path: root/flex.1

.TH FLEX 1 "April 1995" "Version 2.5"
.SH NAME
flex \- fast lexical analyzer generator
.SH SYNOPSIS
.B flex
.B [\-bcdfhilnpstvwBFILTV78+? \-C[aefFmr] \-ooutput \-Pprefix \-Sskeleton]
.B [\-\-help \-\-version]
.I [filename ...]
.SH OVERVIEW
This manual describes
.I flex,
a tool for generating programs that perform pattern-matching on text.  The
manual includes both tutorial and reference sections:
.nf

    Description
        a brief overview of the tool

    Some Simple Examples

    Format Of The Input File

    Patterns
        the extended regular expressions used by flex

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

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

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

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

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

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

    Miscellaneous Macros
        a summary of macros available to the actions

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

    Interfacing With Yacc
        connecting flex scanners together with yacc parsers

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

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

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

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

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

    Files
        files used by flex

    Deficiencies / Bugs
        known problems with flex

    See Also
        other documentation, related tools

    Author
        includes contact information

.fi
.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 \-lfl
library to produce an executable.  When the executable is run,
it analyzes its input for occurrences
of the regular expressions.  Whenever it finds one, it executes
the corresponding C code.
.SH SOME SIMPLE EXAMPLES
.PP
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.
.PP
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).
.PP
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 (%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();
        }

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


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


    r|s        either an r or an s


    r/s        an r but only if it is followed by an s.  The
                 text matched by s is included when determining
                 whether this rule is the "longest match",
                 but is then returned to the input before
                 the action is executed.  So the action only
                 sees the text matched by r.  This type
                 of pattern is called trailing context".
                 (There are some combinations of r/s that flex
                 cannot match correctly; see notes in the
                 Deficiencies / Bugs section below regarding
                 "dangerous trailing context".)
    ^r         an r, but only at the beginning of a line (i.e.,
                 which just starting to scan, or right after a
                 newline has been scanned).
    r$         an r, but only at the end of a line (i.e., just
                 before a newline).  Equivalent to "r/\\n".

               Note that flex's notion of "newline" is exactly
               whatever the C compiler used to compile flex
               interprets '\\n' as; in particular, on some DOS
               systems you must either filter out \\r's in the
               input yourself, or explicitly use r/\\r\\n for "r$".


    <s>r       an r, but only in start condition s (see
                 below for discussion of start conditions)
    <s1,s2,s3>r
               same, but in any of start conditions s1,
                 s2, or s3
    <*>r       an r in any start condition, even an exclusive one.


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

.fi
Note that inside of a character class, all regular expression operators
lose their special meaning except escape ('\\') and the character class
operators, '-', ']', and, at the beginning of the class, '^'.
.PP
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
.PP
In addition to characters and ranges of characters, character classes
can also contain character class
.I expressions.
These are expressions enclosed inside
.B [:
and
.B :]
delimiters (which themselves must appear between the '[' and ']' of the
character class; other elements may occur inside the character class, too).
The valid expressions are:
.nf

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

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

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

.fi
If your scanner is case-insensitive (the
.B \-i
flag), then
.B [:upper:]
and
.B [:lower:]
are equivalent to
.B [:alpha:].
.PP
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 the entire
input unless there's another quote in the input.
.IP -
A rule can have at most one instance of trailing context (the '/' operator
or the '$' operator).  The start condition, '^', and "<<EOF>>" patterns
can only occur at the beginning of a pattern, and, as well as with '/' and '$',
cannot be grouped inside parentheses.  A '^' which does not occur at
the beginning of a rule or a '$' which does not occur at the end of
a rule loses its special properties and is treated as a normal character.
.IP
The following are illegal:
.nf

    foo/bar$
    <sc1>foo<sc2>bar

.fi
Note that the first of these, can be written "foo/bar\\n".
.IP
The following will result in '$' or '^' being treated as a normal character:
.nf

    foo|(bar$)
    foo|^bar

.fi
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):
.nf

    foo      |
    bar$     /* action goes here */

.fi
A similar trick will work for matching a foo or a
bar-at-the-beginning-of-a-line.
.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.
.PP
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.
.PP
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.
.PP
Note that
.B yytext
can be defined in two different ways: either as a character
.I pointer
or as a character
.I array.
You can control which definition
.I flex
uses by including one of the special directives
.B %pointer
or
.B %array
in the first (definitions) section of your flex input.  The default is
.B %pointer,
unless you use the
.B -l
lex compatibility option, in which case
.B yytext
will be an array.
The advantage of using
.B %pointer
is substantially faster scanning and no buffer overflow when matching
very large tokens (unless you run out of dynamic memory).  The disadvantage
is that you are restricted in how your actions can modify
.B yytext
(see the next section), and calls to the
.B unput()
function destroys the present contents of
.B yytext,
which can be a considerable porting headache when moving between different
.I lex
versions.
.PP
The advantage of
.B %array
is that you can then modify
.B yytext
to your heart's content, and calls to
.B unput()
do not destroy
.B yytext
(see below).  Furthermore, existing
.I lex
programs sometimes access
.B yytext
externally using declarations of the form:
.nf
    extern char yytext[];
.fi
This definition is erroneous when used with
.B %pointer,
but correct for
.B %array.
.PP
.B %array
defines
.B yytext
to be an array of
.B YYLMAX
characters, which defaults to a fairly large value.  You can change
the size by simply #define'ing
.B YYLMAX
to a different value in the first section of your
.I flex
input.  As mentioned above, with
.B %pointer
yytext grows dynamically to accommodate large tokens.  While this means your
.B %pointer
scanner can accommodate very large tokens (such as matching entire blocks
of comments), bear in mind that each time the scanner must resize
.B yytext
it also must rescan the entire token from the beginning, so matching such
tokens can prove slow.
.B yytext
presently does
.I not
dynamically grow if a call to
.B unput()
results in too much text being pushed back; instead, a run-time error results.
.PP
Also note that you cannot use
.B %array
with C++ scanner classes
(the
.B c++
option; see below).
.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.)
.PP
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
.PP
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).
.PP
An action consisting solely of a vertical bar ('|') means "same as
the action for the next rule."  See below for an illustration.
.PP
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.
.PP
Actions are free to modify
.B yytext
except for lengthening it (adding
characters to its end--these will overwrite later characters in the
input stream).  This however does not apply when using
.B %array
(see above); in that case,
.B yytext
may be freely modified in any way.
.PP
Actions are free to modify
.B yyleng
except they should not do so if the action also includes use of
.B yymore()
(see below).
.PP
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 of 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.  Furthermore,
.B REJECT
cannot be used with the
.I -Cf
or
.I -CF
options (see below).
.IP
Note also that unlike the other special actions,
.B REJECT
is a
.I branch;
code immediately following it in the action will
.I not
be executed.
.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
.B ECHO
for the "kludge" rule will actually write "mega-kludge".
.PP
Two notes regarding use of
.B yymore().
First,
.B yymore()
depends on the value of
.I yyleng
correctly reflecting the size of the current token, so you must not
modify
.I yyleng
if you are using
.B yymore().
Second, 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 entire 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.
.PP
Note that
.B yyless
is a macro and can only be used in the flex input file, not from
other source files.
.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;
    /* Copy yytext because unput() trashes yytext */
    char *yycopy = strdup( yytext );
    unput( ')' );
    for ( i = yyleng - 1; i >= 0; --i )
        unput( yycopy[i] );
    unput( '(' );
    free( yycopy );
    }

.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.
.PP
An important potential problem when using
.B unput()
is that if you are using
.B %pointer
(the default), a call to
.B unput()
.I destroys
the contents of
.I yytext,
starting with its rightmost character and devouring one character to
the left with each call.  If you need the value of yytext preserved
after a call to
.B unput()
(as in the above example),
you must either first copy it elsewhere, or build your scanner using
.B %array
instead (see How The Input Is Matched).
.PP
Finally, note that you cannot put back
.B EOF
to attempt to mark the input stream with an end-of-file.
.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
(Note that if the scanner is compiled using
.B C++,
then
.B input()
is instead referred to as
.B yyinput(),
in order to avoid a name clash with the
.B C++
stream by the name of
.I input.)
.IP -
.B YY_FLUSH_BUFFER
flushes the scanner's internal buffer
so that the next time the scanner attempts to match a token, it will
first refill the buffer using
.B YY_INPUT
(see The Generated Scanner, below).  This action is a special case
of the more general
.B yy_flush_buffer()
function, described below in the section Multiple Input Buffers.
.IP -
.B 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,
.B 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 auxiliary 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 defining
the "YY_DECL" macro.  For example, you could use:
.nf

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

.fi
to give 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 (;).
.PP
Whenever
.B yylex()
is called, it scans tokens from the global input file
.I 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
.I return
statement.
.PP
If the scanner reaches an end-of-file, subsequent calls are undefined
unless either
.I yyin
is pointed at a new input file (in which case scanning continues from
that file), or
.B yyrestart()
is called.
.B yyrestart()
takes one argument, a
.B FILE *
pointer (which can be nil, if you've set up
.B YY_INPUT
to scan from a source other than
.I yyin),
and initializes
.I yyin
for scanning from that file.  Essentially there is no difference between
just assigning
.I yyin
to a new input file or using
.B yyrestart()
to do so; the latter is available for compatibility with previous versions
of
.I flex,
and because it can be used to switch input files in the middle of scanning.
It can also be used to throw away the current input buffer, by calling
it with an argument of
.I yyin;
but better is to use
.B YY_FLUSH_BUFFER
(see above).
Note that
.B yyrestart()
does
.I not
reset the start condition to
.B INITIAL
(see Start Conditions, below).
.PP
If
.B yylex()
stops scanning due to executing a
.I return
statement in one of the actions, the scanner may then be called again and it
will resume scanning where it left off.
.PP
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 defining 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".
.PP
A sample definition of YY_INPUT (in the definitions
section of the input file):
.nf

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

.fi
This definition will change the input processing to occur
one character at a time.
.PP
When the scanner receives an end-of-file indication from YY_INPUT,
it then checks the
.B yywrap()
function.  If
.B yywrap()
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.  Note that in either case, the start condition remains unchanged;
it does
.I not
revert to
.B INITIAL.
.PP
If you do not supply your own version of
.B yywrap(),
then you must either use
.B %option noyywrap
(in which case the scanner behaves as though
.B yywrap()
returned 1), or you must link with
.B \-lfl
to obtain the default version of the routine, which always returns 1.
.PP
Three routines are available for scanning from in-memory buffers rather
than files:
.B yy_scan_string(), yy_scan_bytes(),
and
.B yy_scan_buffer().
See the discussion of them below in the section Multiple Input Buffers.
.PP
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
.B FILE
pointer.
.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".
.PP
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.
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
.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).
.PP
If the distinction between inclusive and exclusive start conditions
is still a little vague, here's a simple example illustrating the
connection between the two.  The set of rules:
.nf

    %s example
    %%

    <example>foo   do_something();

    bar            something_else();

.fi
is equivalent to
.nf

    %x example
    %%

    <example>foo   do_something();

    <INITIAL,example>bar    something_else();

.fi
Without the
.B <INITIAL,example>
qualifier, the
.I bar
pattern in the second example wouldn't be active (i.e., couldn't match)
when in start condition
.B example.
If we just used
.B <example>
to qualify
.I bar,
though, then it would only be active in
.B example
and not in
.B INITIAL,
while in the first example it's active in both, because in the first
example the
.B example
startion condition is an
.I inclusive
.B (%s)
start condition.
.PP
Also note that the special start-condition specifier
.B <*>
matches every start condition.  Thus, the above example could also
have been written;
.nf

    %x example
    %%

    <example>foo   do_something();

    <*>bar    something_else();

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

    <*>.|\\n     ECHO;

.fi
.PP
.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).
(The parentheses around the start condition name are not required but
are considered good style.)
.PP
.B 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
.B yylex()
is called and the global variable
.I enter_special
is true:
.nf

            int enter_special;

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

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

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

    %{
    #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" );

.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;

    "/*"         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);

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

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

    comment_caller = YY_START;

.fi
Flex provides
.B YYSTATE
as an alias for
.B YY_START
(since that is what's used by AT&T
.I lex).
.PP
Note that start conditions do not have their own name-space; %s's and %x's
declare names in the same fashion as #define's.
.PP
Finally, here's an example of how to match C-style quoted strings using
exclusive start conditions, including expanded escape sequences (but
not including checking for a string that's too long):
.nf

    %x str

    %%
            char string_buf[MAX_STR_CONST];
            char *string_buf_ptr;


    \\"      string_buf_ptr = string_buf; BEGIN(str);

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

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

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

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

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

            *string_buf_ptr++ = result;
            }

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

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

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

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

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

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

    <SCs>{

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

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

.fi
is equivalent to:
.nf

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

.fi
Start condition scopes may be nested.
.PP
Three routines are available for manipulating stacks of start conditions:
.TP
.B void yy_push_state(int new_state)
pushes the current start condition onto the top of the start condition
stack and switches to
.I new_state
as though you had used
.B BEGIN new_state
(recall that start condition names are also integers).
.TP
.B void yy_pop_state()
pops the top of the stack and switches to it via
.B BEGIN.
.TP
.B int yy_top_state()
returns the top of the stack without altering the stack's contents.
.PP
The start condition stack grows dynamically and so has no built-in
size limitation.  If memory is exhausted, program execution aborts.
.PP
To use start condition stacks, your scanner must include a
.B %option stack
directive (see Options below).
.SH MULTIPLE INPUT BUFFERS
Some scanners (such as those which support "include" files)
require reading from several input streams.  As
.I flex
scanners do a large amount of buffering, one cannot control
where the next input will be read from by simply writing a
.B YY_INPUT
which is sensitive to the scanning context.
.B 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.
.PP
To negotiate these sorts of problems,
.I flex
provides a mechanism for creating and switching between multiple
input buffers.  An input buffer is created by using:
.nf

    YY_BUFFER_STATE yy_create_buffer( FILE *file, int size )

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

    void yy_switch_to_buffer( YY_BUFFER_STATE new_buffer )

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

    void yy_delete_buffer( YY_BUFFER_STATE buffer )

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

    void yy_flush_buffer( YY_BUFFER_STATE buffer )

.fi
This function discards the buffer's contents,
so the next time the scanner attempts to match a token from the
buffer, it will first fill the buffer anew using
.B YY_INPUT.
.PP
.B yy_new_buffer()
is an alias for
.B yy_create_buffer(),
provided for compatibility with the C++ use of
.I new
and
.I delete
for creating and destroying dynamic objects.
.PP
Finally, the
.B YY_CURRENT_BUFFER
macro returns a
.B YY_BUFFER_STATE
handle to the current buffer.
.PP
Here is an example of using these features for writing a scanner
which expands include files (the
.B <<EOF>>
feature is discussed below):
.nf

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

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

    %%
    include             BEGIN(incl);

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

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

            include_stack[include_stack_ptr++] =
                YY_CURRENT_BUFFER;

            yyin = fopen( yytext, "r" );

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

            yy_switch_to_buffer(
                yy_create_buffer( yyin, YY_BUF_SIZE ) );

            BEGIN(INITIAL);
            }

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

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

.fi
Three routines are available for setting up input buffers for
scanning in-memory strings instead of files.  All of them create
a new input buffer for scanning the string, and return a corresponding
.B YY_BUFFER_STATE
handle (which you should delete with
.B yy_delete_buffer()
when done with it).  They also switch to the new buffer using
.B yy_switch_to_buffer(),
so the next call to
.B yylex()
will start scanning the string.
.TP
.B yy_scan_string(const char *str)
scans a NUL-terminated string.
.TP
.B yy_scan_bytes(const char *bytes, int len)
scans
.I len
bytes (including possibly NUL's)
starting at location
.I bytes.
.PP
Note that both of these functions create and scan a
.I copy
of the string or bytes.  (This may be desirable, since
.B yylex()
modifies the contents of the buffer it is scanning.)  You can avoid the
copy by using:
.TP
.B yy_scan_buffer(char *base, yy_size_t size)
which scans in place the buffer starting at
.I base,
consisting of
.I size
bytes, the last two bytes of which
.I must
be
.B YY_END_OF_BUFFER_CHAR
(ASCII NUL).
These last two bytes are not scanned; thus, scanning
consists of
.B base[0]
through
.B base[size-2],
inclusive.
.IP
If you fail to set up
.I base
in this manner (i.e., forget the final two
.B YY_END_OF_BUFFER_CHAR
bytes), then
.B yy_scan_buffer()
returns a nil pointer instead of creating a new input buffer.
.IP
The type
.B yy_size_t
is an integral type to which you can cast an integer expression
reflecting the size of the buffer.
.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 must finish
by doing one of four things:
.IP -
assigning
.I yyin
to a new input file (in previous versions of flex, after doing the
assignment you had to call the special action
.B YY_NEW_FILE;
this is no longer necessary);
.IP -
executing a
.I return
statement;
.IP -
executing the special
.B yyterminate()
action;
.IP -
or, switching to a new buffer using
.B yy_switch_to_buffer()
as shown in the example above.
.PP
<<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
.I all
start conditions which do not already have <<EOF>> actions.  To
specify an <<EOF>> rule for only the initial start condition, use
.nf

    <INITIAL><<EOF>>

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

    %x quote
    %%

    ...other rules for dealing with quotes...

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

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

    #define YY_USER_ACTION ++ctr[yy_act]

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

    int ctr[YY_NUM_RULES];

.fi
.PP
The macro
.B YY_USER_INIT
may be defined to provide an action which is always executed before
the first scan (and before the scanner's internal initializations are done).
For example, it could be used to call a routine to read
in a data table or open a logging file.
.PP
The macro
.B yy_set_interactive(is_interactive)
can be used to control whether the current buffer is considered
.I interactive.
An interactive buffer is processed more slowly,
but must be used when the scanner's input source is indeed
interactive to avoid problems due to waiting to fill buffers
(see the discussion of the
.B \-I
flag below).  A non-zero value
in the macro invocation marks the buffer as interactive, a zero  
value as non-interactive.  Note that use of this macro overrides
.B %option always-interactive
or
.B %option never-interactive
(see Options below).
.B yy_set_interactive()
must be invoked prior to beginning to scan the buffer that is
(or is not) to be considered interactive.
.PP
The macro
.B yy_set_bol(at_bol)
can be used to control whether the current buffer's scanning
context for the next token match is done as though at the
beginning of a line.  A non-zero macro argument makes rules anchored with
'^' active, while a zero argument makes '^' rules inactive.
.PP
The macro
.B YY_AT_BOL()
returns true if the next token scanned from the current buffer
will have '^' rules active, false otherwise.
.PP
In the generated scanner, the actions are all gathered in one large
switch statement and separated using
.B 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
.B 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
.B YY_BREAK
is inaccessible.
.SH VALUES AVAILABLE TO THE USER
This section summarizes the various values available to the user
in the rule actions.
.IP -
.B char *yytext
holds the text of the current token.  It may be modified but not lengthened
(you cannot append characters to the end).
.IP
If the special directive
.B %array
appears in the first section of the scanner description, then
.B yytext
is instead declared
.B char yytext[YYLMAX],
where
.B YYLMAX
is a macro definition that you can redefine in the first section
if you don't like the default value (generally 8KB).  Using
.B %array
results in somewhat slower scanners, but the value of
.B yytext
becomes immune to calls to
.I input()
and
.I unput(),
which potentially destroy its value when
.B yytext
is a character pointer.  The opposite of
.B %array
is
.B %pointer,
which is the default.
.IP
You cannot use
.B %array
when generating C++ scanner classes
(the
.B \-+
flag).
.IP -
.B int yyleng
holds the length of the current token.
.IP -
.B FILE *yyin
is the file which by default
.I flex
reads from.  It may be redefined but doing so only makes sense before
scanning begins or after an EOF has been encountered.  Changing it in
the midst of scanning will have unexpected results since
.I flex
buffers its input; use
.B yyrestart()
instead.
Once scanning terminates because an end-of-file
has been seen, you can assign
.I yyin
at the new input file and then call the scanner again to continue scanning.
.IP -
.B void yyrestart( FILE *new_file )
may be called to point
.I yyin
at the new input file.  The switch-over to the new file is immediate
(any previously buffered-up input is lost).  Note that calling
.B yyrestart()
with
.I yyin
as an argument thus throws away the current input buffer and continues
scanning the same input file.
.IP -
.B FILE *yyout
is the file to which
.B ECHO
actions are done.  It can be reassigned by the user.
.IP -
.B YY_CURRENT_BUFFER
returns a
.B YY_BUFFER_STATE
handle to the current buffer.
.IP -
.B YY_START
returns an integer value corresponding to the current start
condition.  You can subsequently use this value with
.B BEGIN
to return to that start condition.
.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 a routine named
.B 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
.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 OPTIONS
.I flex
has the following options:
.TP
.B \-b
Generate backing-up information to
.I lex.backup.
This is a list of scanner states which require backing up
and the input characters on which they do so.  By adding rules one
can remove backing-up states.  If
.I all
backing-up states are eliminated and
.B \-Cf
or
.B \-CF
is used, the generated scanner will run faster (see the
.B \-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.)
.TP
.B \-c
is a do-nothing, deprecated option included for POSIX compliance.
.TP
.B \-d
makes the generated scanner run in
.I debug
mode.  Whenever a pattern is recognized and the global
.B yy_flex_debug
is non-zero (which is the default),
the scanner will write to
.I stderr
a line of the form:
.nf

    --accepting rule at line 53 ("the matched text")

.fi
The line number refers to the location of the rule in the file
defining the scanner (i.e., the file that was fed to flex).  Messages
are also generated when the scanner backs up, accepts the
default rule, reaches the end of its input buffer (or encounters
a NUL; at this point, the two look the same as far as the scanner's concerned),
or reaches an end-of-file.
.TP
.B \-f
specifies
.I fast scanner.
No table compression is done and stdio is bypassed.
The result is large but fast.  This option is equivalent to
.B \-Cfr
(see below).
.TP
.B \-h
generates a "help" summary of
.I flex's
options to
.I stdout 
and then exits.
.B \-?
and
.B \-\-help
are synonyms for
.B \-h.
.TP
.B \-i
instructs
.I flex
to generate a
.I case-insensitive
scanner.  The case of letters given in the
.I flex
input patterns will
be ignored, and tokens in the input will be matched regardless of case.  The
matched text given in
.I yytext
will have the preserved case (i.e., it will not be folded).
.TP
.B \-l
turns on maximum compatibility with the original AT&T
.I lex
implementation.  Note that this does not mean
.I full
compatibility.  Use of this option costs a considerable amount of
performance, and it cannot be used with the
.B \-+, -f, -F, -Cf,
or
.B -CF
options.  For details on the compatibilities it provides, see the section
"Incompatibilities With Lex And POSIX" below.  This option also results
in the name
.B YY_FLEX_LEX_COMPAT
being #define'd in the generated scanner.
.TP
.B \-n
is another do-nothing, deprecated option included only for
POSIX compliance.
.TP
.B \-p
generates a performance report to stderr.  The report
consists of comments regarding features of the
.I flex
input file which will cause a serious loss of performance in the resulting
scanner.  If you give the flag twice, you will also get comments regarding
features that lead to minor performance losses.
.IP
Note that the use of
.B REJECT,
.B %option yylineno,
and variable trailing context (see the Deficiencies / Bugs section below)
entails a substantial performance penalty; use of
.I yymore(),
the
.B ^
operator,
and the
.B \-I
flag entail minor performance penalties.
.TP
.B \-s
causes the
.I default rule
(that unmatched scanner input is echoed to
.I 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.
.TP
.B \-t
instructs
.I flex
to write the scanner it generates to standard output instead
of
.B lex.yy.c.
.TP
.B \-v
specifies that
.I flex
should write to
.I stderr
a summary of statistics regarding the scanner it generates.
Most of the statistics are meaningless to the casual
.I flex
user, but the first line identifies the version of
.I flex
(same as reported by
.B \-V),
and the next line the flags used when generating the scanner, including
those that are on by default.
.TP
.B \-w
suppresses warning messages.
.TP
.B \-B
instructs
.I flex
to generate a
.I batch
scanner, the opposite of
.I interactive
scanners generated by
.B \-I
(see below).  In general, you use
.B \-B
when you are
.I certain
that your scanner will never be used interactively, and you want to
squeeze a
.I little
more performance out of it.  If your goal is instead to squeeze out a
.I lot
more performance, you should  be using the
.B \-Cf
or
.B \-CF
options (discussed below), which turn on
.B \-B
automatically anyway.
.TP
.B \-F
specifies that the
.ul
fast
scanner table representation should be used (and stdio
bypassed).  This representation is
about as fast as the full table representation
.B (-f),
and for some sets of patterns will be considerably smaller (and for
others, larger).  In general, if the pattern set contains both "keywords"
and a catch-all, "identifier" rule, such as in the set:
.nf

    "case"    return TOK_CASE;
    "switch"  return TOK_SWITCH;
    ...
    "default" return TOK_DEFAULT;
    [a-z]+    return TOK_ID;

.fi
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
.B -F.
.IP
This option is equivalent to
.B \-CFr
(see below).  It cannot be used with
.B \-+.
.TP
.B \-I
instructs
.I flex
to generate an
.I interactive
scanner.  An interactive scanner is one that only looks ahead to decide
what token has been matched if it absolutely must.  It turns out that
always looking one extra character ahead, even if the scanner has already
seen enough text to disambiguate the current token, is a bit faster than
only looking ahead when necessary.  But scanners that always look ahead
give dreadful interactive performance; for example, when a user types
a newline, it is not recognized as a newline token until they enter
.I another
token, which often means typing in another whole line.
.IP
.I Flex
scanners default to
.I interactive
unless you use the
.B \-Cf
or
.B \-CF
table-compression options (see below).  That's because if you're looking
for high-performance you should be using one of these options, so if you
didn't,
.I flex
assumes you'd rather trade off a bit of run-time performance for intuitive
interactive behavior.  Note also that you
.I cannot
use
.B \-I
in conjunction with
.B \-Cf
or
.B \-CF.
Thus, this option is not really needed; it is on by default for all those
cases in which it is allowed.
.IP
You can force a scanner to
.I not
be interactive by using
.B \-B
(see above).
.TP
.B \-L
instructs
.I flex
not to generate
.B #line
directives.  Without this option,
.I flex
peppers the generated scanner
with #line directives so error messages in the actions will be correctly
located with respect to either the original
.I flex
input file (if the errors are due to code in the input file), or
.B lex.yy.c
(if the errors are
.I flex's
fault -- you should report these sorts of errors to the email address
given below).
.TP
.B \-T
makes
.I flex
run in
.I trace
mode.  It will generate a lot of messages to
.I stderr
concerning
the form of the input and the resultant non-deterministic and deterministic
finite automata.  This option is mostly for use in maintaining
.I flex.
.TP
.B \-V
prints the version number to
.I stdout
and exits.
.B \-\-version
is a synonym for
.B \-V.
.TP
.B \-7
instructs
.I flex
to generate a 7-bit scanner, i.e., one which can only recognized 7-bit
characters in its input.  The advantage of using
.B \-7
is that the scanner's tables can be up to half the size of those generated
using the
.B \-8
option (see below).  The disadvantage is that such scanners often hang
or crash if their input contains an 8-bit character.
.IP
Note, however, that unless you generate your scanner using the
.B \-Cf
or
.B \-CF
table compression options, use of
.B \-7
will save only a small amount of table space, and make your scanner
considerably less portable.
.I Flex's
default behavior is to generate an 8-bit scanner unless you use the
.B \-Cf
or
.B \-CF,
in which case
.I flex
defaults to generating 7-bit scanners unless your site was always
configured to generate 8-bit scanners (as will often be the case
with non-USA sites).  You can tell whether flex generated a 7-bit
or an 8-bit scanner by inspecting the flag summary in the
.B \-v
output as described above.
.IP
Note that if you use
.B \-Cfe
or
.B \-CFe
(those table compression options, but also using equivalence classes as
discussed see below), flex still defaults to generating an 8-bit
scanner, since usually with these compression options full 8-bit tables
are not much more expensive than 7-bit tables.
.TP
.B \-8
instructs
.I flex
to generate an 8-bit scanner, i.e., one which can recognize 8-bit
characters.  This flag is only needed for scanners generated using
.B \-Cf
or
.B \-CF,
as otherwise flex defaults to generating an 8-bit scanner anyway.
.IP
See the discussion of
.B \-7
above for flex's default behavior and the tradeoffs between 7-bit
and 8-bit scanners.
.TP
.B \-+
specifies that you want flex to generate a C++
scanner class.  See the section on Generating C++ Scanners below for
details.
.TP 
.B \-C[aefFmr]
controls the degree of table compression and, more generally, trade-offs
between small scanners and fast scanners.
.IP
.B \-Ca
("align") instructs flex to trade off larger tables in the
generated scanner for faster performance because the elements of
the tables are better aligned for memory access and computation.  On some
RISC architectures, fetching and manipulating longwords is more efficient
than with smaller-sized units such as shortwords.  This option can
double the size of the tables used by your scanner.
.IP
.B \-Ce
directs
.I flex
to construct
.I equivalence classes,
i.e., sets of characters
which have identical lexical properties (for example, if the only
appearance of digits in the
.I flex
input is in the character class
"[0-9]" then the digits '0', '1', ..., '9' will all be put
in the same equivalence class).  Equivalence classes usually give
dramatic reductions in the final table/object file sizes (typically
a factor of 2-5) and are pretty cheap performance-wise (one array
look-up per character scanned).
.IP
.B \-Cf
specifies that the
.I full
scanner tables should be generated -
.I flex
should not compress the
tables by taking advantages of similar transition functions for
different states.
.IP
.B \-CF
specifies that the alternate fast scanner representation (described
above under the
.B \-F
flag)
should be used.  This option cannot be used with
.B \-+.
.IP
.B \-Cm
directs
.I flex
to construct
.I meta-equivalence classes,
which are sets of equivalence classes (or characters, if equivalence
classes are not being used) that are commonly used together.  Meta-equivalence
classes are often a big win when using compressed tables, but they
have a moderate performance impact (one or two "if" tests and one
array look-up per character scanned).
.IP
.B \-Cr
causes the generated scanner to
.I bypass
use of the standard I/O library (stdio) for input.  Instead of calling
.B fread()
or
.B getc(),
the scanner will use the
.B read()
system call, resulting in a performance gain which varies from system
to system, but in general is probably negligible unless you are also using
.B \-Cf
or
.B \-CF.
Using
.B \-Cr
can cause strange behavior if, for example, you read from
.I yyin
using stdio prior to calling the scanner (because the scanner will miss
whatever text your previous reads left in the stdio input buffer).
.IP
.B \-Cr
has no effect if you define
.B YY_INPUT
(see The Generated Scanner above).
.IP
A lone
.B \-C
specifies that the scanner tables should be compressed but neither
equivalence classes nor meta-equivalence classes should be used.
.IP
The options
.B \-Cf
or
.B \-CF
and
.B \-Cm
do not make sense together - there is no opportunity for meta-equivalence
classes if the table is not being compressed.  Otherwise the options
may be freely mixed, and are cumulative.
.IP
The default setting is
.B \-Cem,
which specifies that
.I 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:
.nf

    slowest & smallest
          -Cem
          -Cm
          -Ce
          -C
          -C{f,F}e
          -C{f,F}
          -C{f,F}a
    fastest & largest

.fi
Note that scanners with the smallest tables are usually generated and
compiled the quickest, so
during development you will usually want to use the default, maximal
compression.
.IP
.B \-Cfe
is often a good compromise between speed and size for production
scanners.
.TP
.B \-ooutput
directs flex to write the scanner to the file
.B output
instead of
.B lex.yy.c.
If you combine
.B \-o
with the
.B \-t
option, then the scanner is written to
.I stdout
but its
.B #line
directives (see the
.B \\-L
option above) refer to the file
.B output.
.TP
.B \-Pprefix
changes the default
.I "yy"
prefix used by
.I flex
for all globally-visible variable and function names to instead be
.I prefix.
For example,
.B \-Pfoo
changes the name of
.B yytext
to
.B footext.
It also changes the name of the default output file from
.B lex.yy.c
to
.B lex.foo.c.
Here are all of the names affected:
.nf

    yy_create_buffer
    yy_delete_buffer
    yy_flex_debug
    yy_init_buffer
    yy_flush_buffer
    yy_load_buffer_state
    yy_switch_to_buffer
    yyin
    yyleng
    yylex
    yylineno
    yyout
    yyrestart
    yytext
    yywrap

.fi
(If you are using a C++ scanner, then only
.B yywrap
and
.B yyFlexLexer
are affected.)
Within your scanner itself, you can still refer to the global variables
and functions using either version of their name; but externally, they
have the modified name.
.IP
This option lets you easily link together multiple
.I flex
programs into the same executable.  Note, though, that using this
option also renames
.B yywrap(),
so you now
.I must
either
provide your own (appropriately-named) version of the routine for your
scanner, or use
.B %option noyywrap,
as linking with
.B \-lfl
no longer provides one for you by default.
.TP
.B \-Sskeleton_file
overrides the default skeleton file from which
.I flex
constructs its scanners.  You'll never need this option unless you are doing
.I flex
maintenance or development.
.PP
.I flex
also provides a mechanism for controlling options within the
scanner specification itself, rather than from the flex command-line.
This is done by including
.B %option
directives in the first section of the scanner specification.
You can specify multiple options with a single
.B %option
directive, and multiple directives in the first section of your flex input
file.
.PP
Most options are given simply as names, optionally preceded by the
word "no" (with no intervening whitespace) to negate their meaning.
A number are equivalent to flex flags or their negation:
.nf

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

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

    case-insensitive or
    caseless        -i option

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

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

.fi
Some
.B %option's
provide features otherwise not available:
.TP
.B always-interactive
instructs flex to generate a scanner which always considers its input
"interactive".  Normally, on each new input file the scanner calls
.B isatty()
in an attempt to determine whether
the scanner's input source is interactive and thus should be read a
character at a time.  When this option is used, however, then no
such call is made.
.TP
.B main
directs flex to provide a default
.B main()
program for the scanner, which simply calls
.B yylex().
This option implies
.B noyywrap
(see below).
.TP
.B never-interactive
instructs flex to generate a scanner which never considers its input
"interactive" (again, no call made to
.B isatty()).
This is the opposite of
.B always-interactive.
.TP
.B stack
enables the use of start condition stacks (see Start Conditions above).
.TP
.B stdinit
if set (i.e.,
.B %option stdinit)
initializes
.I yyin
and
.I yyout
to
.I stdin
and
.I stdout,
instead of the default of
.I nil.
Some existing
.I lex
programs depend on this behavior, even though it is not compliant with
ANSI C, which does not require
.I stdin
and
.I stdout
to be compile-time constant.
.TP
.B yylineno
directs
.I flex
to generate a scanner that maintains the number of the current line
read from its input in the global variable
.B yylineno.
This option is implied by
.B %option lex-compat.
.TP
.B yywrap
if unset (i.e.,
.B %option noyywrap),
makes the scanner not call
.B yywrap()
upon an end-of-file, but simply assume that there are no more
files to scan (until the user points
.I yyin
at a new file and calls
.B yylex()
again).
.PP
.I flex
scans your rule actions to determine whether you use the
.B REJECT
or
.B yymore()
features.  The
.B reject
and
.B yymore
options are available to override its decision as to whether you use the
options, either by setting them (e.g.,
.B %option reject)
to indicate the feature is indeed used, or
unsetting them to indicate it actually is not used
(e.g.,
.B %option noyymore).
.PP
Three options take string-delimited values, offset with '=':
.nf

    %option outfile="ABC"

.fi
is equivalent to
.B -oABC,
and
.nf

    %option prefix="XYZ"

.fi
is equivalent to
.B -PXYZ.
Finally,
.nf

    %option yyclass="foo"

.fi
only applies when generating a C++ scanner (
.B \-+
option).  It informs
.I flex
that you have derived
.B foo
as a subclass of
.B yyFlexLexer,
so
.I flex
will place your actions in the member function
.B foo::yylex()
instead of
.B yyFlexLexer::yylex().
It also generates a
.B yyFlexLexer::yylex()
member function that emits a run-time error (by invoking
.B yyFlexLexer::LexerError())
if called.
See Generating C++ Scanners, below, for additional information.
.PP
A number of options are available for lint purists who want to suppress
the appearance of unneeded routines in the generated scanner.  Each of the
following, if unset
(e.g.,
.B %option nounput
), results in the corresponding routine not appearing in
the generated scanner:
.nf

    input, unput
    yy_push_state, yy_pop_state, yy_top_state
    yy_scan_buffer, yy_scan_bytes, yy_scan_string

.fi
(though
.B yy_push_state()
and friends won't appear anyway unless you use
.B %option stack).
.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 on
scanner speed of the table compression
.B \-C
options outlined above,
there are a number of options/actions which degrade performance.  These
are, from most expensive to least:
.nf

    REJECT
    %option yylineno
    arbitrary trailing context

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

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

.fi
with the first three all being quite expensive and the last two
being quite cheap.  Note also that
.B unput()
is implemented as a routine call that potentially does quite a bit of
work, while
.B yyless()
is a quite-cheap macro; so if just putting back some excess text you
scanned, use
.B yyless().
.PP
.B REJECT
should be avoided at all costs when performance is important.
It is a particularly expensive option.
.PP
Getting rid of backing up is messy and often may be an enormous
amount of work for a complicated scanner.  In principal, one begins
by using the
.B \-b 
flag to generate a
.I lex.backup
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 rule line numbers:
           2       3
     out-transitions: [ o ]
     jam-transitions: EOF [ \\001-n  p-\\177 ]

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

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

    Compressed tables always back up.

.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 that in that state the currently scanned text does not match
any rule.  The state occurs when trying to match the rules found
at lines 2 and 3 in the input file.
If the scanner is in that state and then reads
something other than an 'o', it will have to back up to find
a rule which is matched.  With
a bit of headscratching one can see that this must be the
state it's in when it has seen "fo".  When this has happened,
if anything other than another 'o' is seen, the scanner will
have to back up to simply match the 'f' (by the default rule).
.PP
The comment regarding State #8 indicates there's a problem
when "foob" has been scanned.  Indeed, on any character other
than an 'a', the scanner will have to back up to accept "foo".
Similarly, the comment for State #9 concerns when "fooba" has
been scanned and an 'r' does not follow.
.PP
The final comment reminds us that there's no point going to
all the trouble of removing backing up from the rules unless
we're using
.B \-Cf
or
.B \-CF,
since there's no performance gain doing so with compressed scanners.
.PP
The way to remove the backing up 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
.PP
Eliminating backing up among a list of keywords can also be
done using a "catch-all" rule:
.nf

    %%
    foo         return TOK_KEYWORD;
    foobar      return TOK_KEYWORD;

    [a-z]+      return TOK_ID;

.fi
This is usually the best solution when appropriate.
.PP
Backing up messages tend to cascade.
With a complicated set of rules it's not uncommon to get hundreds
of messages.  If one can decipher them, though, it often
only takes a dozen or so rules to eliminate the backing up (though
it's easy to make a mistake and have an error rule accidentally match
a valid token.  A possible future
.I flex
feature will be to automatically add rules to eliminate backing up).
.PP
It's important to keep in mind that you gain the benefits of eliminating
backing up only if you eliminate
.I every
instance of backing up.  Leaving just one means you gain nothing.
.PP
.I Variable
trailing context (where both the leading and trailing parts do not have
a fixed length) entails almost the same performance loss as
.B REJECT
(i.e., substantial).  So when possible a rule like:
.nf

    %%
    mouse|rat/(cat|dog)   run();

.fi
is better written:
.nf

    %%
    mouse/cat|dog         run();
    rat/cat|dog           run();

.fi
or as
.nf

    %%
    mouse|rat/cat         run();
    mouse|rat/dog         run();

.fi
Note that here the special '|' action does
.I not
provide any savings, and can even make things worse (see
Deficiencies / Bugs below).
.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;

    "/*"         BEGIN(comment);

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

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

    %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);

.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 '|'.
.PP
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:
.nf

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

    .|\\n     /* it's not a keyword */

.fi
To eliminate the back-tracking, introduce a catch-all rule:
.nf

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

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

.fi
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:
.nf

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

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

.fi
One has to be careful here, as we have now reintroduced backing up
into the scanner.  In particular, while
.I we
know that there will never be any characters in the input stream
other than letters or newlines,
.I flex
can't figure this out, and it will plan for possibly needing to back up
when it has scanned a token like "auto" and then the next character
is something other than a newline or a letter.  Previously it would
then just match the "auto" rule and be done, but now it has no "auto"
rule, only a "auto\\n" rule.  To eliminate the possibility of backing up,
we could either duplicate all rules but without final newlines, or,
since we never expect to encounter such an input and therefore don't
how it's classified, we can introduce one more catch-all rule, this
one which doesn't include a newline:
.nf

    %%
    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 */

.fi
Compiled with
.B \-Cf,
this is about as fast as one can get a
.I flex 
scanner to go for this particular problem.
.PP
A final note:
.I flex
is slow when matching NUL's, particularly when a token contains
multiple NUL's.
It's best to write rules which match
.I short
amounts of text if it's anticipated that the text will often include NUL's.
.PP
Another final note regarding performance: as mentioned above in the section
How the Input is Matched, dynamically resizing
.B yytext
to accommodate huge tokens is a slow process because it presently requires that
the (huge) token be rescanned from the beginning.  Thus if performance is
vital, you should attempt to match "large" quantities of text but not
"huge" quantities, where the cutoff between the two is at about 8K
characters/token.
.SH GENERATING C++ SCANNERS
.I flex
provides two different ways to generate scanners for use with C++.  The
first way is to simply compile a scanner generated by
.I flex
using a C++ compiler instead of a C compiler.  You should not encounter
any compilations errors (please report any you find to the email address
given in the Author section below).  You can then use C++ code in your
rule actions instead of C code.  Note that the default input source for
your scanner remains
.I yyin,
and default echoing is still done to
.I yyout.
Both of these remain
.I FILE *
variables and not C++
.I streams.
.PP
You can also use
.I flex
to generate a C++ scanner class, using the
.B \-+
option (or, equivalently,
.B %option c++),
which is automatically specified if the name of the flex
executable ends in a '+', such as
.I flex++.
When using this option, flex defaults to generating the scanner to the file
.B lex.yy.cc
instead of
.B lex.yy.c.
The generated scanner includes the header file
.I FlexLexer.h,
which defines the interface to two C++ classes.
.PP
The first class,
.B FlexLexer,
provides an abstract base class defining the general scanner class
interface.  It provides the following member functions:
.TP
.B const char* YYText()
returns the text of the most recently matched token, the equivalent of
.B yytext.
.TP
.B int YYLeng()
returns the length of the most recently matched token, the equivalent of
.B yyleng.
.TP
.B int lineno() const
returns the current input line number
(see
.B %option yylineno),
or
.B 1
if
.B %option yylineno
was not used.
.TP
.B void set_debug( int flag )
sets the debugging flag for the scanner, equivalent to assigning to
.B yy_flex_debug
(see the Options section above).  Note that you must build the scanner
using
.B %option debug
to include debugging information in it.
.TP
.B int debug() const
returns the current setting of the debugging flag.
.PP
Also provided are member functions equivalent to
.B yy_switch_to_buffer(),
.B yy_create_buffer()
(though the first argument is an
.B istream*
object pointer and not a
.B FILE*),
.B yy_flush_buffer(),
.B yy_delete_buffer(),
and
.B yyrestart()
(again, the first argument is a
.B istream*
object pointer).
.PP
The second class defined in
.I FlexLexer.h
is
.B yyFlexLexer,
which is derived from
.B FlexLexer.
It defines the following additional member functions:
.TP
.B
yyFlexLexer( istream* arg_yyin = 0, ostream* arg_yyout = 0 )
constructs a
.B yyFlexLexer
object using the given streams for input and output.  If not specified,
the streams default to
.B cin
and
.B cout,
respectively.
.TP
.B virtual int yylex()
performs the same role is
.B yylex()
does for ordinary flex scanners: it scans the input stream, consuming
tokens, until a rule's action returns a value.  If you derive a subclass
.B S
from
.B yyFlexLexer
and want to access the member functions and variables of
.B S
inside
.B yylex(),
then you need to use
.B %option yyclass="S"
to inform
.I flex
that you will be using that subclass instead of
.B yyFlexLexer.
In this case, rather than generating
.B yyFlexLexer::yylex(),
.I flex
generates
.B S::yylex()
(and also generates a dummy
.B yyFlexLexer::yylex()
that calls
.B yyFlexLexer::LexerError()
if called).
.TP
.B
virtual void switch_streams(istream* new_in = 0,
.B
ostream* new_out = 0)
reassigns
.B yyin
to
.B new_in
(if non-nil)
and
.B yyout
to
.B new_out
(ditto), deleting the previous input buffer if
.B yyin
is reassigned.
.TP
.B
int yylex( istream* new_in, ostream* new_out = 0 )
first switches the input streams via
.B switch_streams( new_in, new_out )
and then returns the value of
.B yylex().
.PP
In addition,
.B yyFlexLexer
defines the following protected virtual functions which you can redefine
in derived classes to tailor the scanner:
.TP
.B
virtual int LexerInput( char* buf, int max_size )
reads up to
.B max_size
characters into
.B buf
and returns the number of characters read.  To indicate end-of-input,
return 0 characters.  Note that "interactive" scanners (see the
.B \-B
and
.B \-I
flags) define the macro
.B YY_INTERACTIVE.
If you redefine
.B LexerInput()
and need to take different actions depending on whether or not
the scanner might be scanning an interactive input source, you can
test for the presence of this name via
.B #ifdef.
.TP
.B
virtual void LexerOutput( const char* buf, int size )
writes out
.B size
characters from the buffer
.B buf,
which, while NUL-terminated, may also contain "internal" NUL's if
the scanner's rules can match text with NUL's in them.
.TP
.B
virtual void LexerError( const char* msg )
reports a fatal error message.  The default version of this function
writes the message to the stream
.B cerr
and exits.
.PP
Note that a
.B yyFlexLexer
object contains its
.I entire
scanning state.  Thus you can use such objects to create reentrant
scanners.  You can instantiate multiple instances of the same
.B yyFlexLexer
class, and you can also combine multiple C++ scanner classes together
in the same program using the
.B \-P
option discussed above.
.PP
Finally, note that the
.B %array
feature is not available to C++ scanner classes; you must use
.B %pointer
(the default).
.PP
Here is an example of a simple C++ scanner:
.nf

        // An example of using the flex C++ scanner class.

    %{
    int mylineno = 0;
    %}

    string  \\"[^\\n"]+\\"

    ws      [ \\t]+

    alpha   [A-Za-z]
    dig     [0-9]
    name    ({alpha}|{dig}|\\$)({alpha}|{dig}|[_.\\-/$])*
    num1    [-+]?{dig}+\\.?([eE][-+]?{dig}+)?
    num2    [-+]?{dig}*\\.{dig}+([eE][-+]?{dig}+)?
    number  {num1}|{num2}

    %%

    {ws}    /* skip blanks and tabs */

    "/*"    {
            int c;

            while((c = yyinput()) != 0)
                {
                if(c == '\\n')
                    ++mylineno;

                else if(c == '*')
                    {
                    if((c = yyinput()) == '/')
                        break;
                    else
                        unput(c);
                    }
                }
            }

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

    \\n        mylineno++;

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

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

    %%

    int main( int /* argc */, char** /* argv */ )
        {
        FlexLexer* lexer = new yyFlexLexer;
        while(lexer->yylex() != 0)
            ;
        return 0;
        }
.fi
If you want to create multiple (different) lexer classes, you use the
.B \-P
flag (or the
.B prefix=
option) to rename each
.B yyFlexLexer
to some other
.B xxFlexLexer.
You then can include
.B <FlexLexer.h>
in your other sources once per lexer class, first renaming
.B yyFlexLexer
as follows:
.nf

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

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

.fi
if, for example, you used
.B %option prefix="xx"
for one of your scanners and
.B %option prefix="zz"
for the other.
.PP
IMPORTANT: the present form of the scanning class is
.I experimental
and may change considerably between major releases. 
.SH INCOMPATIBILITIES WITH LEX AND POSIX
.I flex
is a rewrite of the AT&T Unix
.I lex
tool (the two implementations do not share any code, though),
with some extensions and incompatibilities, both of which
are of concern to those who wish to write scanners acceptable
to either implementation.  Flex is fully compliant with the POSIX
.I lex
specification, except that when using
.B %pointer
(the default), a call to
.B unput()
destroys the contents of
.B yytext,
which is counter to the POSIX specification.
.PP
In this section we discuss all of the known areas of incompatibility
between flex, AT&T lex, and the POSIX specification.
.PP
.I flex's
.B \-l
option turns on maximum compatibility with the original AT&T
.I lex
implementation, at the cost of a major loss in the generated scanner's
performance.  We note below which incompatibilities can be overcome
using the
.B \-l
option.
.PP
.I flex
is fully compatible with
.I lex
with the following exceptions:
.IP -
The undocumented
.I lex
scanner internal variable
.B yylineno
is not supported unless
.B \-l
or
.B %option yylineno
is used.
.IP
.B yylineno
should be maintained on a per-buffer basis, rather than a per-scanner
(single global variable) basis.
.IP
.B yylineno
is not part of the POSIX specification.
.IP -
The
.B input()
routine is not redefinable, though it 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 by
.B input()
as
.I EOF.
.IP
Input is instead controlled by defining the
.B YY_INPUT
macro.
.IP
The
.I flex
restriction that
.B input()
cannot be redefined is in accordance with the POSIX specification,
which simply does not specify any way of controlling the
scanner's input other than by making an initial assignment to
.I yyin.
.IP -
The
.B unput()
routine is not redefinable.  This restriction is in accordance with POSIX.
.IP -
.I flex
scanners are not as reentrant as
.I lex
scanners.  In particular, if you have an interactive scanner and
an interrupt handler which long-jumps out of the scanner, and
the scanner is subsequently called again, you may get the following
message:
.nf

    fatal flex scanner internal error--end of buffer missed

.fi
To reenter the scanner, first use
.nf

    yyrestart( yyin );

.fi
Note that this call will throw away any buffered input; usually this
isn't a problem with an interactive scanner.
.IP
Also note that flex C++ scanner classes
.I are
reentrant, so if using C++ is an option for you, you should use
them instead.  See "Generating C++ Scanners" above for details.
.IP -
.B output()
is not supported.
Output from the
.B ECHO
macro is done to the file-pointer
.I yyout
(default
.I stdout).
.IP
.B output()
is not part of the POSIX specification.
.IP -
.I lex
does not support exclusive start conditions (%x), though they
are in the POSIX specification.
.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.
.IP
Note that if the definition begins with
.B ^
or ends with
.B $
then it is
.I not
expanded with parentheses, to allow these operators to appear in
definitions without losing their special meanings.  But the
.B <s>, /,
and
.B <<EOF>>
operators cannot be used in a
.I flex
definition.
.IP
Using
.B \-l
results in the
.I lex
behavior of no parentheses around the definition.
.IP
The POSIX specification is that the definition be enclosed in parentheses.
.IP -
Some implementations of
.I lex
allow a rule's action to begin on a separate line, if the rule's pattern
has trailing whitespace:
.nf

    %%
    foo|bar<space here>
      { foobar_action(); }

.fi
.I flex
does not support this feature.
.IP -
The
.I lex
.B %r
(generate a Ratfor scanner) option is not supported.  It is not part
of the POSIX specification.
.IP -
After a call to
.B unput(),
.I yytext
is undefined until the next token is matched, unless the scanner
was built using
.B %array.
This is not the case with
.I lex
or the POSIX specification.  The
.B \-l
option does away with this incompatibility.
.IP -
The precedence of the
.B {}
(numeric range) operator is different.
.I 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 POSIX specification.
.IP -
The precedence of the
.B ^
operator is different.
.I lex
interprets "^foo|bar" as "match either 'foo' at the beginning of a line,
or 'bar' anywhere", whereas
.I flex
interprets it as "match either 'foo' or 'bar' if they come at the beginning
of a line".  The latter is in agreement with the POSIX specification.
.IP -
The special table-size declarations such as
.B %a
supported by
.I lex
are not required by
.I flex
scanners;
.I flex
ignores them.
.IP -
The name
.bd
FLEX_SCANNER
is #define'd so scanners may be written for use with either
.I flex
or
.I lex.
Scanners also include
.B YY_FLEX_MAJOR_VERSION
and
.B YY_FLEX_MINOR_VERSION
indicating which version of
.I flex
generated the scanner
(for example, for the 2.5 release, these defines would be 2 and 5
respectively).
.PP
The following
.I flex
features are not included in
.I lex
or the POSIX specification:
.nf

    C++ scanners
    %option
    start condition scopes
    start condition stacks
    interactive/non-interactive scanners
    yy_scan_string() and friends
    yyterminate()
    yy_set_interactive()
    yy_set_bol()
    YY_AT_BOL()
    <<EOF>>
    <*>
    YY_DECL
    YY_START
    YY_USER_ACTION
    YY_USER_INIT
    #line directives
    %{}'s around actions
    multiple actions on a line

.fi
plus almost all of the flex flags.
The last feature in the list refers to the fact that with
.I flex
you can put multiple actions on the same line, separated with
semi-colons, while with
.I lex,
the following
.nf

    foo    handle_foo(); ++num_foos_seen;

.fi
is (rather surprisingly) truncated to
.nf

    foo    handle_foo();

.fi
.I flex
does not truncate the action.  Actions that are not enclosed in
braces are simply terminated at the end of the line.
.SH DIAGNOSTICS
.PP
.I warning, rule cannot be matched
indicates that the given rule
cannot be matched because it follows other rules that will
always match the same text as it.  For
example, in the following "foo" cannot be matched because it comes after
an identifier "catch-all" rule:
.nf

    [a-z]+    got_identifier();
    foo       got_foo();

.fi
Using
.B REJECT
in a scanner suppresses this warning.
.PP
.I warning,
.B \-s
.I
option given but default rule can be matched
means that it is possible (perhaps only in a particular start condition)
that the default rule (match any single character) is the only one
that will match a particular input.  Since
.B \-s
was given, presumably this is not intended.
.PP
.I reject_used_but_not_detected undefined
or
.I yymore_used_but_not_detected undefined -
These errors can occur at compile time.  They indicate that the
scanner uses
.B REJECT
or
.B yymore()
but that
.I flex
failed to notice the fact, meaning that
.I flex
scanned the first two sections looking for occurrences of these actions
and failed to find any, but somehow you snuck some in (via a #include
file, for example).  Use
.B %option reject
or
.B %option yymore
to indicate to flex that you really do use these features.
.PP
.I flex scanner jammed -
a scanner compiled with
.B \-s
has encountered an input string which wasn't matched by
any of its rules.  This error can also occur due to internal problems.
.PP
.I token too large, exceeds YYLMAX -
your scanner uses
.B %array
and one of its rules matched a string longer than the
.B YYLMAX
constant (8K bytes by default).  You can increase the value by
#define'ing
.B YYLMAX
in the definitions section of your
.I flex
input.
.PP
.I scanner requires \-8 flag to
.I use the character 'x' -
Your scanner specification includes recognizing the 8-bit character
.I 'x'
and you did not specify the \-8 flag, and your scanner defaulted to 7-bit
because you used the
.B \-Cf
or
.B \-CF
table compression options.  See the discussion of the
.B \-7
flag for details.
.PP
.I flex scanner push-back overflow -
you used
.B unput()
to push back so much text that the scanner's buffer could not hold
both the pushed-back text and the current token in
.B yytext.
Ideally the scanner should dynamically resize the buffer in this case, but at
present it does not.
.PP
.I
input buffer overflow, can't enlarge buffer because scanner uses REJECT -
the scanner was working on matching an extremely large token and needed
to expand the input buffer.  This doesn't work with scanners that use
.B
REJECT.
.PP
.I
fatal flex scanner internal error--end of buffer missed -
This can occur in an scanner which is reentered after a long-jump
has jumped out (or over) the scanner's activation frame.  Before
reentering the scanner, use:
.nf

    yyrestart( yyin );

.fi
or, as noted above, switch to using the C++ scanner class.
.PP
.I too many start conditions in <> construct! -
you listed more start conditions in a <> construct than exist (so
you must have listed at least one of them twice).
.SH FILES
.TP
.B \-lfl
library with which scanners must be linked.
.TP
.I lex.yy.c
generated scanner (called
.I lexyy.c
on some systems).
.TP
.I lex.yy.cc
generated C++ scanner class, when using
.B -+.
.TP
.I <FlexLexer.h>
header file defining the C++ scanner base class,
.B FlexLexer,
and its derived class,
.B yyFlexLexer.
.TP
.I flex.skl
skeleton scanner.  This file is only used when building flex, not when
flex executes.
.TP
.I lex.backup
backing-up information for
.B \-b
flag (called
.I lex.bck
on some systems).
.SH DEFICIENCIES / BUGS
.PP
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.)
.PP
For some trailing context rules, parts which are actually fixed-length are
not recognized as such, leading to the abovementioned performance loss.
In particular, parts using '|' or {n} (such as "foo{3}") are always
considered variable-length.
.PP
Combining trailing context with the special '|' action can result in
.I fixed
trailing context being turned into the more expensive
.I variable
trailing context.  For example, in the following:
.nf

    %%
    abc      |
    xyz/def

.fi
.PP
Use of
.B unput()
invalidates yytext and yyleng, unless the
.B %array
directive
or the
.B \-l
option has been used.
.PP
Pattern-matching of NUL's is substantially slower than matching other
characters.
.PP
Dynamic resizing of the input buffer is slow, as it entails rescanning
all the text matched so far by the current (generally huge) token.
.PP
Due to both buffering of input and read-ahead, you cannot intermix
calls to <stdio.h> routines, such as, for example,
.B getchar(),
with
.I flex
rules and expect it to work.  Call
.B input()
instead.
.PP
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
.B REJECT,
and somewhat greater than the number of states if it does.
.PP
.B REJECT
cannot be used with the
.B \-f
or
.B \-F
options.
.PP
The
.I flex
internal algorithms need documentation.
.SH SEE ALSO
.PP
lex(1), yacc(1), sed(1), awk(1).
.PP
John Levine, Tony Mason, and Doug Brown,
.I Lex & Yacc,
O'Reilly and Associates.  Be sure to get the 2nd edition.
.PP
M. E. Lesk and E. Schmidt,
.I LEX \- Lexical Analyzer Generator
.PP
Alfred Aho, Ravi Sethi and Jeffrey Ullman,
.I Compilers: Principles, Techniques and Tools,
Addison-Wesley (1986).  Describes the pattern-matching techniques used by
.I flex
(deterministic finite automata).
.SH AUTHOR
Vern Paxson, with the help of many ideas and much inspiration from
Van Jacobson.  Original version by Jef Poskanzer.  The fast table
representation is a partial implementation of a design done by Van
Jacobson.  The implementation was done by Kevin Gong and Vern Paxson.
.PP
Thanks to the many
.I flex
beta-testers, feedbackers, and contributors, especially Francois Pinard,
Casey Leedom,
Robert Abramovitz,
Stan Adermann, Terry Allen, David Barker-Plummer, John Basrai,
Nelson H.F. Beebe, benson@odi.com,
Karl Berry, Peter A. Bigot, Simon Blanchard,
Keith Bostic, Frederic Brehm, Ian Brockbank, Kin Cho, Nick Christopher,
Brian Clapper, J.T. Conklin,
Jason Coughlin, Bill Cox, Nick Cropper, Dave Curtis, Scott David
Daniels, Chris G. Demetriou, Theo Deraadt,
Mike Donahue, Chuck Doucette, Tom Epperly, Leo Eskin,
Chris Faylor, Chris Flatters, Jon Forrest, Jeffrey Friedl,
Joe Gayda, Kaveh R. Ghazi, Wolfgang Glunz,
Eric Goldman, Christopher M. Gould, Ulrich Grepel, Peer Griebel,
Jan Hajic, Charles Hemphill, NORO Hideo,
Jarkko Hietaniemi, Scott Hofmann,
Jeff Honig, Dana Hudes, Eric Hughes, John Interrante,
Ceriel Jacobs, Michal Jaegermann, Sakari Jalovaara, Jeffrey R. Jones,
Henry Juengst, Klaus Kaempf, Jonathan I. Kamens, Terrence O Kane,
Amir Katz, ken@ken.hilco.com, Kevin B. Kenny,
Steve Kirsch, Winfried Koenig, Marq Kole, Ronald Lamprecht,
Greg Lee, Rohan Lenard, Craig Leres, John Levine, Steve Liddle,
David Loffredo, Mike Long,
Mohamed el Lozy, Brian Madsen, Malte, Joe Marshall,
Bengt Martensson, Chris Metcalf,
Luke Mewburn, Jim Meyering, R. Alexander Milowski, Erik Naggum,
G.T. Nicol, Landon Noll, James Nordby, Marc Nozell,
Richard Ohnemus, Karsten Pahnke,
Sven Panne, Roland Pesch, Walter Pelissero, Gaumond
Pierre, Esmond Pitt, Jef Poskanzer, Joe Rahmeh, Jarmo Raiha,
Frederic Raimbault, Pat Rankin, Rick Richardson,
Kevin Rodgers, Kai Uwe Rommel, Jim Roskind, Alberto Santini,
Andreas Scherer, Darrell Schiebel, Raf Schietekat,
Doug Schmidt, Philippe Schnoebelen, Andreas Schwab,
Larry Schwimmer, Alex Siegel, Eckehard Stolz, Jan-Erik Strvmquist,
Mike Stump, Paul Stuart, Dave Tallman, Ian Lance Taylor,
Chris Thewalt, Richard M. Timoney, Jodi Tsai,
Paul Tuinenga, Gary Weik, Frank Whaley, Gerhard Wilhelms, Kent Williams, Ken
Yap, Ron Zellar, Nathan Zelle, David Zuhn,
and those whose names have slipped my marginal
mail-archiving skills but whose contributions are appreciated all the
same.
.PP
Thanks to Keith Bostic, Jon Forrest, Noah Friedman,
John Gilmore, Craig Leres, John Levine, Bob Mulcahy, G.T.
Nicol, Francois Pinard, Rich Salz, and Richard Stallman for help with various
distribution headaches.
.PP
Thanks to Esmond Pitt and Earle Horton for 8-bit character support; to
Benson Margulies and Fred Burke for C++ support; to Kent Williams and Tom
Epperly for C++ class support; to Ove Ewerlid for support of NUL's; and to
Eric Hughes for support of multiple buffers.
.PP
This work was primarily done when I was with the Real Time Systems Group
at the Lawrence Berkeley Laboratory in Berkeley, CA.  Many thanks to all there
for the support I received.
.PP
Send comments to vern@ee.lbl.gov.