libregex/doc/xregex.texi

\input texinfo
@c %**start of header
@setfilename regex.info
@settitle Regex
@c %**end of header

@c \\{fill-paragraph} works better (for me, anyway) if the text in the
@c source file isn't indented.
@paragraphindent 2

@c Define a new index for our magic constants.
@defcodeindex cn

@c Put everything in one index (arbitrarily chosen to be the concept index).
@syncodeindex cn cp
@syncodeindex ky cp
@syncodeindex pg cp
@syncodeindex tp cp
@syncodeindex vr cp

@c Here is what we use in the Info `dir' file:
@c * Regex: (regex).	Regular expression library.


@ifinfo
This file documents the GNU regular expression library.

Copyright (C) 1992, 1993 Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

@ignore
Permission is granted to process this file through TeX and print the
results, provided the printed document carries a copying permission
notice identical to this one except for the removal of this paragraph
(this paragraph not being relevant to the printed manual).
@end ignore

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided also that the
section entitled ``GNU General Public License'' is included exactly as
in the original, and provided that the entire resulting derived work is
distributed under the terms of a permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions,
except that the section entitled ``GNU General Public License'' may be
included in a translation approved by the Free Software Foundation
instead of in the original English.
@end ifinfo


@titlepage

@title Regex
@subtitle edition 0.12a
@subtitle 19 September 1992
@author Kathryn A. Hargreaves
@author Karl Berry

@page

@vskip 0pt plus 1filll
Copyright @copyright{} 1992 Free Software Foundation.

Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided also that the
section entitled ``GNU General Public License'' is included exactly as
in the original, and provided that the entire resulting derived work is
distributed under the terms of a permission notice identical to this
one.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions,
except that the section entitled ``GNU General Public License'' may be
included in a translation approved by the Free Software Foundation
instead of in the original English.

@end titlepage


@ifinfo
@node Top, Overview, (dir), (dir)
@top Regular Expression Library

This manual documents how to program with the GNU regular expression
library.  This is edition 0.12a of the manual, 19 September 1992.

The first part of this master menu lists the major nodes in this Info
document, including the index.  The rest of the menu lists all the
lower level nodes in the document.

@menu
* Overview::
* Regular Expression Syntax::
* Common Operators::
* GNU Operators::
* GNU Emacs Operators::
* What Gets Matched?::
* Programming with Regex::
* Copying::			Copying and sharing Regex.
* Index::			General index.
 --- The Detailed Node Listing ---

Regular Expression Syntax

* Syntax Bits::
* Predefined Syntaxes::
* Collating Elements vs. Characters::
* The Backslash Character::

Common Operators

* Match-self Operator::			Ordinary characters.
* Match-any-character Operator::	.
* Concatenation Operator::		Juxtaposition.
* Repetition Operators::		*  +  ? @{@}
* Alternation Operator::		|
* List Operators::			[...]  [^...]
* Grouping Operators::			(...)
* Back-reference Operator::		\digit
* Anchoring Operators::			^  $

Repetition Operators

* Match-zero-or-more Operator::  *
* Match-one-or-more Operator::   +
* Match-zero-or-one Operator::   ?
* Interval Operators::           @{@}

List Operators (@code{[} @dots{} @code{]} and @code{[^} @dots{} @code{]})

* Character Class Operators::   [:class:]
* Range Operator::          start-end

Anchoring Operators

* Match-beginning-of-line Operator::  ^
* Match-end-of-line Operator::        $

GNU Operators

* Word Operators::
* Buffer Operators::

Word Operators

* Non-Emacs Syntax Tables::
* Match-word-boundary Operator::	\b
* Match-within-word Operator::		\B
* Match-beginning-of-word Operator::	\<
* Match-end-of-word Operator::		\>
* Match-word-constituent Operator::	\w
* Match-non-word-constituent Operator::	\W

Buffer Operators

* Match-beginning-of-buffer Operator::	\`
* Match-end-of-buffer Operator::	\'

GNU Emacs Operators

* Syntactic Class Operators::

Syntactic Class Operators

* Emacs Syntax Tables::
* Match-syntactic-class Operator::	\sCLASS
* Match-not-syntactic-class Operator::  \SCLASS

Programming with Regex

* GNU Regex Functions::
* POSIX Regex Functions::
* BSD Regex Functions::

GNU Regex Functions

* GNU Pattern Buffers::         The re_pattern_buffer type.
* GNU Regular Expression Compiling::  re_compile_pattern ()
* GNU Matching::                re_match ()
* GNU Searching::               re_search ()
* Matching/Searching with Split Data::  re_match_2 (), re_search_2 ()
* Searching with Fastmaps::     re_compile_fastmap ()
* GNU Translate Tables::        The `translate' field.
* Using Registers::             The re_registers type and related fns.
* Freeing GNU Pattern Buffers::  regfree ()

POSIX Regex Functions

* POSIX Pattern Buffers::		The regex_t type.
* POSIX Regular Expression Compiling::	regcomp ()
* POSIX Matching::			regexec ()
* Reporting Errors::			regerror ()
* Using Byte Offsets::			The regmatch_t type.
* Freeing POSIX Pattern Buffers::	regfree ()

BSD Regex Functions

* BSD Regular Expression Compiling::	re_comp ()
* BSD Searching::			re_exec ()
@end menu
@end ifinfo
@node Overview, Regular Expression Syntax, Top, Top
@chapter Overview

A @dfn{regular expression} (or @dfn{regexp}, or @dfn{pattern}) is a text
string that describes some (mathematical) set of strings.  A regexp
@var{r} @dfn{matches} a string @var{s} if @var{s} is in the set of
strings described by @var{r}.

Using the Regex library, you can:

@itemize @bullet

@item
see if a string matches a specified pattern as a whole, and

@item
search within a string for a substring matching a specified pattern.

@end itemize

Some regular expressions match only one string, i.e., the set they
describe has only one member.  For example, the regular expression
@samp{foo} matches the string @samp{foo} and no others.  Other regular
expressions match more than one string, i.e., the set they describe has
more than one member.  For example, the regular expression @samp{f*}
matches the set of strings made up of any number (including zero) of
@samp{f}s.  As you can see, some characters in regular expressions match
themselves (such as @samp{f}) and some don't (such as @samp{*}); the
ones that don't match themselves instead let you specify patterns that
describe many different strings.

To either match or search for a regular expression with the Regex
library functions, you must first compile it with a Regex pattern
compiling function.  A @dfn{compiled pattern} is a regular expression
converted to the internal format used by the library functions.  Once
you've compiled a pattern, you can use it for matching or searching any
number of times.

The Regex library consists of two source files: @file{regex.h} and
@file{regex.c}.
@pindex regex.h
@pindex regex.c
Regex provides three groups of functions with which you can operate on
regular expressions.  One group---the @sc{gnu} group---is more powerful
but not completely compatible with the other two, namely the @sc{posix}
and Berkeley @sc{unix} groups; its interface was designed specifically
for @sc{gnu}.  The other groups have the same interfaces as do the
regular expression functions in @sc{posix} and Berkeley
@sc{unix}.

We wrote this chapter with programmers in mind, not users of
programs---such as Emacs---that use Regex.  We describe the Regex
library in its entirety, not how to write regular expressions that a
particular program understands.


@node Regular Expression Syntax, Common Operators, Overview, Top
@chapter Regular Expression Syntax

@cindex regular expressions, syntax of
@cindex syntax of regular expressions

@dfn{Characters} are things you can type.  @dfn{Operators} are things in
a regular expression that match one or more characters.  You compose
regular expressions from operators, which in turn you specify using one
or more characters.

Most characters represent what we call the match-self operator, i.e.,
they match themselves; we call these characters @dfn{ordinary}.  Other
characters represent either all or parts of fancier operators; e.g.,
@samp{.} represents what we call the match-any-character operator
(which, no surprise, matches (almost) any character); we call these
characters @dfn{special}.  Two different things determine what
characters represent what operators:

@enumerate
@item
the regular expression syntax your program has told the Regex library to
recognize, and

@item
the context of the character in the regular expression.
@end enumerate

In the following sections, we describe these things in more detail.

@menu
* Syntax Bits::
* Predefined Syntaxes::
* Collating Elements vs. Characters::
* The Backslash Character::
@end menu


@node Syntax Bits, Predefined Syntaxes,  , Regular Expression Syntax
@section Syntax Bits

@cindex syntax bits

In any particular syntax for regular expressions, some characters are
always special, others are sometimes special, and others are never
special.  The particular syntax that Regex recognizes for a given
regular expression depends on the value in the @code{syntax} field of
the pattern buffer of that regular expression.

You get a pattern buffer by compiling a regular expression.  @xref{GNU
Pattern Buffers}, and @ref{POSIX Pattern Buffers}, for more information
on pattern buffers.  @xref{GNU Regular Expression Compiling}, @ref{POSIX
Regular Expression Compiling}, and @ref{BSD Regular Expression
Compiling}, for more information on compiling.

Regex considers the value of the @code{syntax} field to be a collection
of bits; we refer to these bits as @dfn{syntax bits}.  In most cases,
they affect what characters represent what operators.  We describe the
meanings of the operators to which we refer in @ref{Common Operators},
@ref{GNU Operators}, and @ref{GNU Emacs Operators}.

For reference, here is the complete list of syntax bits, in alphabetical
order:

@table @code

@cnindex RE_BACKSLASH_ESCAPE_IN_LIST
@item RE_BACKSLASH_ESCAPE_IN_LISTS
If this bit is set, then @samp{\} inside a list (@pxref{List Operators}
quotes (makes ordinary, if it's special) the following character; if
this bit isn't set, then @samp{\} is an ordinary character inside lists.
(@xref{The Backslash Character}, for what `\' does outside of lists.)

@cnindex RE_BK_PLUS_QM
@item RE_BK_PLUS_QM
If this bit is set, then @samp{\+} represents the match-one-or-more
operator and @samp{\?} represents the match-zero-or-more operator; if
this bit isn't set, then @samp{+} represents the match-one-or-more
operator and @samp{?} represents the match-zero-or-one operator.  This
bit is irrelevant if @code{RE_LIMITED_OPS} is set.

@cnindex RE_CHAR_CLASSES
@item RE_CHAR_CLASSES
If this bit is set, then you can use character classes in lists; if this
bit isn't set, then you can't.

@cnindex RE_CONTEXT_INDEP_ANCHORS
@item RE_CONTEXT_INDEP_ANCHORS
If this bit is set, then @samp{^} and @samp{$} are special anywhere outside
a list; if this bit isn't set, then these characters are special only in
certain contexts.  @xref{Match-beginning-of-line Operator}, and
@ref{Match-end-of-line Operator}.

@cnindex RE_CONTEXT_INDEP_OPS
@item RE_CONTEXT_INDEP_OPS
If this bit is set, then certain characters are special anywhere outside
a list; if this bit isn't set, then those characters are special only in
some contexts and are ordinary elsewhere.  Specifically, if this bit
isn't set then @samp{*}, and (if the syntax bit @code{RE_LIMITED_OPS}
isn't set) @samp{+} and @samp{?} (or @samp{\+} and @samp{\?}, depending
on the syntax bit @code{RE_BK_PLUS_QM}) represent repetition operators
only if they're not first in a regular expression or just after an
open-group or alternation operator.  The same holds for @samp{@{} (or
@samp{\@{}, depending on the syntax bit @code{RE_NO_BK_BRACES}) if
it is the beginning of a valid interval and the syntax bit
@code{RE_INTERVALS} is set.

@cnindex RE_CONTEXT_INVALID_OPS
@item RE_CONTEXT_INVALID_OPS
If this bit is set, then repetition and alternation operators can't be
in certain positions within a regular expression.  Specifically, the
regular expression is invalid if it has:

@itemize @bullet

@item
a repetition operator first in the regular expression or just after a
match-beginning-of-line, open-group, or alternation operator; or

@item
an alternation operator first or last in the regular expression, just
before a match-end-of-line operator, or just after an alternation or
open-group operator.

@end itemize

If this bit isn't set, then you can put the characters representing the
repetition and alternation characters anywhere in a regular expression.
Whether or not they will in fact be operators in certain positions
depends on other syntax bits.

@cnindex RE_DOT_NEWLINE
@item RE_DOT_NEWLINE
If this bit is set, then the match-any-character operator matches
a newline; if this bit isn't set, then it doesn't.

@cnindex RE_DOT_NOT_NULL
@item RE_DOT_NOT_NULL
If this bit is set, then the match-any-character operator doesn't match
a null character; if this bit isn't set, then it does.

@cnindex RE_INTERVALS
@item RE_INTERVALS
If this bit is set, then Regex recognizes interval operators; if this bit
isn't set, then it doesn't.

@cnindex RE_LIMITED_OPS
@item RE_LIMITED_OPS
If this bit is set, then Regex doesn't recognize the match-one-or-more,
match-zero-or-one or alternation operators; if this bit isn't set, then
it does.

@cnindex RE_NEWLINE_ALT
@item RE_NEWLINE_ALT
If this bit is set, then newline represents the alternation operator; if
this bit isn't set, then newline is ordinary.

@cnindex RE_NO_BK_BRACES
@item RE_NO_BK_BRACES
If this bit is set, then @samp{@{} represents the open-interval operator
and @samp{@}} represents the close-interval operator; if this bit isn't
set, then @samp{\@{} represents the open-interval operator and
@samp{\@}} represents the close-interval operator.  This bit is relevant
only if @code{RE_INTERVALS} is set.

@cnindex RE_NO_BK_PARENS
@item RE_NO_BK_PARENS
If this bit is set, then @samp{(} represents the open-group operator and
@samp{)} represents the close-group operator; if this bit isn't set, then
@samp{\(} represents the open-group operator and @samp{\)} represents
the close-group operator.

@cnindex RE_NO_BK_REFS
@item RE_NO_BK_REFS
If this bit is set, then Regex doesn't recognize @samp{\}@var{digit} as
the back reference operator; if this bit isn't set, then it does.

@cnindex RE_NO_BK_VBAR
@item RE_NO_BK_VBAR
If this bit is set, then @samp{|} represents the alternation operator;
if this bit isn't set, then @samp{\|} represents the alternation
operator.  This bit is irrelevant if @code{RE_LIMITED_OPS} is set.

@cnindex RE_NO_EMPTY_RANGES
@item RE_NO_EMPTY_RANGES
If this bit is set, then a regular expression with a range whose ending
point collates lower than its starting point is invalid; if this bit
isn't set, then Regex considers such a range to be empty.

@cnindex RE_UNMATCHED_RIGHT_PAREN_ORD
@item RE_UNMATCHED_RIGHT_PAREN_ORD
If this bit is set and the regular expression has no matching open-group
operator, then Regex considers what would otherwise be a close-group
operator (based on how @code{RE_NO_BK_PARENS} is set) to match @samp{)}.

@end table


@node Predefined Syntaxes, Collating Elements vs. Characters, Syntax Bits, Regular Expression Syntax
@section Predefined Syntaxes

If you're programming with Regex, you can set a pattern buffer's
(@pxref{GNU Pattern Buffers}, and @ref{POSIX Pattern Buffers})
@code{syntax} field either to an arbitrary combination of syntax bits
(@pxref{Syntax Bits}) or else to the configurations defined by Regex.
These configurations define the syntaxes used by certain
programs---@sc{gnu} Emacs,
@cindex Emacs
@sc{posix} Awk,
@cindex POSIX Awk
traditional Awk,
@cindex Awk
Grep,
@cindex Grep
@cindex Egrep
Egrep---in addition to syntaxes for @sc{posix} basic and extended
regular expressions.

The predefined syntaxes--taken directly from @file{regex.h}---are:

@example
[[[ syntaxes ]]]
@end example

@node Collating Elements vs. Characters, The Backslash Character, Predefined Syntaxes, Regular Expression Syntax
@section Collating Elements vs.@: Characters

@sc{posix} generalizes the notion of a character to that of a
collating element.  It defines a @dfn{collating element} to be ``a
sequence of one or more bytes defined in the current collating sequence
as a unit of collation.''

This generalizes the notion of a character in
two ways.  First, a single character can map into two or more collating
elements.  For example, the German
@tex
`\ss'
@end tex
@ifinfo
``es-zet''
@end ifinfo
collates as the collating element @samp{s} followed by another collating
element @samp{s}.  Second, two or more characters can map into one
collating element.  For example, the Spanish @samp{ll} collates after
@samp{l} and before @samp{m}.

Since @sc{posix}'s ``collating element'' preserves the essential idea of
a ``character,'' we use the latter, more familiar, term in this document.

@node The Backslash Character,  , Collating Elements vs. Characters, Regular Expression Syntax
@section The Backslash Character

@cindex \
The @samp{\} character has one of four different meanings, depending on
the context in which you use it and what syntax bits are set
(@pxref{Syntax Bits}).  It can: 1) stand for itself, 2) quote the next
character, 3) introduce an operator, or 4) do nothing.

@enumerate
@item
It stands for itself inside a list
(@pxref{List Operators}) if the syntax bit
@code{RE_BACKSLASH_ESCAPE_IN_LISTS} is not set.  For example, @samp{[\]}
would match @samp{\}.

@item
It quotes (makes ordinary, if it's special) the next character when you
use it either:

@itemize @bullet
@item
outside a list,@footnote{Sometimes
you don't have to explicitly quote special characters to make
them ordinary.  For instance, most characters lose any special meaning
inside a list (@pxref{List Operators}).  In addition, if the syntax bits
@code{RE_CONTEXT_INVALID_OPS} and @code{RE_CONTEXT_INDEP_OPS}
aren't set, then (for historical reasons) the matcher considers special
characters ordinary if they are in contexts where the operations they
represent make no sense; for example, then the match-zero-or-more
operator (represented by @samp{*}) matches itself in the regular
expression @samp{*foo} because there is no preceding expression on which
it can operate.  It is poor practice, however, to depend on this
behavior; if you want a special character to be ordinary outside a list,
it's better to always quote it, regardless.} or

@item
inside a list and the syntax bit @code{RE_BACKSLASH_ESCAPE_IN_LISTS} is set.

@end itemize

@item
It introduces an operator when followed by certain ordinary
characters---sometimes only when certain syntax bits are set.  See the
cases @code{RE_BK_PLUS_QM}, @code{RE_NO_BK_BRACES}, @code{RE_NO_BK_VAR},
@code{RE_NO_BK_PARENS}, @code{RE_NO_BK_REF} in @ref{Syntax Bits}.  Also:

@itemize @bullet
@item
@samp{\b} represents the match-word-boundary operator
(@pxref{Match-word-boundary Operator}).

@item
@samp{\B} represents the match-within-word operator
(@pxref{Match-within-word Operator}).

@item
@samp{\<} represents the match-beginning-of-word operator @*
(@pxref{Match-beginning-of-word Operator}).

@item
@samp{\>} represents the match-end-of-word operator
(@pxref{Match-end-of-word Operator}).

@item
@samp{\w} represents the match-word-constituent operator
(@pxref{Match-word-constituent Operator}).

@item
@samp{\W} represents the match-non-word-constituent operator
(@pxref{Match-non-word-constituent Operator}).

@item
@samp{\`} represents the match-beginning-of-buffer
operator and @samp{\'} represents the match-end-of-buffer operator
(@pxref{Buffer Operators}).

@item
If Regex was compiled with the C preprocessor symbol @code{emacs}
defined, then @samp{\s@var{class}} represents the match-syntactic-class
operator and @samp{\S@var{class}} represents the
match-not-syntactic-class operator (@pxref{Syntactic Class Operators}).

@end itemize

@item
In all other cases, Regex ignores @samp{\}.  For example,
@samp{\n} matches @samp{n}.

@end enumerate

@node Common Operators, GNU Operators, Regular Expression Syntax, Top
@chapter Common Operators

You compose regular expressions from operators.  In the following
sections, we describe the regular expression operators specified by
@sc{posix}; @sc{gnu} also uses these.  Most operators have more than one
representation as characters.  @xref{Regular Expression Syntax}, for
what characters represent what operators under what circumstances.

For most operators that can be represented in two ways, one
representation is a single character and the other is that character
preceded by @samp{\}.  For example, either @samp{(} or @samp{\(}
represents the open-group operator.  Which one does depends on the
setting of a syntax bit, in this case @code{RE_NO_BK_PARENS}.  Why is
this so?  Historical reasons dictate some of the varying
representations, while @sc{posix} dictates others.

Finally, almost all characters lose any special meaning inside a list
(@pxref{List Operators}).

@menu
* Match-self Operator::			Ordinary characters.
* Match-any-character Operator::	.
* Concatenation Operator::		Juxtaposition.
* Repetition Operators::		*  +  ? @{@}
* Alternation Operator::		|
* List Operators::			[...]  [^...]
* Grouping Operators::			(...)
* Back-reference Operator::		\digit
* Anchoring Operators::			^  $
@end menu

@node Match-self Operator, Match-any-character Operator,  , Common Operators
@section The Match-self Operator (@var{ordinary character})

This operator matches the character itself.  All ordinary characters
(@pxref{Regular Expression Syntax}) represent this operator.  For
example, @samp{f} is always an ordinary character, so the regular
expression @samp{f} matches only the string @samp{f}.  In
particular, it does @emph{not} match the string @samp{ff}.

@node Match-any-character Operator, Concatenation Operator, Match-self Operator, Common Operators
@section The Match-any-character Operator (@code{.})

@cindex @samp{.}

This operator matches any single printing or nonprinting character
except it won't match a:

@table @asis
@item newline
if the syntax bit @code{RE_DOT_NEWLINE} isn't set.

@item null
if the syntax bit @code{RE_DOT_NOT_NULL} is set.

@end table

The @samp{.} (period) character represents this operator.  For example,
@samp{a.b} matches any three-character string beginning with @samp{a}
and ending with @samp{b}.

@node Concatenation Operator, Repetition Operators, Match-any-character Operator, Common Operators
@section The Concatenation Operator

This operator concatenates two regular expressions @var{a} and @var{b}.
No character represents this operator; you simply put @var{b} after
@var{a}.  The result is a regular expression that will match a string if
@var{a} matches its first part and @var{b} matches the rest.  For
example, @samp{xy} (two match-self operators) matches @samp{xy}.

@node Repetition Operators, Alternation Operator, Concatenation Operator, Common Operators
@section Repetition Operators

Repetition operators repeat the preceding regular expression a specified
number of times.

@menu
* Match-zero-or-more Operator::  *
* Match-one-or-more Operator::   +
* Match-zero-or-one Operator::   ?
* Interval Operators::           @{@}
@end menu

@node Match-zero-or-more Operator, Match-one-or-more Operator,  , Repetition Operators
@subsection The Match-zero-or-more Operator (@code{*})

@cindex @samp{*}

This operator repeats the smallest possible preceding regular expression
as many times as necessary (including zero) to match the pattern.
@samp{*} represents this operator.  For example, @samp{o*}
matches any string made up of zero or more @samp{o}s.  Since this
operator operates on the smallest preceding regular expression,
@samp{fo*} has a repeating @samp{o}, not a repeating @samp{fo}.  So,
@samp{fo*} matches @samp{f}, @samp{fo}, @samp{foo}, and so on.

Since the match-zero-or-more operator is a suffix operator, it may be
useless as such when no regular expression precedes it.  This is the
case when it:

@itemize @bullet
@item
is first in a regular expression, or

@item
follows a match-beginning-of-line, open-group, or alternation
operator.

@end itemize

@noindent
Three different things can happen in these cases:

@enumerate
@item
If the syntax bit @code{RE_CONTEXT_INVALID_OPS} is set, then the
regular expression is invalid.

@item
If @code{RE_CONTEXT_INVALID_OPS} isn't set, but
@code{RE_CONTEXT_INDEP_OPS} is, then @samp{*} represents the
match-zero-or-more operator (which then operates on the empty string).

@item
Otherwise, @samp{*} is ordinary.

@end enumerate

@cindex backtracking
The matcher processes a match-zero-or-more operator by first matching as
many repetitions of the smallest preceding regular expression as it can.
Then it continues to match the rest of the pattern.

If it can't match the rest of the pattern, it backtracks (as many times
as necessary), each time discarding one of the matches until it can
either match the entire pattern or be certain that it cannot get a
match.  For example, when matching @samp{ca*ar} against @samp{caaar},
the matcher first matches all three @samp{a}s of the string with the
@samp{a*} of the regular expression.  However, it cannot then match the
final @samp{ar} of the regular expression against the final @samp{r} of
the string.  So it backtracks, discarding the match of the last @samp{a}
in the string.  It can then match the remaining @samp{ar}.


@node Match-one-or-more Operator, Match-zero-or-one Operator, Match-zero-or-more Operator, Repetition Operators
@subsection The Match-one-or-more Operator (@code{+} or @code{\+})

@cindex @samp{+}

If the syntax bit @code{RE_LIMITED_OPS} is set, then Regex doesn't recognize
this operator.  Otherwise, if the syntax bit @code{RE_BK_PLUS_QM} isn't
set, then @samp{+} represents this operator; if it is, then @samp{\+}
does.

This operator is similar to the match-zero-or-more operator except that
it repeats the preceding regular expression at least once;
@pxref{Match-zero-or-more Operator}, for what it operates on, how some
syntax bits affect it, and how Regex backtracks to match it.

For example, supposing that @samp{+} represents the match-one-or-more
operator; then @samp{ca+r} matches, e.g., @samp{car} and
@samp{caaaar}, but not @samp{cr}.

@node Match-zero-or-one Operator, Interval Operators, Match-one-or-more Operator, Repetition Operators
@subsection The Match-zero-or-one Operator (@code{?} or @code{\?})
@cindex @samp{?}

If the syntax bit @code{RE_LIMITED_OPS} is set, then Regex doesn't
recognize this operator.  Otherwise, if the syntax bit
@code{RE_BK_PLUS_QM} isn't set, then @samp{?} represents this operator;
if it is, then @samp{\?} does.

This operator is similar to the match-zero-or-more operator except that
it repeats the preceding regular expression once or not at all;
@pxref{Match-zero-or-more Operator}, to see what it operates on, how
some syntax bits affect it, and how Regex backtracks to match it.

For example, supposing that @samp{?} represents the match-zero-or-one
operator; then @samp{ca?r} matches both @samp{car} and @samp{cr}, but
nothing else.

@node Interval Operators,  , Match-zero-or-one Operator, Repetition Operators
@subsection Interval Operators (@code{@{} @dots{} @code{@}} or @code{\@{} @dots{} @code{\@}})

@cindex interval expression
@cindex @samp{@{}
@cindex @samp{@}}
@cindex @samp{\@{}
@cindex @samp{\@}}

If the syntax bit @code{RE_INTERVALS} is set, then Regex recognizes
@dfn{interval expressions}.  They repeat the smallest possible preceding
regular expression a specified number of times.

If the syntax bit @code{RE_NO_BK_BRACES} is set, @samp{@{} represents
the @dfn{open-interval operator} and @samp{@}} represents the
@dfn{close-interval operator} ; otherwise, @samp{\@{} and @samp{\@}} do.

Specifically, supposing that @samp{@{} and @samp{@}} represent the
open-interval and close-interval operators; then:

@table @code
@item  @{@var{count}@}
matches exactly @var{count} occurrences of the preceding regular
expression.

@item @{@var{min,}@}
matches @var{min} or more occurrences of the preceding regular
expression.

@item  @{@var{min, max}@}
matches at least @var{min} but no more than @var{max} occurrences of
the preceding regular expression.

@end table

The interval expression (but not necessarily the regular expression that
contains it) is invalid if:

@itemize @bullet
@item
@var{min} is greater than @var{max}, or

@item
any of @var{count}, @var{min}, or @var{max} are outside the range
zero to @code{RE_DUP_MAX} (which symbol @file{regex.h}
defines).

@end itemize

If the interval expression is invalid and the syntax bit
@code{RE_NO_BK_BRACES} is set, then Regex considers all the
characters in the would-be interval to be ordinary.  If that bit
isn't set, then the regular expression is invalid.

If the interval expression is valid but there is no preceding regular
expression on which to operate, then if the syntax bit
@code{RE_CONTEXT_INVALID_OPS} is set, the regular expression is invalid.
If that bit isn't set, then Regex considers all the characters---other
than backslashes, which it ignores---in the would-be interval to be
ordinary.


@node Alternation Operator, List Operators, Repetition Operators, Common Operators
@section The Alternation Operator (@code{|} or @code{\|})

@kindex |
@kindex \|
@cindex alternation operator
@cindex or operator

If the syntax bit @code{RE_LIMITED_OPS} is set, then Regex doesn't
recognize this operator.  Otherwise, if the syntax bit
@code{RE_NO_BK_VBAR} is set, then @samp{|} represents this operator;
otherwise, @samp{\|} does.

Alternatives match one of a choice of regular expressions:
if you put the character(s) representing the alternation operator between
any two regular expressions @var{a} and @var{b}, the result matches
the union of the strings that @var{a} and @var{b} match.  For
example, supposing that @samp{|} is the alternation operator, then
@samp{foo|bar|quux} would match any of @samp{foo}, @samp{bar} or
@samp{quux}.

@ignore
@c Nobody needs to disallow empty alternatives any more.
If the syntax bit @code{RE_NO_EMPTY_ALTS} is set, then if either of the regular
expressions @var{a} or @var{b} is empty, the
regular expression is invalid.  More precisely, if this syntax bit is
set, then the alternation operator can't:

@itemize @bullet
@item
be first or last in a regular expression;

@item
follow either another alternation operator or an open-group operator
(@pxref{Grouping Operators}); or

@item
precede a close-group operator.

@end itemize

@noindent
For example, supposing @samp{(} and @samp{)} represent the open and
close-group operators, then @samp{|foo}, @samp{foo|}, @samp{foo||bar},
@samp{foo(|bar)}, and @samp{(foo|)bar} would all be invalid.
@end ignore

The alternation operator operates on the @emph{largest} possible
surrounding regular expressions.  (Put another way, it has the lowest
precedence of any regular expression operator.)
Thus, the only way you can
delimit its arguments is to use grouping.  For example, if @samp{(} and
@samp{)} are the open and close-group operators, then @samp{fo(o|b)ar}
would match either @samp{fooar} or @samp{fobar}.  (@samp{foo|bar} would
match @samp{foo} or @samp{bar}.)

@cindex backtracking
The matcher usually tries all combinations of alternatives so as to
match the longest possible string.  For example, when matching
@samp{(fooq|foo)*(qbarquux|bar)} against @samp{fooqbarquux}, it cannot
take, say, the first (``depth-first'') combination it could match, since
then it would be content to match just @samp{fooqbar}.

@comment xx something about leftmost-longest


@node List Operators, Grouping Operators, Alternation Operator, Common Operators
@section List Operators (@code{[} @dots{} @code{]} and @code{[^} @dots{} @code{]})

@cindex matching list
@cindex @samp{[}
@cindex @samp{]}
@cindex @samp{^}
@cindex @samp{-}
@cindex @samp{\}
@cindex @samp{[^}
@cindex nonmatching list
@cindex matching newline
@cindex bracket expression

@dfn{Lists}, also called @dfn{bracket expressions}, are a set of one or
more items.  An @dfn{item} is a character,
@ignore
(These get added when they get implemented.)
a collating symbol, an equivalence class expression,
@end ignore
a character class expression, or a range expression.  The syntax bits
affect which kinds of items you can put in a list.  We explain the last
two items in subsections below.  Empty lists are invalid.

A @dfn{matching list} matches a single character represented by one of
the list items.  You form a matching list by enclosing one or more items
within an @dfn{open-matching-list operator} (represented by @samp{[})
and a @dfn{close-list operator} (represented by @samp{]}).

For example, @samp{[ab]} matches either @samp{a} or @samp{b}.
@samp{[ad]*} matches the empty string and any string composed of just
@samp{a}s and @samp{d}s in any order.  Regex considers invalid a regular
expression with a @samp{[} but no matching
@samp{]}.

@dfn{Nonmatching lists} are similar to matching lists except that they
match a single character @emph{not} represented by one of the list
items.  You use an @dfn{open-nonmatching-list operator} (represented by
@samp{[^}@footnote{Regex therefore doesn't consider the @samp{^} to be
the first character in the list.  If you put a @samp{^} character first
in (what you think is) a matching list, you'll turn it into a
nonmatching list.}) instead of an open-matching-list operator to start a
nonmatching list.

For example, @samp{[^ab]} matches any character except @samp{a} or
@samp{b}.

If the @code{posix_newline} field in the pattern buffer (@pxref{GNU
Pattern Buffers} is set, then nonmatching lists do not match a newline.

Most characters lose any special meaning inside a list.  The special
characters inside a list follow.

@table @samp
@item ]
ends the list if it's not the first list item.  So, if you want to make
the @samp{]} character a list item, you must put it first.

@item \
quotes the next character if the syntax bit @code{RE_BACKSLASH_ESCAPE_IN_LISTS} is
set.

@ignore
Put these in if they get implemented.

@item [.
represents the open-collating-symbol operator (@pxref{Collating Symbol
Operators}).

@item .]
represents the close-collating-symbol operator.

@item [=
represents the open-equivalence-class operator (@pxref{Equivalence Class
Operators}).

@item =]
represents the close-equivalence-class operator.

@end ignore

@item [:
represents the open-character-class operator (@pxref{Character Class
Operators}) if the syntax bit @code{RE_CHAR_CLASSES} is set and what
follows is a valid character class expression.

@item :]
represents the close-character-class operator if the syntax bit
@code{RE_CHAR_CLASSES} is set and what precedes it is an
open-character-class operator followed by a valid character class name.

@item -
represents the range operator (@pxref{Range Operator}) if it's
not first or last in a list or the ending point of a range.

@end table

@noindent
All other characters are ordinary.  For example, @samp{[.*]} matches
@samp{.} and @samp{*}.

@menu
* Character Class Operators::   [:class:]
* Range Operator::          start-end
@end menu

@ignore
(If collating symbols and equivalence class expressions get implemented,
then add this.)

node Collating Symbol Operators
subsubsection Collating Symbol Operators (@code{[.} @dots{} @code{.]})

If the syntax bit @code{XX} is set, then you can represent
collating symbols inside lists.  You form a @dfn{collating symbol} by
putting a collating element between an @dfn{open-collating-symbol
operator} and an @dfn{close-collating-symbol operator}.  @samp{[.}
represents the open-collating-symbol operator and @samp{.]} represents
the close-collating-symbol operator.  For example, if @samp{ll} is a
collating element, then @samp{[[.ll.]]} would match @samp{ll}.

node Equivalence Class Operators
subsubsection Equivalence Class Operators (@code{[=} @dots{} @code{=]})
@cindex equivalence class expression in regex
@cindex @samp{[=} in regex
@cindex @samp{=]} in regex

If the syntax bit @code{XX} is set, then Regex recognizes equivalence class
expressions inside lists.  A @dfn{equivalence class expression} is a set
of collating elements which all belong to the same equivalence class.
You form an equivalence class expression by putting a collating
element between an @dfn{open-equivalence-class operator} and a
@dfn{close-equivalence-class operator}.  @samp{[=} represents the
open-equivalence-class operator and @samp{=]} represents the
close-equivalence-class operator.  For example, if @samp{a} and @samp{A}
were an equivalence class, then both @samp{[[=a=]]} and @samp{[[=A=]]}
would match both @samp{a} and @samp{A}.  If the collating element in an
equivalence class expression isn't part of an equivalence class, then
the matcher considers the equivalence class expression to be a collating
symbol.

@end ignore

@node Character Class Operators, Range Operator,  , List Operators
@subsection Character Class Operators (@code{[:} @dots{} @code{:]})

@cindex character classes
@cindex @samp{[:} in regex
@cindex @samp{:]} in regex

If the syntax bit @code{RE_CHARACTER_CLASSES} is set, then Regex
recognizes character class expressions inside lists.  A @dfn{character
class expression} matches one character from a given class.  You form a
character class expression by putting a character class name between an
@dfn{open-character-class operator} (represented by @samp{[:}) and a
@dfn{close-character-class operator} (represented by @samp{:]}).  The
character class names and their meanings are:

@table @code

@item alnum
letters and digits

@item alpha
letters

@item blank
system-dependent; for @sc{gnu}, a space or tab

@item cntrl
control characters (in the @sc{ascii} encoding, code 0177 and codes
less than 040)

@item digit
digits

@item graph
same as @code{print} except omits space

@item lower
lowercase letters

@item print
printable characters (in the @sc{ascii} encoding, space
tilde---codes 040 through 0176)

@item punct
neither control nor alphanumeric characters

@item space
space, carriage return, newline, vertical tab, and form feed

@item upper
uppercase letters

@item xdigit
hexadecimal digits: @code{0}--@code{9}, @code{a}--@code{f}, @code{A}--@code{F}

@end table

@noindent
These correspond to the definitions in the C library's @file{<ctype.h>}
facility.  For example, @samp{[:alpha:]} corresponds to the standard
facility @code{isalpha}.  Regex recognizes character class expressions
only inside of lists; so @samp{[[:alpha:]]} matches any letter, but
@samp{[:alpha:]} outside of a bracket expression and not followed by a
repetition operator matches just itself.

@node Range Operator,  , Character Class Operators, List Operators
@subsection The Range Operator (@code{-})

Regex recognizes @dfn{range expressions} inside a list. They represent
those characters
that fall between two elements in the current collating sequence.  You
form a range expression by putting a @dfn{range operator} between two
@ignore
(If these get implemented, then substitute this for ``characters.'')
of any of the following: characters, collating elements, collating symbols,
and equivalence class expressions.  The starting point of the range and
the ending point of the range don't have to be the same kind of item,
e.g., the starting point could be a collating element and the ending
point could be an equivalence class expression.  If a range's ending
point is an equivalence class, then all the collating elements in that
class will be in the range.
@end ignore
characters.@footnote{You can't use a character class for the starting
or ending point of a range, since a character class is not a single
character.} @samp{-} represents the range operator.  For example,
@samp{a-f} within a list represents all the characters from @samp{a}
through @samp{f}
inclusively.

If the syntax bit @code{RE_NO_EMPTY_RANGES} is set, then if the range's
ending point collates less than its starting point, the range (and the
regular expression containing it) is invalid.  For example, the regular
expression @samp{[z-a]} would be invalid.  If this bit isn't set, then
Regex considers such a range to be empty.

Since @samp{-} represents the range operator, if you want to make a
@samp{-} character itself
a list item, you must do one of the following:

@itemize @bullet
@item
Put the @samp{-} either first or last in the list.

@item
Include a range whose starting point collates strictly lower than
@samp{-} and whose ending point collates equal or higher.  Unless a
range is the first item in a list, a @samp{-} can't be its starting
point, but @emph{can} be its ending point.  That is because Regex
considers @samp{-} to be the range operator unless it is preceded by
another @samp{-}.  For example, in the @sc{ascii} encoding, @samp{)},
@samp{*}, @samp{+}, @samp{,}, @samp{-}, @samp{.}, and @samp{/} are
contiguous characters in the collating sequence.  You might think that
@samp{[)-+--/]} has two ranges: @samp{)-+} and @samp{--/}.  Rather, it
has the ranges @samp{)-+} and @samp{+--}, plus the character @samp{/}, so
it matches, e.g., @samp{,}, not @samp{.}.

@item
Put a range whose starting point is @samp{-} first in the list.

@end itemize

For example, @samp{[-a-z]} matches a lowercase letter or a hyphen (in
English, in @sc{ascii}).


@node Grouping Operators, Back-reference Operator, List Operators, Common Operators
@section Grouping Operators (@code{(} @dots{} @code{)} or @code{\(} @dots{} @code{\)})

@kindex (
@kindex )
@kindex \(
@kindex \)
@cindex grouping
@cindex subexpressions
@cindex parenthesizing

A @dfn{group}, also known as a @dfn{subexpression}, consists of an
@dfn{open-group operator}, any number of other operators, and a
@dfn{close-group operator}.  Regex treats this sequence as a unit, just
as mathematics and programming languages treat a parenthesized
expression as a unit.

Therefore, using @dfn{groups}, you can:

@itemize @bullet
@item
delimit the argument(s) to an alternation operator (@pxref{Alternation
Operator}) or a repetition operator (@pxref{Repetition
Operators}).

@item
keep track of the indices of the substring that matched a given group.
@xref{Using Registers}, for a precise explanation.
This lets you:

@itemize @bullet
@item
use the back-reference operator (@pxref{Back-reference Operator}).

@item
use registers (@pxref{Using Registers}).

@end itemize

@end itemize

If the syntax bit @code{RE_NO_BK_PARENS} is set, then @samp{(} represents
the open-group operator and @samp{)} represents the
close-group operator; otherwise, @samp{\(} and @samp{\)} do.

If the syntax bit @code{RE_UNMATCHED_RIGHT_PAREN_ORD} is set and a
close-group operator has no matching open-group operator, then Regex
considers it to match @samp{)}.


@node Back-reference Operator, Anchoring Operators, Grouping Operators, Common Operators
@section The Back-reference Operator (@dfn{\}@var{digit})

@cindex back references

If the syntax bit @code{RE_NO_BK_REF} isn't set, then Regex recognizes
back references.  A back reference matches a specified preceding group.
The back reference operator is represented by @samp{\@var{digit}}
anywhere after the end of a regular expression's @w{@var{digit}-th}
group (@pxref{Grouping Operators}).

@var{digit} must be between @samp{1} and @samp{9}.  The matcher assigns
numbers 1 through 9 to the first nine groups it encounters.  By using
one of @samp{\1} through @samp{\9} after the corresponding group's
close-group operator, you can match a substring identical to the
one that the group does.

Back references match according to the following (in all examples below,
@samp{(} represents the open-group, @samp{)} the close-group, @samp{@{}
the open-interval and @samp{@}} the close-interval operator):

@itemize @bullet
@item
If the group matches a substring, the back reference matches an
identical substring.  For example, @samp{(a)\1} matches @samp{aa} and
@samp{(bana)na\1bo\1} matches @samp{bananabanabobana}.  Likewise,
@samp{(.*)\1} matches any (newline-free if the syntax bit
@code{RE_DOT_NEWLINE} isn't set) string that is composed of two
identical halves; the @samp{(.*)} matches the first half and the
@samp{\1} matches the second half.

@item
If the group matches more than once (as it might if followed
by, e.g., a repetition operator), then the back reference matches the
substring the group @emph{last} matched.  For example,
@samp{((a*)b)*\1\2} matches @samp{aabababa}; first @w{group 1} (the
outer one) matches @samp{aab} and @w{group 2} (the inner one) matches
@samp{aa}.  Then @w{group 1} matches @samp{ab} and @w{group 2} matches
@samp{a}.  So, @samp{\1} matches @samp{ab} and @samp{\2} matches
@samp{a}.

@item
If the group doesn't participate in a match, i.e., it is part of an
alternative not taken or a repetition operator allows zero repetitions
of it, then the back reference makes the whole match fail.  For example,
@samp{(one()|two())-and-(three\2|four\3)} matches @samp{one-and-three}
and @samp{two-and-four}, but not @samp{one-and-four} or
@samp{two-and-three}.  For example, if the pattern matches
@samp{one-and-}, then its @w{group 2} matches the empty string and its
@w{group 3} doesn't participate in the match.  So, if it then matches
@samp{four}, then when it tries to back reference @w{group 3}---which it
will attempt to do because @samp{\3} follows the @samp{four}---the match
will fail because @w{group 3} didn't participate in the match.

@end itemize

You can use a back reference as an argument to a repetition operator.  For
example, @samp{(a(b))\2*} matches @samp{a} followed by two or more
@samp{b}s.  Similarly, @samp{(a(b))\2@{3@}} matches @samp{abbbb}.

If there is no preceding @w{@var{digit}-th} subexpression, the regular
expression is invalid.


@node Anchoring Operators,  , Back-reference Operator, Common Operators
@section Anchoring Operators

@cindex anchoring
@cindex regexp anchoring

These operators can constrain a pattern to match only at the beginning or
end of the entire string or at the beginning or end of a line.

@menu
* Match-beginning-of-line Operator::  ^
* Match-end-of-line Operator::        $
@end menu


@node Match-beginning-of-line Operator, Match-end-of-line Operator,  , Anchoring Operators
@subsection The Match-beginning-of-line Operator (@code{^})

@kindex ^
@cindex beginning-of-line operator
@cindex anchors

This operator can match the empty string either at the beginning of the
string or after a newline character.  Thus, it is said to @dfn{anchor}
the pattern to the beginning of a line.

In the cases following, @samp{^} represents this operator.  (Otherwise,
@samp{^} is ordinary.)

@itemize @bullet

@item
It (the @samp{^}) is first in the pattern, as in @samp{^foo}.

@cnindex RE_CONTEXT_INDEP_ANCHORS @r{(and @samp{^})}
@item
The syntax bit @code{RE_CONTEXT_INDEP_ANCHORS} is set, and it is outside
a bracket expression.

@cindex open-group operator and @samp{^}
@cindex alternation operator and @samp{^}
@item
It follows an open-group or alternation operator, as in @samp{a\(^b\)}
and @samp{a\|^b}.  @xref{Grouping Operators}, and @ref{Alternation
Operator}.

@end itemize

These rules imply that some valid patterns containing @samp{^} cannot be
matched; for example, @samp{foo^bar} if @code{RE_CONTEXT_INDEP_ANCHORS}
is set.

@vindex not_bol @r{field in pattern buffer}
If the @code{not_bol} field is set in the pattern buffer (@pxref{GNU
Pattern Buffers}), then @samp{^} fails to match at the beginning of the
string.  @xref{POSIX Matching}, for when you might find this useful.

@vindex newline_anchor @r{field in pattern buffer}
If the @code{newline_anchor} field is set in the pattern buffer, then
@samp{^} fails to match after a newline.  This is useful when you do not
regard the string to be matched as broken into lines.


@node Match-end-of-line Operator,  , Match-beginning-of-line Operator, Anchoring Operators
@subsection The Match-end-of-line Operator (@code{$})

@kindex $
@cindex end-of-line operator
@cindex anchors

This operator can match the empty string either at the end of
the string or before a newline character in the string.  Thus, it is
said to @dfn{anchor} the pattern to the end of a line.

It is always represented by @samp{$}.  For example, @samp{foo$} usually
matches, e.g., @samp{foo} and, e.g., the first three characters of
@samp{foo\nbar}.

Its interaction with the syntax bits and pattern buffer fields is
exactly the dual of @samp{^}'s; see the previous section.  (That is,
``beginning'' becomes ``end'', ``next'' becomes ``previous'', and
``after'' becomes ``before''.)


@node GNU Operators, GNU Emacs Operators, Common Operators, Top
@chapter GNU Operators

Following are operators that @sc{gnu} defines (and @sc{posix} doesn't).

@menu
* Word Operators::
* Buffer Operators::
@end menu

@node Word Operators, Buffer Operators,  , GNU Operators
@section Word Operators

The operators in this section require Regex to recognize parts of words.
Regex uses a syntax table to determine whether or not a character is
part of a word, i.e., whether or not it is @dfn{word-constituent}.

@menu
* Non-Emacs Syntax Tables::
* Match-word-boundary Operator::	\b
* Match-within-word Operator::		\B
* Match-beginning-of-word Operator::	\<
* Match-end-of-word Operator::		\>
* Match-word-constituent Operator::	\w
* Match-non-word-constituent Operator::	\W
@end menu

@node Non-Emacs Syntax Tables, Match-word-boundary Operator,  , Word Operators
@subsection Non-Emacs Syntax Tables

A @dfn{syntax table} is an array indexed by the characters in your
character set.  In the @sc{ascii} encoding, therefore, a syntax table
has 256 elements.  Regex always uses a @code{char *} variable
@code{re_syntax_table} as its syntax table.  In some cases, it
initializes this variable and in others it expects you to initialize it.

@itemize @bullet
@item
If Regex is compiled with the preprocessor symbols @code{emacs} and
@code{SYNTAX_TABLE} both undefined, then Regex allocates
@code{re_syntax_table} and initializes an element @var{i} either to
@code{Sword} (which it defines) if @var{i} is a letter, number, or
@samp{_}, or to zero if it's not.

@item
If Regex is compiled with @code{emacs} undefined but @code{SYNTAX_TABLE}
defined, then Regex expects you to define a @code{char *} variable
@code{re_syntax_table} to be a valid syntax table.

@item
@xref{Emacs Syntax Tables}, for what happens when Regex is compiled with
the preprocessor symbol @code{emacs} defined.

@end itemize

@node Match-word-boundary Operator, Match-within-word Operator, Non-Emacs Syntax Tables, Word Operators
@subsection The Match-word-boundary Operator (@code{\b})

@cindex @samp{\b}
@cindex word boundaries, matching

This operator (represented by @samp{\b}) matches the empty string at
either the beginning or the end of a word.  For example, @samp{\brat\b}
matches the separate word @samp{rat}.

@node Match-within-word Operator, Match-beginning-of-word Operator, Match-word-boundary Operator, Word Operators
@subsection The Match-within-word Operator (@code{\B})

@cindex @samp{\B}

This operator (represented by @samp{\B}) matches the empty string within
a word. For example, @samp{c\Brat\Be} matches @samp{crate}, but
@samp{dirty \Brat} doesn't match @samp{dirty rat}.

@node Match-beginning-of-word Operator, Match-end-of-word Operator, Match-within-word Operator, Word Operators
@subsection The Match-beginning-of-word Operator (@code{\<})

@cindex @samp{\<}

This operator (represented by @samp{\<}) matches the empty string at the
beginning of a word.

@node Match-end-of-word Operator, Match-word-constituent Operator, Match-beginning-of-word Operator, Word Operators
@subsection The Match-end-of-word Operator (@code{\>})

@cindex @samp{\>}

This operator (represented by @samp{\>}) matches the empty string at the
end of a word.

@node Match-word-constituent Operator, Match-non-word-constituent Operator, Match-end-of-word Operator, Word Operators
@subsection The Match-word-constituent Operator (@code{\w})

@cindex @samp{\w}

This operator (represented by @samp{\w}) matches any word-constituent
character.

@node Match-non-word-constituent Operator,  , Match-word-constituent Operator, Word Operators
@subsection The Match-non-word-constituent Operator (@code{\W})

@cindex @samp{\W}

This operator (represented by @samp{\W}) matches any character that is
not word-constituent.


@node Buffer Operators,  , Word Operators, GNU Operators
@section Buffer Operators

Following are operators which work on buffers.  In Emacs, a @dfn{buffer}
is, naturally, an Emacs buffer.  For other programs, Regex considers the
entire string to be matched as the buffer.

@menu
* Match-beginning-of-buffer Operator::	\`
* Match-end-of-buffer Operator::	\'
@end menu


@node Match-beginning-of-buffer Operator, Match-end-of-buffer Operator,  , Buffer Operators
@subsection The Match-beginning-of-buffer Operator (@code{\`})

@cindex @samp{\`}

This operator (represented by @samp{\`}) matches the empty string at the
beginning of the buffer.

@node Match-end-of-buffer Operator,  , Match-beginning-of-buffer Operator, Buffer Operators
@subsection The Match-end-of-buffer Operator (@code{\'})

@cindex @samp{\'}

This operator (represented by @samp{\'}) matches the empty string at the
end of the buffer.


@node GNU Emacs Operators, What Gets Matched?, GNU Operators, Top
@chapter GNU Emacs Operators

Following are operators that @sc{gnu} defines (and @sc{posix} doesn't)
that you can use only when Regex is compiled with the preprocessor
symbol @code{emacs} defined.

@menu
* Syntactic Class Operators::
@end menu


@node Syntactic Class Operators,  ,  , GNU Emacs Operators
@section Syntactic Class Operators

The operators in this section require Regex to recognize the syntactic
classes of characters.  Regex uses a syntax table to determine this.

@menu
* Emacs Syntax Tables::
* Match-syntactic-class Operator::	\sCLASS
* Match-not-syntactic-class Operator::  \SCLASS
@end menu

@node Emacs Syntax Tables, Match-syntactic-class Operator,  , Syntactic Class Operators
@subsection Emacs Syntax Tables

A @dfn{syntax table} is an array indexed by the characters in your
character set.  In the @sc{ascii} encoding, therefore, a syntax table
has 256 elements.

If Regex is compiled with the preprocessor symbol @code{emacs} defined,
then Regex expects you to define and initialize the variable
@code{re_syntax_table} to be an Emacs syntax table.  Emacs' syntax
tables are more complicated than Regex's own (@pxref{Non-Emacs Syntax
Tables}).  @xref{Syntax, , Syntax, emacs, The GNU Emacs User's Manual},
for a description of Emacs' syntax tables.

@node Match-syntactic-class Operator, Match-not-syntactic-class Operator, Emacs Syntax Tables, Syntactic Class Operators
@subsection The Match-syntactic-class Operator (@code{\s}@var{class})

@cindex @samp{\s}

This operator matches any character whose syntactic class is represented
by a specified character.  @samp{\s@var{class}} represents this operator
where @var{class} is the character representing the syntactic class you
want.  For example, @samp{w} represents the syntactic
class of word-constituent characters, so @samp{\sw} matches any
word-constituent character.

@node Match-not-syntactic-class Operator,  , Match-syntactic-class Operator, Syntactic Class Operators
@subsection The Match-not-syntactic-class Operator (@code{\S}@var{class})

@cindex @samp{\S}

This operator is similar to the match-syntactic-class operator except
that it matches any character whose syntactic class is @emph{not}
represented by the specified character.  @samp{\S@var{class}} represents
this operator.  For example, @samp{w} represents the syntactic class of
word-constituent characters, so @samp{\Sw} matches any character that is
not word-constituent.


@node What Gets Matched?, Programming with Regex, GNU Emacs Operators, Top
@chapter What Gets Matched?

Regex usually matches strings according to the ``leftmost longest''
rule; that is, it chooses the longest of the leftmost matches.  This
does not mean that for a regular expression containing subexpressions
that it simply chooses the longest match for each subexpression, left to
right; the overall match must also be the longest possible one.

For example, @samp{(ac*)(c*d[ac]*)\1} matches @samp{acdacaaa}, not
@samp{acdac}, as it would if it were to choose the longest match for the
first subexpression.


@node Programming with Regex, Copying, What Gets Matched?, Top
@chapter Programming with Regex

Here we describe how you use the Regex data structures and functions in
C programs.  Regex has three interfaces: one designed for @sc{gnu}, one
compatible with @sc{posix} and one compatible with Berkeley @sc{unix}.

@menu
* GNU Regex Functions::
* POSIX Regex Functions::
* BSD Regex Functions::
@end menu


@node GNU Regex Functions, POSIX Regex Functions,  , Programming with Regex
@section GNU Regex Functions

If you're writing code that doesn't need to be compatible with either
@sc{posix} or Berkeley @sc{unix}, you can use these functions.  They
provide more options than the other interfaces.

@menu
* GNU Pattern Buffers::         The re_pattern_buffer type.
* GNU Regular Expression Compiling::  re_compile_pattern ()
* GNU Matching::                re_match ()
* GNU Searching::               re_search ()
* Matching/Searching with Split Data::  re_match_2 (), re_search_2 ()
* Searching with Fastmaps::     re_compile_fastmap ()
* GNU Translate Tables::        The `translate' field.
* Using Registers::             The re_registers type and related fns.
* Freeing GNU Pattern Buffers::  regfree ()
@end menu


@node GNU Pattern Buffers, GNU Regular Expression Compiling,  , GNU Regex Functions
@subsection GNU Pattern Buffers

@cindex pattern buffer, definition of
@tindex re_pattern_buffer @r{definition}
@tindex struct re_pattern_buffer @r{definition}

To compile, match, or search for a given regular expression, you must
supply a pattern buffer.  A @dfn{pattern buffer} holds one compiled
regular expression.@footnote{Regular expressions are also referred to as
``patterns,'' hence the name ``pattern buffer.''}

You can have several different pattern buffers simultaneously, each
holding a compiled pattern for a different regular expression.

@file{regex.h} defines the pattern buffer @code{struct} as follows:

@example
[[[ pattern_buffer ]]]
@end example


@node GNU Regular Expression Compiling, GNU Matching, GNU Pattern Buffers, GNU Regex Functions
@subsection GNU Regular Expression Compiling

In @sc{gnu}, you can both match and search for a given regular
expression.  To do either, you must first compile it in a pattern buffer
(@pxref{GNU Pattern Buffers}).

@cindex syntax initialization
@vindex re_syntax_options @r{initialization}
Regular expressions match according to the syntax with which they were
compiled; with @sc{gnu}, you indicate what syntax you want by setting
the variable @code{re_syntax_options} (declared in @file{regex.h} and
defined in @file{regex.c}) before calling the compiling function,
@code{re_compile_pattern} (see below).  @xref{Syntax Bits}, and
@ref{Predefined Syntaxes}.

You can change the value of @code{re_syntax_options} at any time.
Usually, however, you set its value once and then never change it.

@cindex pattern buffer initialization
@code{re_compile_pattern} takes a pattern buffer as an argument.  You
must initialize the following fields:

@table @code

@item translate @r{initialization}

@item translate
@vindex translate @r{initialization}
Initialize this to point to a translate table if you want one, or to
zero if you don't.  We explain translate tables in @ref{GNU Translate
Tables}.

@item fastmap
@vindex fastmap @r{initialization}
Initialize this to nonzero if you want a fastmap, or to zero if you
don't.

@item buffer
@itemx allocated
@vindex buffer @r{initialization}
@vindex allocated @r{initialization}
@findex malloc
If you want @code{re_compile_pattern} to allocate memory for the
compiled pattern, set both of these to zero.  If you have an existing
block of memory (allocated with @code{malloc}) you want Regex to use,
set @code{buffer} to its address and @code{allocated} to its size (in
bytes).

@code{re_compile_pattern} uses @code{realloc} to extend the space for
the compiled pattern as necessary.

@end table

To compile a pattern buffer, use:

@findex re_compile_pattern
@example
char *
re_compile_pattern (const char *@var{regex}, const int @var{regex_size},
                    struct re_pattern_buffer *@var{pattern_buffer})
@end example

@noindent
@var{regex} is the regular expression's address, @var{regex_size} is its
length, and @var{pattern_buffer} is the pattern buffer's address.

If @code{re_compile_pattern} successfully compiles the regular
expression, it returns zero and sets @code{*@var{pattern_buffer}} to the
compiled pattern.  It sets the pattern buffer's fields as follows:

@table @code
@item buffer
@vindex buffer @r{field, set by @code{re_compile_pattern}}
to the compiled pattern.

@item used
@vindex used @r{field, set by @code{re_compile_pattern}}
to the number of bytes the compiled pattern in @code{buffer} occupies.

@item syntax
@vindex syntax @r{field, set by @code{re_compile_pattern}}
to the current value of @code{re_syntax_options}.

@item re_nsub
@vindex re_nsub @r{field, set by @code{re_compile_pattern}}
to the number of subexpressions in @var{regex}.

@item fastmap_accurate
@vindex fastmap_accurate @r{field, set by @code{re_compile_pattern}}
to zero on the theory that the pattern you're compiling is different
than the one previously compiled into @code{buffer}; in that case (since
you can't make a fastmap without a compiled pattern),
@code{fastmap} would either contain an incompatible fastmap, or nothing
at all.

@c xx what else?
@end table

If @code{re_compile_pattern} can't compile @var{regex}, it returns an
error string corresponding to one of the errors listed in @ref{POSIX
Regular Expression Compiling}.


@node GNU Matching, GNU Searching, GNU Regular Expression Compiling, GNU Regex Functions
@subsection GNU Matching

@cindex matching with GNU functions

Matching the @sc{gnu} way means trying to match as much of a string as
possible starting at a position within it you specify.  Once you've compiled
a pattern into a pattern buffer (@pxref{GNU Regular Expression
Compiling}), you can ask the matcher to match that pattern against a
string using:

@findex re_match
@example
int
re_match (struct re_pattern_buffer *@var{pattern_buffer},
          const char *@var{string}, const int @var{size},
          const int @var{start}, struct re_registers *@var{regs})
@end example

@noindent
@var{pattern_buffer} is the address of a pattern buffer containing a
compiled pattern.  @var{string} is the string you want to match; it can
contain newline and null characters.  @var{size} is the length of that
string.  @var{start} is the string index at which you want to
begin matching; the first character of @var{string} is at index zero.
@xref{Using Registers}, for a explanation of @var{regs}; you can safely
pass zero.

@code{re_match} matches the regular expression in @var{pattern_buffer}
against the string @var{string} according to the syntax in
@var{pattern_buffers}'s @code{syntax} field.  (@xref{GNU Regular
Expression Compiling}, for how to set it.)  The function returns
@math{-1} if the compiled pattern does not match any part of
@var{string} and @math{-2} if an internal error happens; otherwise, it
returns how many (possibly zero) characters of @var{string} the pattern
matched.

An example: suppose @var{pattern_buffer} points to a pattern buffer
containing the compiled pattern for @samp{a*}, and @var{string} points
to @samp{aaaaab} (whereupon @var{size} should be 6). Then if @var{start}
is 2, @code{re_match} returns 3, i.e., @samp{a*} would have matched the
last three @samp{a}s in @var{string}.  If @var{start} is 0,
@code{re_match} returns 5, i.e., @samp{a*} would have matched all the
@samp{a}s in @var{string}.  If @var{start} is either 5 or 6, it returns
zero.

If @var{start} is not between zero and @var{size}, then
@code{re_match} returns @math{-1}.


@node GNU Searching, Matching/Searching with Split Data, GNU Matching, GNU Regex Functions
@subsection GNU Searching

@cindex searching with GNU functions

@dfn{Searching} means trying to match starting at successive positions
within a string.  The function @code{re_search} does this.

Before calling @code{re_search}, you must compile your regular
expression.  @xref{GNU Regular Expression Compiling}.

Here is the function declaration:

@findex re_search
@example
int
re_search (struct re_pattern_buffer *@var{pattern_buffer},
           const char *@var{string}, const int @var{size},
           const int @var{start}, const int @var{range},
           struct re_registers *@var{regs})
@end example

@noindent
@vindex start @r{argument to @code{re_search}}
@vindex range @r{argument to @code{re_search}}
whose arguments are the same as those to @code{re_match} (@pxref{GNU
Matching}) except that the two arguments @var{start} and @var{range}
replace @code{re_match}'s argument @var{start}.

If @var{range} is positive, then @code{re_search} attempts a match
starting first at index @var{start}, then at @math{@var{start} + 1} if
that fails, and so on, up to @math{@var{start} + @var{range}}; if
@var{range} is negative, then it attempts a match starting first at
index @var{start}, then at @math{@var{start} -1} if that fails, and so
on.

If @var{start} is not between zero and @var{size}, then @code{re_search}
returns @math{-1}.  When @var{range} is positive, @code{re_search}
adjusts @var{range} so that @math{@var{start} + @var{range} - 1} is
between zero and @var{size}, if necessary; that way it won't search
outside of @var{string}.  Similarly, when @var{range} is negative,
@code{re_search} adjusts @var{range} so that @math{@var{start} +
@var{range} + 1} is between zero and @var{size}, if necessary.

If the @code{fastmap} field of @var{pattern_buffer} is zero,
@code{re_search} matches starting at consecutive positions; otherwise,
it uses @code{fastmap} to make the search more efficient.
@xref{Searching with Fastmaps}.

If no match is found, @code{re_search} returns @math{-1}.  If
a match is found, it returns the index where the match began.  If an
internal error happens, it returns @math{-2}.


@node Matching/Searching with Split Data, Searching with Fastmaps, GNU Searching, GNU Regex Functions
@subsection Matching and Searching with Split Data

Using the functions @code{re_match_2} and @code{re_search_2}, you can
match or search in data that is divided into two strings.

The function:

@findex re_match_2
@example
int
re_match_2 (struct re_pattern_buffer *@var{buffer},
            const char *@var{string1}, const int @var{size1},
            const char *@var{string2}, const int @var{size2},
            const int @var{start},
            struct re_registers *@var{regs},
            const int @var{stop})
@end example

@noindent
is similar to @code{re_match} (@pxref{GNU Matching}) except that you
pass @emph{two} data strings and sizes, and an index @var{stop} beyond
which you don't want the matcher to try matching.  As with
@code{re_match}, if it succeeds, @code{re_match_2} returns how many
characters of @var{string} it matched.  Regard @var{string1} and
@var{string2} as concatenated when you set the arguments @var{start} and
@var{stop} and use the contents of @var{regs}; @code{re_match_2} never
returns a value larger than @math{@var{size1} + @var{size2}}.

The function:

@findex re_search_2
@example
int
re_search_2 (struct re_pattern_buffer *@var{buffer},
             const char *@var{string1}, const int @var{size1},
             const char *@var{string2}, const int @var{size2},
             const int @var{start}, const int @var{range},
             struct re_registers *@var{regs},
             const int @var{stop})
@end example

@noindent
is similarly related to @code{re_search}.


@node Searching with Fastmaps, GNU Translate Tables, Matching/Searching with Split Data, GNU Regex Functions
@subsection Searching with Fastmaps

@cindex fastmaps
If you're searching through a long string, you should use a fastmap.
Without one, the searcher tries to match at consecutive positions in the
string.  Generally, most of the characters in the string could not start
a match.  It takes much longer to try matching at a given position in the
string than it does to check in a table whether or not the character at
that position could start a match.  A @dfn{fastmap} is such a table.

More specifically, a fastmap is an array indexed by the characters in
your character set.  Under the @sc{ascii} encoding, therefore, a fastmap
has 256 elements.  If you want the searcher to use a fastmap with a
given pattern buffer, you must allocate the array and assign the array's
address to the pattern buffer's @code{fastmap} field.  You either can
compile the fastmap yourself or have @code{re_search} do it for you;
when @code{fastmap} is nonzero, it automatically compiles a fastmap the
first time you search using a particular compiled pattern.

To compile a fastmap yourself, use:

@findex re_compile_fastmap
@example
int
re_compile_fastmap (struct re_pattern_buffer *@var{pattern_buffer})
@end example

@noindent
@var{pattern_buffer} is the address of a pattern buffer.  If the
character @var{c} could start a match for the pattern,
@code{re_compile_fastmap} makes
@code{@var{pattern_buffer}->fastmap[@var{c}]} nonzero.  It returns
@math{0} if it can compile a fastmap and @math{-2} if there is an
internal error.  For example, if @samp{|} is the alternation operator
and @var{pattern_buffer} holds the compiled pattern for @samp{a|b}, then
@code{re_compile_fastmap} sets @code{fastmap['a']} and
@code{fastmap['b']} (and no others).

@code{re_search} uses a fastmap as it moves along in the string: it
checks the string's characters until it finds one that's in the fastmap.
Then it tries matching at that character.  If the match fails, it
repeats the process.  So, by using a fastmap, @code{re_search} doesn't
waste time trying to match at positions in the string that couldn't
start a match.

If you don't want @code{re_search} to use a fastmap,
store zero in the @code{fastmap} field of the pattern buffer before
calling @code{re_search}.

Once you've initialized a pattern buffer's @code{fastmap} field, you
need never do so again---even if you compile a new pattern in
it---provided the way the field is set still reflects whether or not you
want a fastmap.  @code{re_search} will still either do nothing if
@code{fastmap} is null or, if it isn't, compile a new fastmap for the
new pattern.

@node GNU Translate Tables, Using Registers, Searching with Fastmaps, GNU Regex Functions
@subsection GNU Translate Tables

If you set the @code{translate} field of a pattern buffer to a translate
table, then the @sc{gnu} Regex functions to which you've passed that
pattern buffer use it to apply a simple transformation
to all the regular expression and string characters at which they look.

A @dfn{translate table} is an array indexed by the characters in your
character set.  Under the @sc{ascii} encoding, therefore, a translate
table has 256 elements.  The array's elements are also characters in
your character set.  When the Regex functions see a character @var{c},
they use @code{translate[@var{c}]} in its place, with one exception: the
character after a @samp{\} is not translated.  (This ensures that, the
operators, e.g., @samp{\B} and @samp{\b}, are always distinguishable.)

For example, a table that maps all lowercase letters to the
corresponding uppercase ones would cause the matcher to ignore
differences in case.@footnote{A table that maps all uppercase letters to
the corresponding lowercase ones would work just as well for this
purpose.}  Such a table would map all characters except lowercase letters
to themselves, and lowercase letters to the corresponding uppercase
ones.  Under the @sc{ascii} encoding, here's how you could initialize
such a table (we'll call it @code{case_fold}):

@example
for (i = 0; i < 256; i++)
  case_fold[i] = i;
for (i = 'a'; i <= 'z'; i++)
  case_fold[i] = i - ('a' - 'A');
@end example

You tell Regex to use a translate table on a given pattern buffer by
assigning that table's address to the @code{translate} field of that
buffer.  If you don't want Regex to do any translation, put zero into
this field.  You'll get weird results if you change the table's contents
anytime between compiling the pattern buffer, compiling its fastmap, and
matching or searching with the pattern buffer.

@node Using Registers, Freeing GNU Pattern Buffers, GNU Translate Tables, GNU Regex Functions
@subsection Using Registers

A group in a regular expression can match a (posssibly empty) substring
of the string that regular expression as a whole matched.  The matcher
remembers the beginning and end of the substring matched by
each group.

To find out what they matched, pass a nonzero @var{regs} argument to a
@sc{gnu} matching or searching function (@pxref{GNU Matching} and
@ref{GNU Searching}), i.e., the address of a structure of this type, as
defined in @file{regex.h}:

@c We don't bother to include this directly from regex.h,
@c since it changes so rarely.
@example
@tindex re_registers
@vindex num_regs @r{in @code{struct re_registers}}
@vindex start @r{in @code{struct re_registers}}
@vindex end @r{in @code{struct re_registers}}
struct re_registers
@{
  unsigned num_regs;
  regoff_t *start;
  regoff_t *end;
@};
@end example

Except for (possibly) the @var{num_regs}'th element (see below), the
@var{i}th element of the @code{start} and @code{end} arrays records
information about the @var{i}th group in the pattern.  (They're declared
as C pointers, but this is only because not all C compilers accept
zero-length arrays; conceptually, it is simplest to think of them as
arrays.)

The @code{start} and @code{end} arrays are allocated in various ways,
depending on the value of the @code{regs_allocated}
@vindex regs_allocated
field in the pattern buffer passed to the matcher.

The simplest and perhaps most useful is to let the matcher (re)allocate
enough space to record information for all the groups in the regular
expression.  If @code{regs_allocated} is @code{REGS_UNALLOCATED},
@vindex REGS_UNALLOCATED
the matcher allocates @math{1 + @var{re_nsub}} (another field in the
pattern buffer; @pxref{GNU Pattern Buffers}).  The extra element is set
to @math{-1}, and sets @code{regs_allocated} to @code{REGS_REALLOCATE}.
@vindex REGS_REALLOCATE
Then on subsequent calls with the same pattern buffer and @var{regs}
arguments, the matcher reallocates more space if necessary.

It would perhaps be more logical to make the @code{regs_allocated} field
part of the @code{re_registers} structure, instead of part of the
pattern buffer.  But in that case the caller would be forced to
initialize the structure before passing it.  Much existing code doesn't
do this initialization, and it's arguably better to avoid it anyway.

@code{re_compile_pattern} sets @code{regs_allocated} to
@code{REGS_UNALLOCATED},
so if you use the GNU regular expression
functions, you get this behavior by default.

xx document re_set_registers

@sc{posix}, on the other hand, requires a different interface:  the
caller is supposed to pass in a fixed-length array which the matcher
fills.  Therefore, if @code{regs_allocated} is @code{REGS_FIXED}
@vindex REGS_FIXED
the matcher simply fills that array.

The following examples illustrate the information recorded in the
@code{re_registers} structure.  (In all of them, @samp{(} represents the
open-group and @samp{)} the close-group operator.  The first character
in the string @var{string} is at index 0.)

@c xx i'm not sure this is all true anymore.

@itemize @bullet

@item
If the regular expression has an @w{@var{i}-th}
group not contained within another group that matches a
substring of @var{string}, then the function sets
@code{@w{@var{regs}->}start[@var{i}]} to the index in @var{string} where
the substring matched by the @w{@var{i}-th} group begins, and
@code{@w{@var{regs}->}end[@var{i}]} to the index just beyond that
substring's end.  The function sets @code{@w{@var{regs}->}start[0]} and
@code{@w{@var{regs}->}end[0]} to analogous information about the entire
pattern.

For example, when you match @samp{((a)(b))} against @samp{ab}, you get:

@itemize @bullet
@item
0 in @code{@w{@var{regs}->}start[0]} and 2 in @code{@w{@var{regs}->}end[0]}

@item
0 in @code{@w{@var{regs}->}start[1]} and 2 in @code{@w{@var{regs}->}end[1]}

@item
0 in @code{@w{@var{regs}->}start[2]} and 1 in @code{@w{@var{regs}->}end[2]}

@item
1 in @code{@w{@var{regs}->}start[3]} and 2 in @code{@w{@var{regs}->}end[3]}
@end itemize

@item
If a group matches more than once (as it might if followed by,
e.g., a repetition operator), then the function reports the information
about what the group @emph{last} matched.

For example, when you match the pattern @samp{(a)*} against the string
@samp{aa}, you get:

@itemize @bullet
@item
0 in @code{@w{@var{regs}->}start[0]} and 2 in @code{@w{@var{regs}->}end[0]}

@item
1 in @code{@w{@var{regs}->}start[1]} and 2 in @code{@w{@var{regs}->}end[1]}
@end itemize

@item
If the @w{@var{i}-th} group does not participate in a
successful match, e.g., it is an alternative not taken or a
repetition operator allows zero repetitions of it, then the function
sets @code{@w{@var{regs}->}start[@var{i}]} and
@code{@w{@var{regs}->}end[@var{i}]} to @math{-1}.

For example, when you match the pattern @samp{(a)*b} against
the string @samp{b}, you get:

@itemize @bullet
@item
0 in @code{@w{@var{regs}->}start[0]} and 1 in @code{@w{@var{regs}->}end[0]}

@item
@math{-1} in @code{@w{@var{regs}->}start[1]} and @math{-1} in @code{@w{@var{regs}->}end[1]}
@end itemize

@item
If the @w{@var{i}-th} group matches a zero-length string, then the
function sets @code{@w{@var{regs}->}start[@var{i}]} and
@code{@w{@var{regs}->}end[@var{i}]} to the index just beyond that
zero-length string.

For example, when you match the pattern @samp{(a*)b} against the string
@samp{b}, you get:

@itemize @bullet
@item
0 in @code{@w{@var{regs}->}start[0]} and 1 in @code{@w{@var{regs}->}end[0]}

@item
0 in @code{@w{@var{regs}->}start[1]} and 0 in @code{@w{@var{regs}->}end[1]}
@end itemize

@ignore
The function sets @code{@w{@var{regs}->}start[0]} and
@code{@w{@var{regs}->}end[0]} to analogous information about the entire
pattern.

For example, when you match the pattern @samp{(a*)} against the empty
string, you get:

@itemize @bullet
@item
0 in @code{@w{@var{regs}->}start[0]} and 0 in @code{@w{@var{regs}->}end[0]}

@item
0 in @code{@w{@var{regs}->}start[1]} and 0 in @code{@w{@var{regs}->}end[1]}
@end itemize
@end ignore

@item
If an @w{@var{i}-th} group contains a @w{@var{j}-th} group
in turn not contained within any other group within group @var{i} and
the function reports a match of the @w{@var{i}-th} group, then it
records in @code{@w{@var{regs}->}start[@var{j}]} and
@code{@w{@var{regs}->}end[@var{j}]} the last match (if it matched) of
the @w{@var{j}-th} group.

For example, when you match the pattern @samp{((a*)b)*} against the
string @samp{abb}, @w{group 2} last matches the empty string, so you
get what it previously matched:

@itemize @bullet
@item
0 in @code{@w{@var{regs}->}start[0]} and 3 in @code{@w{@var{regs}->}end[0]}

@item
2 in @code{@w{@var{regs}->}start[1]} and 3 in @code{@w{@var{regs}->}end[1]}

@item
2 in @code{@w{@var{regs}->}start[2]} and 2 in @code{@w{@var{regs}->}end[2]}
@end itemize

When you match the pattern @samp{((a)*b)*} against the string
@samp{abb}, @w{group 2} doesn't participate in the last match, so you
get:

@itemize @bullet
@item
0 in @code{@w{@var{regs}->}start[0]} and 3 in @code{@w{@var{regs}->}end[0]}

@item
2 in @code{@w{@var{regs}->}start[1]} and 3 in @code{@w{@var{regs}->}end[1]}

@item
0 in @code{@w{@var{regs}->}start[2]} and 1 in @code{@w{@var{regs}->}end[2]}
@end itemize

@item
If an @w{@var{i}-th} group contains a @w{@var{j}-th} group
in turn not contained within any other group within group @var{i}
and the function sets
@code{@w{@var{regs}->}start[@var{i}]} and
@code{@w{@var{regs}->}end[@var{i}]} to @math{-1}, then it also sets
@code{@w{@var{regs}->}start[@var{j}]} and
@code{@w{@var{regs}->}end[@var{j}]} to @math{-1}.

For example, when you match the pattern @samp{((a)*b)*c} against the
string @samp{c}, you get:

@itemize @bullet
@item
0 in @code{@w{@var{regs}->}start[0]} and 1 in @code{@w{@var{regs}->}end[0]}

@item
@math{-1} in @code{@w{@var{regs}->}start[1]} and @math{-1} in @code{@w{@var{regs}->}end[1]}

@item
@math{-1} in @code{@w{@var{regs}->}start[2]} and @math{-1} in @code{@w{@var{regs}->}end[2]}
@end itemize

@end itemize

@node Freeing GNU Pattern Buffers,  , Using Registers, GNU Regex Functions
@subsection Freeing GNU Pattern Buffers

To free any allocated fields of a pattern buffer, you can use the
@sc{posix} function described in @ref{Freeing POSIX Pattern Buffers},
since the type @code{regex_t}---the type for @sc{posix} pattern
buffers---is equivalent to the type @code{re_pattern_buffer}.  After
freeing a pattern buffer, you need to again compile a regular expression
in it (@pxref{GNU Regular Expression Compiling}) before passing it to
a matching or searching function.


@node POSIX Regex Functions, BSD Regex Functions, GNU Regex Functions, Programming with Regex
@section POSIX Regex Functions

If you're writing code that has to be @sc{posix} compatible, you'll need
to use these functions. Their interfaces are as specified by @sc{posix},
draft 1003.2/D11.2.

@menu
* POSIX Pattern Buffers::		The regex_t type.
* POSIX Regular Expression Compiling::	regcomp ()
* POSIX Matching::			regexec ()
* Reporting Errors::			regerror ()
* Using Byte Offsets::			The regmatch_t type.
* Freeing POSIX Pattern Buffers::	regfree ()
@end menu


@node POSIX Pattern Buffers, POSIX Regular Expression Compiling,  , POSIX Regex Functions
@subsection POSIX Pattern Buffers

To compile or match a given regular expression the @sc{posix} way, you
must supply a pattern buffer exactly the way you do for @sc{gnu}
(@pxref{GNU Pattern Buffers}).  @sc{posix} pattern buffers have type
@code{regex_t}, which is equivalent to the @sc{gnu} pattern buffer
type @code{re_pattern_buffer}.


@node POSIX Regular Expression Compiling, POSIX Matching, POSIX Pattern Buffers, POSIX Regex Functions
@subsection POSIX Regular Expression Compiling

With @sc{posix}, you can only search for a given regular expression; you
can't match it.  To do this, you must first compile it in a
pattern buffer, using @code{regcomp}.

@ignore
Before calling @code{regcomp}, you must initialize this pattern buffer
as you do for @sc{gnu} (@pxref{GNU Regular Expression Compiling}).  See
below, however, for how to choose a syntax with which to compile.
@end ignore

To compile a pattern buffer, use:

@findex regcomp
@example
int
regcomp (regex_t *@var{preg}, const char *@var{regex}, int @var{cflags})
@end example

@noindent
@var{preg} is the initialized pattern buffer's address, @var{regex} is
the regular expression's address, and @var{cflags} is the compilation
flags, which Regex considers as a collection of bits.  Here are the
valid bits, as defined in @file{regex.h}:

@table @code

@item REG_EXTENDED
@vindex REG_EXTENDED
says to use @sc{posix} Extended Regular Expression syntax; if this isn't
set, then says to use @sc{posix} Basic Regular Expression syntax.
@code{regcomp} sets @var{preg}'s @code{syntax} field accordingly.

@item REG_ICASE
@vindex REG_ICASE
@cindex ignoring case
says to ignore case; @code{regcomp} sets @var{preg}'s @code{translate}
field to a translate table which ignores case, replacing anything you've
put there before.

@item REG_NOSUB
@vindex REG_NOSUB
says to set @var{preg}'s @code{no_sub} field; @pxref{POSIX Matching},
for what this means.

@item REG_NEWLINE
@vindex REG_NEWLINE
says that a:

@itemize @bullet

@item
match-any-character operator (@pxref{Match-any-character
Operator}) doesn't match a newline.

@item
nonmatching list not containing a newline (@pxref{List
Operators}) matches a newline.

@item
match-beginning-of-line operator (@pxref{Match-beginning-of-line
Operator}) matches the empty string immediately after a newline,
regardless of how @code{REG_NOTBOL} is set (@pxref{POSIX Matching}, for
an explanation of @code{REG_NOTBOL}).

@item
match-end-of-line operator (@pxref{Match-beginning-of-line
Operator}) matches the empty string immediately before a newline,
regardless of how @code{REG_NOTEOL} is set (@pxref{POSIX Matching},
for an explanation of @code{REG_NOTEOL}).

@end itemize

@end table

If @code{regcomp} successfully compiles the regular expression, it
returns zero and sets @code{*@var{pattern_buffer}} to the compiled
pattern. Except for @code{syntax} (which it sets as explained above), it
also sets the same fields the same way as does the @sc{gnu} compiling
function (@pxref{GNU Regular Expression Compiling}).

If @code{regcomp} can't compile the regular expression, it returns one
of the error codes listed here.  (Except when noted differently, the
syntax of in all examples below is basic regular expression syntax.)

@table @code

@comment repetitions
@item REG_BADRPT
For example, the consecutive repetition operators @samp{**} in
@samp{a**} are invalid.  As another example, if the syntax is extended
regular expression syntax, then the repetition operator @samp{*} with
nothing on which to operate in @samp{*} is invalid.

@item REG_BADBR
For example, the @var{count} @samp{-1} in @samp{a\@{-1} is invalid.

@item REG_EBRACE
For example, @samp{a\@{1} is missing a close-interval operator.

@comment lists
@item REG_EBRACK
For example, @samp{[a} is missing a close-list operator.

@item REG_ERANGE
For example, the range ending point @samp{z} that collates lower than
does its starting point @samp{a} in @samp{[z-a]} is invalid.  Also, the
range with the character class @samp{[:alpha:]} as its starting point in
@samp{[[:alpha:]-|]}.

@item REG_ECTYPE
For example, the character class name @samp{foo} in @samp{[[:foo:]} is
invalid.

@comment groups
@item REG_EPAREN
For example, @samp{a\)} is missing an open-group operator and @samp{\(a}
is missing a close-group operator.

@item REG_ESUBREG
For example, the back reference @samp{\2} that refers to a nonexistent
subexpression in @samp{\(a\)\2} is invalid.

@comment unfinished business

@item REG_EEND
Returned when a regular expression causes no other more specific error.

@item REG_EESCAPE
For example, the trailing backslash @samp{\} in @samp{a\} is invalid, as is the
one in @samp{\}.

@comment kitchen sink
@item REG_BADPAT
For example, in the extended regular expression syntax, the empty group
@samp{()} in @samp{a()b} is invalid.

@comment internal
@item REG_ESIZE
Returned when a regular expression needs a pattern buffer larger than
65536 bytes.

@item REG_ESPACE
Returned when a regular expression makes Regex to run out of memory.

@end table


@node POSIX Matching, Reporting Errors, POSIX Regular Expression Compiling, POSIX Regex Functions
@subsection POSIX Matching

Matching the @sc{posix} way means trying to match a null-terminated
string starting at its first character.  Once you've compiled a pattern
into a pattern buffer (@pxref{POSIX Regular Expression Compiling}), you
can ask the matcher to match that pattern against a string using:

@findex regexec
@example
int
regexec (const regex_t *@var{preg}, const char *@var{string},
         size_t @var{nmatch}, regmatch_t @var{pmatch}[], int @var{eflags})
@end example

@noindent
@var{preg} is the address of a pattern buffer for a compiled pattern.
@var{string} is the string you want to match.

@xref{Using Byte Offsets}, for an explanation of @var{pmatch}.  If you
pass zero for @var{nmatch} or you compiled @var{preg} with the
compilation flag @code{REG_NOSUB} set, then @code{regexec} will ignore
@var{pmatch}; otherwise, you must allocate it to have at least
@var{nmatch} elements.  @code{regexec} will record @var{nmatch} byte
offsets in @var{pmatch}, and set to @math{-1} any unused elements up to
@math{@var{pmatch}@code{[@var{nmatch}]} - 1}.

@var{eflags} specifies @dfn{execution flags}---namely, the two bits
@code{REG_NOTBOL} and @code{REG_NOTEOL} (defined in @file{regex.h}).  If
you set @code{REG_NOTBOL}, then the match-beginning-of-line operator
(@pxref{Match-beginning-of-line Operator}) always fails to match.
This lets you match against pieces of a line, as you would need to if,
say, searching for repeated instances of a given pattern in a line; it
would work correctly for patterns both with and without
match-beginning-of-line operators.  @code{REG_NOTEOL} works analogously
for the match-end-of-line operator (@pxref{Match-end-of-line
Operator}); it exists for symmetry.

@code{regexec} tries to find a match for @var{preg} in @var{string}
according to the syntax in @var{preg}'s @code{syntax} field.
(@xref{POSIX Regular Expression Compiling}, for how to set it.)  The
function returns zero if the compiled pattern matches @var{string} and
@code{REG_NOMATCH} (defined in @file{regex.h}) if it doesn't.

@node Reporting Errors, Using Byte Offsets, POSIX Matching, POSIX Regex Functions
@subsection Reporting Errors

If either @code{regcomp} or @code{regexec} fail, they return a nonzero
error code, the possibilities for which are defined in @file{regex.h}.
@xref{POSIX Regular Expression Compiling}, and @ref{POSIX Matching}, for
what these codes mean.  To get an error string corresponding to these
codes, you can use:

@findex regerror
@example
size_t
regerror (int @var{errcode},
          const regex_t *@var{preg},
          char *@var{errbuf},
          size_t @var{errbuf_size})
@end example

@noindent
@var{errcode} is an error code, @var{preg} is the address of the pattern
buffer which provoked the error, @var{errbuf} is the error buffer, and
@var{errbuf_size} is @var{errbuf}'s size.

@code{regerror} returns the size in bytes of the error string
corresponding to @var{errcode} (including its terminating null).  If
@var{errbuf} and @var{errbuf_size} are nonzero, it also returns in
@var{errbuf} the first @math{@var{errbuf_size} - 1} characters of the
error string, followed by a null.
@var{errbuf_size} must be a nonnegative number less than or equal to the
size in bytes of @var{errbuf}.

You can call @code{regerror} with a null @var{errbuf} and a zero
@var{errbuf_size} to determine how large @var{errbuf} need be to
accommodate @code{regerror}'s error string.

@node Using Byte Offsets, Freeing POSIX Pattern Buffers, Reporting Errors, POSIX Regex Functions
@subsection Using Byte Offsets

In @sc{posix}, variables of type @code{regmatch_t} hold analogous
information, but are not identical to, @sc{gnu}'s registers (@pxref{Using
Registers}).  To get information about registers in @sc{posix}, pass to
@code{regexec} a nonzero @var{pmatch} of type @code{regmatch_t}, i.e.,
the address of a structure of this type, defined in
@file{regex.h}:

@tindex regmatch_t
@example
typedef struct
@{
  regoff_t rm_so;
  regoff_t rm_eo;
@} regmatch_t;
@end example

When reading in @ref{Using Registers}, about how the matching function
stores the information into the registers, substitute @var{pmatch} for
@var{regs}, @code{@w{@var{pmatch}[@var{i}]->}rm_so} for
@code{@w{@var{regs}->}start[@var{i}]} and
@code{@w{@var{pmatch}[@var{i}]->}rm_eo} for
@code{@w{@var{regs}->}end[@var{i}]}.

@node Freeing POSIX Pattern Buffers,  , Using Byte Offsets, POSIX Regex Functions
@subsection Freeing POSIX Pattern Buffers

To free any allocated fields of a pattern buffer, use:

@findex regfree
@example
void
regfree (regex_t *@var{preg})
@end example

@noindent
@var{preg} is the pattern buffer whose allocated fields you want freed.
@code{regfree} also sets @var{preg}'s @code{allocated} and @code{used}
fields to zero.  After freeing a pattern buffer, you need to again
compile a regular expression in it (@pxref{POSIX Regular Expression
Compiling}) before passing it to the matching function (@pxref{POSIX
Matching}).


@node BSD Regex Functions,  , POSIX Regex Functions, Programming with Regex
@section BSD Regex Functions

If you're writing code that has to be Berkeley @sc{unix} compatible,
you'll need to use these functions whose interfaces are the same as those
in Berkeley @sc{unix}.

@menu
* BSD Regular Expression Compiling::	re_comp ()
* BSD Searching::			re_exec ()
@end menu

@node BSD Regular Expression Compiling, BSD Searching,  , BSD Regex Functions
@subsection  BSD Regular Expression Compiling

With Berkeley @sc{unix}, you can only search for a given regular
expression; you can't match one.  To search for it, you must first
compile it.  Before you compile it, you must indicate the regular
expression syntax you want it compiled according to by setting the
variable @code{re_syntax_options} (declared in @file{regex.h} to some
syntax (@pxref{Regular Expression Syntax}).

To compile a regular expression use:

@findex re_comp
@example
char *
re_comp (char *@var{regex})
@end example

@noindent
@var{regex} is the address of a null-terminated regular expression.
@code{re_comp} uses an internal pattern buffer, so you can use only the
most recently compiled pattern buffer.  This means that if you want to
use a given regular expression that you've already compiled---but it
isn't the latest one you've compiled---you'll have to recompile it.  If
you call @code{re_comp} with the null string (@emph{not} the empty
string) as the argument, it doesn't change the contents of the pattern
buffer.

If @code{re_comp} successfully compiles the regular expression, it
returns zero.  If it can't compile the regular expression, it returns
an error string.  @code{re_comp}'s error messages are identical to those
of @code{re_compile_pattern} (@pxref{GNU Regular Expression
Compiling}).

@node BSD Searching,  , BSD Regular Expression Compiling, BSD Regex Functions
@subsection BSD Searching

Searching the Berkeley @sc{unix} way means searching in a string
starting at its first character and trying successive positions within
it to find a match.  Once you've compiled a pattern using @code{re_comp}
(@pxref{BSD Regular Expression Compiling}), you can ask Regex
to search for that pattern in a string using:

@findex re_exec
@example
int
re_exec (char *@var{string})
@end example

@noindent
@var{string} is the address of the null-terminated string in which you
want to search.

@code{re_exec} returns either 1 for success or 0 for failure.  It
automatically uses a @sc{gnu} fastmap (@pxref{Searching with Fastmaps}).


@node Copying, Index, Programming with Regex, Top
@appendix GNU GENERAL PUBLIC LICENSE
@center Version 2, June 1991

@display
Copyright @copyright{} 1989, 1991 Free Software Foundation, Inc.
675 Mass Ave, Cambridge, MA 02139, USA

Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
@end display

@unnumberedsec Preamble

  The licenses for most software are designed to take away your
freedom to share and change it.  By contrast, the GNU General Public
License is intended to guarantee your freedom to share and change free
software---to make sure the software is free for all its users.  This
General Public License applies to most of the Free Software
Foundation's software and to any other program whose authors commit to
using it.  (Some other Free Software Foundation software is covered by
the GNU Library General Public License instead.)  You can apply it to
your programs, too.

  When we speak of free software, we are referring to freedom, not
price.  Our General Public Licenses are designed to make sure that you
have the freedom to distribute copies of free software (and charge for
this service if you wish), that you receive source code or can get it
if you want it, that you can change the software or use pieces of it
in new free programs; and that you know you can do these things.

  To protect your rights, we need to make restrictions that forbid
anyone to deny you these rights or to ask you to surrender the rights.
These restrictions translate to certain responsibilities for you if you
distribute copies of the software, or if you modify it.

  For example, if you distribute copies of such a program, whether
gratis or for a fee, you must give the recipients all the rights that
you have.  You must make sure that they, too, receive or can get the
source code.  And you must show them these terms so they know their
rights.

  We protect your rights with two steps: (1) copyright the software, and
(2) offer you this license which gives you legal permission to copy,
distribute and/or modify the software.

  Also, for each author's protection and ours, we want to make certain
that everyone understands that there is no warranty for this free
software.  If the software is modified by someone else and passed on, we
want its recipients to know that what they have is not the original, so
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  Finally, any free program is threatened constantly by software
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program proprietary.  To prevent this, we have made it clear that any
patent must be licensed for everyone's free use or not licensed at all.

  The precise terms and conditions for copying, distribution and
modification follow.

@iftex
@unnumberedsec TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
@end iftex
@ifinfo
@center TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
@end ifinfo

@enumerate
@item
This License applies to any program or other work which contains
a notice placed by the copyright holder saying it may be distributed
under the terms of this General Public License.  The ``Program'', below,
refers to any such program or work, and a ``work based on the Program''
means either the Program or any derivative work under copyright law:
that is to say, a work containing the Program or a portion of it,
either verbatim or with modifications and/or translated into another
language.  (Hereinafter, translation is included without limitation in
the term ``modification''.)  Each licensee is addressed as ``you''.

Activities other than copying, distribution and modification are not
covered by this License; they are outside its scope.  The act of
running the Program is not restricted, and the output from the Program
is covered only if its contents constitute a work based on the
Program (independent of having been made by running the Program).
Whether that is true depends on what the Program does.

@item
You may copy and distribute verbatim copies of the Program's
source code as you receive it, in any medium, provided that you
conspicuously and appropriately publish on each copy an appropriate
copyright notice and disclaimer of warranty; keep intact all the
notices that refer to this License and to the absence of any warranty;
and give any other recipients of the Program a copy of this License
along with the Program.

You may charge a fee for the physical act of transferring a copy, and
you may at your option offer warranty protection in exchange for a fee.

@item
You may modify your copy or copies of the Program or any portion
of it, thus forming a work based on the Program, and copy and
distribute such modifications or work under the terms of Section 1
above, provided that you also meet all of these conditions:

@enumerate a
@item
You must cause the modified files to carry prominent notices
stating that you changed the files and the date of any change.

@item
You must cause any work that you distribute or publish, that in
whole or in part contains or is derived from the Program or any
part thereof, to be licensed as a whole at no charge to all third
parties under the terms of this License.

@item
If the modified program normally reads commands interactively
when run, you must cause it, when started running for such
interactive use in the most ordinary way, to print or display an
announcement including an appropriate copyright notice and a
notice that there is no warranty (or else, saying that you provide
a warranty) and that users may redistribute the program under
these conditions, and telling the user how to view a copy of this
License.  (Exception: if the Program itself is interactive but
does not normally print such an announcement, your work based on
the Program is not required to print an announcement.)
@end enumerate

These requirements apply to the modified work as a whole.  If
identifiable sections of that work are not derived from the Program,
and can be reasonably considered independent and separate works in
themselves, then this License, and its terms, do not apply to those
sections when you distribute them as separate works.  But when you
distribute the same sections as part of a whole which is a work based
on the Program, the distribution of the whole must be on the terms of
this License, whose permissions for other licensees extend to the
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Thus, it is not the intent of this section to claim rights or contest
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collective works based on the Program.

In addition, mere aggregation of another work not based on the Program
with the Program (or with a work based on the Program) on a volume of
a storage or distribution medium does not bring the other work under
the scope of this License.

@item
You may copy and distribute the Program (or a work based on it,
under Section 2) in object code or executable form under the terms of
Sections 1 and 2 above provided that you also do one of the following:

@enumerate a
@item
Accompany it with the complete corresponding machine-readable
source code, which must be distributed under the terms of Sections
1 and 2 above on a medium customarily used for software interchange; or,

@item
Accompany it with a written offer, valid for at least three
years, to give any third party, for a charge no more than your
cost of physically performing source distribution, a complete
machine-readable copy of the corresponding source code, to be
distributed under the terms of Sections 1 and 2 above on a medium
customarily used for software interchange; or,

@item
Accompany it with the information you received as to the offer
to distribute corresponding source code.  (This alternative is
allowed only for noncommercial distribution and only if you
received the program in object code or executable form with such
an offer, in accord with Subsection b above.)
@end enumerate

The source code for a work means the preferred form of the work for
making modifications to it.  For an executable work, complete source
code means all the source code for all modules it contains, plus any
associated interface definition files, plus the scripts used to
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If distribution of executable or object code is made by offering
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@item
You may not copy, modify, sublicense, or distribute the Program
except as expressly provided under this License.  Any attempt
otherwise to copy, modify, sublicense or distribute the Program is
void, and will automatically terminate your rights under this License.
However, parties who have received copies, or rights, from you under
this License will not have their licenses terminated so long as such
parties remain in full compliance.

@item
You are not required to accept this License, since you have not
signed it.  However, nothing else grants you permission to modify or
distribute the Program or its derivative works.  These actions are
prohibited by law if you do not accept this License.  Therefore, by
modifying or distributing the Program (or any work based on the
Program), you indicate your acceptance of this License to do so, and
all its terms and conditions for copying, distributing or modifying
the Program or works based on it.

@item
Each time you redistribute the Program (or any work based on the
Program), the recipient automatically receives a license from the
original licensor to copy, distribute or modify the Program subject to
these terms and conditions.  You may not impose any further
restrictions on the recipients' exercise of the rights granted herein.
You are not responsible for enforcing compliance by third parties to
this License.

@item
If, as a consequence of a court judgment or allegation of patent
infringement or for any other reason (not limited to patent issues),
conditions are imposed on you (whether by court order, agreement or
otherwise) that contradict the conditions of this License, they do not
excuse you from the conditions of this License.  If you cannot
distribute so as to satisfy simultaneously your obligations under this
License and any other pertinent obligations, then as a consequence you
may not distribute the Program at all.  For example, if a patent
license would not permit royalty-free redistribution of the Program by
all those who receive copies directly or indirectly through you, then
the only way you could satisfy both it and this License would be to
refrain entirely from distribution of the Program.

If any portion of this section is held invalid or unenforceable under
any particular circumstance, the balance of the section is intended to
apply and the section as a whole is intended to apply in other
circumstances.

It is not the purpose of this section to induce you to infringe any
patents or other property right claims or to contest validity of any
such claims; this section has the sole purpose of protecting the
integrity of the free software distribution system, which is
implemented by public license practices.  Many people have made
generous contributions to the wide range of software distributed
through that system in reliance on consistent application of that
system; it is up to the author/donor to decide if he or she is willing
to distribute software through any other system and a licensee cannot
impose that choice.

This section is intended to make thoroughly clear what is believed to
be a consequence of the rest of this License.

@item
If the distribution and/or use of the Program is restricted in
certain countries either by patents or by copyrighted interfaces, the
original copyright holder who places the Program under this License
may add an explicit geographical distribution limitation excluding
those countries, so that distribution is permitted only in or among
countries not thus excluded.  In such case, this License incorporates
the limitation as if written in the body of this License.

@item
The Free Software Foundation may publish revised and/or new versions
of the General Public License from time to time.  Such new versions will
be similar in spirit to the present version, but may differ in detail to
address new problems or concerns.

Each version is given a distinguishing version number.  If the Program
specifies a version number of this License which applies to it and ``any
later version'', you have the option of following the terms and conditions
either of that version or of any later version published by the Free
Software Foundation.  If the Program does not specify a version number of
this License, you may choose any version ever published by the Free Software
Foundation.

@item
If you wish to incorporate parts of the Program into other free
programs whose distribution conditions are different, write to the author
to ask for permission.  For software which is copyrighted by the Free
Software Foundation, write to the Free Software Foundation; we sometimes
make exceptions for this.  Our decision will be guided by the two goals
of preserving the free status of all derivatives of our free software and
of promoting the sharing and reuse of software generally.

@iftex
@heading NO WARRANTY
@end iftex
@ifinfo
@center NO WARRANTY
@end ifinfo

@item
BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY
FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW.  EXCEPT WHEN
OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES
PROVIDE THE PROGRAM ``AS IS'' WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED
OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.  THE ENTIRE RISK AS
TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU.  SHOULD THE
PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING,
REPAIR OR CORRECTION.

@item
IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR
REDISTRIBUTE THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES,
INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING
OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED
TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY
YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER
PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE
POSSIBILITY OF SUCH DAMAGES.
@end enumerate

@iftex
@heading END OF TERMS AND CONDITIONS
@end iftex
@ifinfo
@center END OF TERMS AND CONDITIONS
@end ifinfo

@page
@unnumberedsec Appendix: How to Apply These Terms to Your New Programs

  If you develop a new program, and you want it to be of the greatest
possible use to the public, the best way to achieve this is to make it
free software which everyone can redistribute and change under these terms.

  To do so, attach the following notices to the program.  It is safest
to attach them to the start of each source file to most effectively
convey the exclusion of warranty; and each file should have at least
the ``copyright'' line and a pointer to where the full notice is found.

@smallexample
@var{one line to give the program's name and a brief idea of what it does.}
Copyright (C) 19@var{yy}  @var{name of author}

This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
@end smallexample

Also add information on how to contact you by electronic and paper mail.

If the program is interactive, make it output a short notice like this
when it starts in an interactive mode:

@smallexample
Gnomovision version 69, Copyright (C) 19@var{yy} @var{name of author}
Gnomovision comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
This is free software, and you are welcome to redistribute it
under certain conditions; type `show c' for details.
@end smallexample

The hypothetical commands @samp{show w} and @samp{show c} should show
the appropriate parts of the General Public License.  Of course, the
commands you use may be called something other than @samp{show w} and
@samp{show c}; they could even be mouse-clicks or menu items---whatever
suits your program.

You should also get your employer (if you work as a programmer) or your
school, if any, to sign a ``copyright disclaimer'' for the program, if
necessary.  Here is a sample; alter the names:

@example
Yoyodyne, Inc., hereby disclaims all copyright interest in the program
`Gnomovision' (which makes passes at compilers) written by James Hacker.

@var{signature of Ty Coon}, 1 April 1989
Ty Coon, President of Vice
@end example

This General Public License does not permit incorporating your program into
proprietary programs.  If your program is a subroutine library, you may
consider it more useful to permit linking proprietary applications with the
library.  If this is what you want to do, use the GNU Library General
Public License instead of this License.


@node Index,  , Copying, Top
@unnumbered Index

@printindex cp

@contents

@bye