MacFormat 1994 November/macformat-018.iso/Utility Spectacular/Developer/flex-2.4.6/flexdoc.1

Text File | 1994-07-23 | 78.1 KB | 3,046 lines | [TEXT/KAHL]

.TH FLEXDOC 1 "November 1993" "Version 2.4"
.SH NAME
flexdoc \- documentation for flex, fast lexical analyzer generator
.SH SYNOPSIS
.B flex
.B [\-bcdfhilnpstvwBFILTV78+ \-C[aefFmr] \-Pprefix \-Sskeleton]
.I [filename ...]
.SH DESCRIPTION
.I flex
is a tool for generating
.I scanners:
programs which recognized lexical patterns in text.
.I flex
reads
the given input files, or its standard input if no file names are given,
for a description of a scanner to generate.  The description is in
the form of pairs
of regular expressions and C code, called
.I rules.  flex
generates as output a C source file,
.B lex.yy.c,
which defines a routine
.B yylex().
This file is compiled and linked with the
.B \-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 except newline
    [xyz]      a "character class"; in this case, the pattern
                 matches either an 'x', a 'y', or a 'z'
    [abj-oZ]   a "character class" with a range in it; matches
                 an 'a', a 'b', any letter from 'j' through 'o',
                 or a 'Z'
    [^A-Z]     a "negated character class", i.e., any character
                 but those in the class.  In this case, any
                 character EXCEPT an uppercase letter.
    [^A-Z\\n]   any character EXCEPT an uppercase letter or
                 a newline
    r*         zero or more r's, where r is any regular expression
    r+         one or more r's
    r?         zero or one r's (that is, "an optional r")
    r{2,5}     anywhere from two to five r's
    r{2,}      two or more r's
    r{4}       exactly 4 r's
    {name}     the expansion of the "name" definition
               (see above)
    "[xyz]\\"foo"
               the literal string: [xyz]"foo
    \\X         if X is an 'a', 'b', 'f', 'n', 'r', 't', or 'v',
                 then the ANSI-C interpretation of \\x.
                 Otherwise, a literal 'X' (used to escape
                 operators such as '*')
    \\123       the character with octal value 123
    \\x2a       the character with hexadecimal value 2a
    (r)        match an r; parentheses are used to override
                 precedence (see below)


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


    r|s        either an r or an s


    r/s        an r but only if it is followed by an s.  The
                 s is not part of the matched text.  This type
                 of pattern is called as "trailing context".
    ^r         an r, but only at the beginning of a line
    r$         an r, but only at the end of a line.  Equivalent
                 to "r/\\n".


    <s>r       an r, but only in start condition s (see
               below for discussion of start conditions)
    <s1,s2,s3>r
               same, but in any of start conditions s1,
               s2, or s3
    <*>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
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 input()
and
.B unput()
functions destroy 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 input()
and
.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 accomodate large tokens.  While this means your
.B %pointer
scanner can accomodate 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 \-+
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).  Modifying the final character of yytext may alter
whether when scanning resumes rules anchored with '^' are active.
Specifically, changing the final character of yytext to a newline will
activate such rules on the next scan, and changing it to anything else
will deactivate the rules.  Users should not rely on this behavior being
present in future releases.  Finally, note that none of this paragraph
applies when using
.B %array
(see above).
.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 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".
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;
    unput( ')' );
    for ( i = yyleng - 1; i >= 0; --i )
        unput( yytext[i] );
    unput( '(' );
    }

.fi
Note that since each
.B unput()
puts the given character back at the
.I beginning
of the input stream, pushing back strings must be done back-to-front.
Also 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 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, 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.
.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
You also can add in things like keeping track of the
input line number this way; but don't expect your scanner to
go very fast.
.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.
.PP
The default
.B yywrap()
always returns 1.
.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 */

.fi
is equivalent to
.nf

    %x example
    %%
    <INITIAL,example>foo   /* do something */

.fi
.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
    %%
    <*>foo   /* do something */

.fi
.PP
The default rule (to
.B ECHO
any unmatched character) remains active in start conditions.
.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
.I 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
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
.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 *yytext_ptr = yytext;

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

.fi
.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:
.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.
.nf

    void yy_delete_buffer( YY_BUFFER_STATE buffer )

.fi
is used to reclaim the storage associated with a buffer.
.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
.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
.bd
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.
.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
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 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 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.
.IP
.B NOTE:
in previous releases of
.I flex
.B \-c
specified table-compression options.  This functionality is
now given by the
.B \-C
flag.  To ease the the impact of this change, when
.I flex
encounters
.B \-c,
it currently issues a warning message and assumes that
.B \-C
was desired instead.  In the future this "promotion" of
.B \-c
to
.B \-C
will go away in the name of full POSIX compliance (unless
the POSIX meaning is removed first).
.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 stderr
and then exits.
.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.
.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
and variable trailing context (see the Bugs section in flex(1))
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 the original
.I flex
input file, and not to
the fairly meaningless line numbers of
.B lex.yy.c.
(Unfortunately
.I flex
does not presently generate the necessary directives
to "retarget" the line numbers for those parts of
.B lex.yy.c
which it generated.  So if there is an error in the generated code,
a meaningless line number is reported.)
.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 stderr
and exits.
.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 datums 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 \-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

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

.fi
Within your scanner itself, you can still refer to the global variables
and functions using either version of their name; but eternally, 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
provide your own (appropriately-named) version of the routine for your
scanner, 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.
.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

    pattern sets that require backing up
    arbitrary trailing context

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

.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
.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
.PP
A final note regarding performance: as mentioned above in the section
How the Input is Matched, dynamically resizing
.B yytext
to accomodate 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.
.PP
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.
.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, 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.
.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_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.
.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
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.  The POSIX
.I lex
specification is closer to
.I flex's
behavior than that of the original
.I lex
implementation, but there also remain some incompatibilities between
.I flex
and POSIX.  The intent is that ultimately
.I flex
will be fully POSIX-conformant.  In this section we discuss all of
the known areas of incompatibility.
.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
is used.
.IP
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 -
.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 -
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
and
.I yyleng
are 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 -
.I yyin
is
.I initialized
by
.I lex
to be
.I stdin;
.I flex,
on the other hand,
initializes
.I yyin
to NULL
and then
.I assigns
it to
.I stdin
the first time the scanner is called, providing
.I yyin
has not already been assigned to a non-NULL value.  The difference is
subtle, but the net effect is that with
.I flex
scanners,
.I yyin
does not have a valid value until the scanner has been called.
.IP
The
.B \-l
option does away with this incompatibility.
.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.
.PP
The following
.I flex
features are not included in
.I lex
or the POSIX specification:
.nf

    yyterminate()
    <<EOF>>
    <*>
    YY_DECL
    YY_START
    YY_USER_ACTION
    #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).  Make an explicit reference to the action in your
.I flex
input file.  (Note that previously
.I flex
supported a
.B %used/%unused
mechanism for dealing with this problem; this feature is still supported
but now deprecated, and will go away soon unless the author hears from
people who can argue compellingly that they need it.)
.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
See flex(1).
.SH DEFICIENCIES / BUGS
Again, see flex(1).
.SH "SEE ALSO"
.PP
flex(1), lex(1), yacc(1), sed(1), awk(1).
.PP
M. E. Lesk and E. Schmidt,
.I LEX \- Lexical Analyzer Generator
.SH AUTHOR
Vern Paxson, with the help of many ideas and much inspiration from
Van Jacobson.  Original version by Jef Poskanzer.  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,
Nelson H.F. Beebe, [e-mail], Peter A. Bigot, Keith Bostic, Frederic
Brehm, Nick Christopher, Jason Coughlin, Bill Cox, Dave Curtis, Scott David
Daniels, Chris G. Demetriou, Mike Donahue, Chuck Doucette, Tom Epperly, Leo
Eskin, Chris Faylor, Jon Forrest, Kaveh R. Ghazi,
Eric Goldman, Ulrich Grepel, Jan Hajic,
Jarkko Hietaniemi, Eric Hughes, John Interrante,
Ceriel Jacobs, Jeffrey R. Jones, Henry
Juengst, Amir Katz, [e-mail], Kevin B. Kenny, Marq Kole, Ronald
Lamprecht, Greg Lee, Craig Leres, John Levine, Steve Liddle,
Mohamed el Lozy, Brian Madsen, Chris
Metcalf, Luke Mewburn, Jim Meyering, G.T. Nicol, Landon Noll, Marc Nozell,
Richard Ohnemus, Sven Panne, Roland Pesch, Walter Pelissero, Gaumond
Pierre, Esmond Pitt, Jef Poskanzer, Joe Rahmeh, Frederic Raimbault,
Rick Richardson,
Kevin Rodgers, Jim Roskind,
Doug Schmidt, Philippe Schnoebelen, Andreas Schwab,
Alex Siegel, Mike Stump, Paul Stuart, Dave Tallman, Chris Thewalt,
Paul Tuinenga, Gary Weik, Frank Whaley, Gerhard Wilhelms, Kent Williams, Ken
Yap, 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:
.nf

     Vern Paxson
     Systems Engineering
     Bldg. 46A, Room 1123
     Lawrence Berkeley Laboratory
     University of California
     Berkeley, CA 94720

     [e-mail]

.fi