreasing order of performance impact:
.nf

    REJECT
    pattern sets that require backtracking
    arbitrary trailing context
    %T
    '^' beginning-of-line operator
    yymore()
    start conditions

.fi
.LP
Getting rid of backtracking is messy and often may be too much
work for a complicated scanner's rules.  In principal, one begins
by using the
.B -b 
flag to generate a
.I lex.backtrack
file.  For example, on the input
.nf

    %%
    foo        return TOK_KEYWORD;
    foobar     return TOK_KEYWORD;

.fi
the file looks like:
.nf

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

    State #8 is non-accepting -
     associated rules:
           3
     out-transitions: [ a ]
     jam-transitions: EOF [ \\001-`  b-\\177 ]

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

    Compressed tables always backtrack.

.fi
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 the currently scanned text does not match any rule.
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 happened,
if anything other than another 'o' is seen, the scanner will
have to back up to simply match the 'f' (by the default rule).
.LP
The comment regarding State #8 indicates there's a problem
when "foob" has been scanned.  Indeed, on any character other
than a 'b', the scanner will have to back up to accept "foo".
Similarly, the comment for State #9 concerns when "fooba" has
been scanned.
.LP
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
.B -f
or
.B -F,
since there's no performance gain doing so with compressed scanners.
.LP
The way to remove the backtracking is to add "error" rules:
.nf

    %%
    foo         return TOK_KEYWORD;
    foobar      return TOK_KEYWORD;

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

.fi
.LP
Unfortunately backtracking messages tend to cascade and
with a complicated input set 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.
(A possible future
.I flex
feature will be to automatically add rules to eliminate backtracking.
The problem is that while it's easy for
.I flex
to figure out what rules are needed, it's very hard for it to
know what the proper action is.  Currently I'm thinking that it
will simply invoke a user-redefinable macro and that's it ...)
.LP
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.,
.B yytext)
for the action.  Recall the scanner for C comments:
.nf

    %x comment
    %%
            int line_num = 1;

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

    "/*"         BEGIN(comment);

.fi
This could be sped up by writing it as:
.nf

    %x comment
    %%
            int line_num = 1;

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

    "/*"         BEGIN(comment);

.fi
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
.I adding
rules does
.I 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 '|'.
.SH INCOMPATIBILITIES WITH LEX AND POSIX
.I flex
is a rewrite of the Unix
.I lex
tool (the two implementations do not share any code, though),
which dates to the late 1970's.  There are some incompatibilities
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,
.I flex
as it presently stands will
ultimately be POSIX comformant (i.e., that those areas of conflict with
the POSIX draft will be resolved in
.I flex's
favor).  Please bare in
mind that all the comments are with regard to the POSIX
.I draft
standard and not the final document; they are included so
.I flex
users can be aware of the standardization issues and those areas where
.I flex
may in the near future be incompatibly changed with
its current definition.
.LP
.I flex
is fully compatible with
.I lex
with the following exceptions:
.IP -
When definitions are expanded,
.I flex
encloses them in parentheses.
With lex, the following
.nf

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

.fi
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
.I 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
.B ^, $, <s>,
and
.B /
operators cannot be used in a definition.
.IP
Note that the POSIX draft interpretation here is the same as
.I flex's.
.IP -
The undocumented lex-scanner internal variable
.B yylineno
is not supported.  (The variable is not part of the POSIX draft.)
.IP -
The
.B input()
routine is not redefinable, though may be called to read characters
following whatever has been matched by a rule.  If
.B input()
encounters an end-of-file the normal
.B yywrap()
processing is done.  A ``real'' end-of-file is returned as
.I EOF.
.IP
Input is instead controlled by redefining the
.B YY_INPUT
macro.
.IP
The
.I flex
restriction that
.B input()
cannot be redefined is in accordance with the POSIX draft, but
.B YY_INPUT
has not yet been accepted into the draft.
.IP -
.B output()
is not supported.
Output from the ECHO macro is done to the file-pointer
"yyout" (default
.I stdout).
.IP
The POSIX draft mentions that an
.B output()
routine exists but currently gives no details as to what it does.
.IP -
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.
.IP
The POSIX draft
specifies that yywrap() is a function and this is unlikely to change; so
.I flex users are warned
that
.I yywrap()
is likely to be changed to a function in the near future.
.IP -
The precedence of the
.B {}
operator is different.  lex interprets "abc{1,3}" as "match one, two, or
three occurrences of 'abc'", whereas
.I flex
interprets it as "match 'ab'
followed by one, two, or three occurrences of 'c'".  The latter is
in agreement with the current POSIX draft.
.IP -
To refer to yytext outside of your scanner source file, use
"extern char *yytext;" rather than "extern char yytext[];".
This is contrary to the POSIX draft but a point on which I refuse
to budge, as the array representation entails a serious performance penalty.
.IP -
The name
.bd
FLEX_SCANNER
is #define'd so scanners may be written for use with either
.I flex
or
.I lex.
.SH DEFICIENCES / BUGS
.LP
Some trailing context
patterns cannot be properly matched and generate
warning messages ("Dangerous trailing context").  These are
patterns where the ending of the
first part of the rule matches the beginning of the second
part, such as "zx*/xy*", where the 'x*' matches the 'x' at
the beginning of the trailing context.  (Note that the POSIX draft
states that the text matched by such patterns is undefined.)
If desperate, you can use
.B yyless()
to effect arbitrary trailing context.
.LP
.I variable
trailing context (where both the leading and trailing parts do not have
a fixed length) entails the same performance loss as
.I REJECT
(i.e., substantial).
.LP
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} are always considered variable-length.
.LP
Use of unput() or input() invalidates yytext and yyleng.
.LP
Use of unput() to push back more text than was matched can
result in the pushed-back text matching a beginning-of-line ('^')
rule even though it didn't come at the beginning of the line
(though this is rare!).
.LP
Nulls are not allowed in
.I flex
inputs or in the inputs to
scanners generated by
.I flex.
Their presence generates fatal errors.
.LP
.I flex
does not generate correct #line directives for code internal
to the scanner; thus, bugs in
.I
flex.skel
yield bogus line numbers.
.LP
Due to both buffering of input and read-ahead, you cannot intermix
calls to stdio routines, such as, for example,
.B getchar()
with
.I flex
rules and expect it to work.  Call
.B input()
instead.
.LP
The total table entries listed by the
.B -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.
.LP
It would be useful if
.I flex
wrote to lex.yy.c a summary of the flags used in
its generation (such as which table compression options).
.LP
Some of the macros, such as
.B yywrap(),
may in the future become functions which live in the
.B -ll
library.  This will doubtless break a lot of code, but may be
required for POSIX-compliance.
.LP
The
.I flex
internal algorithms need documentation.
.SH "SEE ALSO"
.LP
lex(1), yacc(1), sed(1), awk(1).
.LP
M. E. Lesk and E. Schmidt,
.I LEX - Lexical Analyzer Generator
.SH AUTHOR
Vern Paxson, with the help of many ideas and much inspiration from
Van Jacobson.  Original version by Jef Poskanzer.  Fast table
representation is a partial implementation of a design done by Van
Jacobson.  The implementation was done by Kevin Gong and Vern Paxson.
.LP
Thanks to the many
.I flex
beta-testers and feedbackers, especially Casey
Leedom, benson@odi.com,
Frederic Brehm, Nick Christopher, Jason Coughlin,
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, Dave Tallman, Frank Whaley, Ken Yap, and others whose names
have slipped my marginal mail-archiving skills but whose contributions
are appreciated all the same.
.LP
Thanks to Keith Bostic, John Gilmore, Bob
Mulcahy, Rich Salz, and Richard Stallman for help with various distribution
headaches.
.LP
Thanks to Esmond Pitt for 8-bit character support, Benson Margulies and Fred
Burke for C++ support, and Ove Ewerlid for supporting NUL's (as well as for
impressive efforts regarding generating extremely high-performance
scanners, which with luck will be soon forthcoming).
.LP
This work was primarily done when I was a member of the Real Time System Group
at the Lawrence Berkeley Laboratory in Berkeley, CA.  Many thanks to all there
for the support I received.
.LP
Send comments to:
.nf

     Vern Paxson
     Computer Science Department
     4126 Upson Hall
     Cornell University
     Ithaca, NY 14853-7501

     vern@cs.cornell.edu
     decvax!cornell!vern
     vern@LBL (bitnet)

.fi