(M-40132'): Unify GT-53970.

[chise/xemacs-chise.git-] / info / lispref.info-32

This is ../info/lispref.info, produced by makeinfo version 4.0b from
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INFO-DIR-SECTION XEmacs Editor
START-INFO-DIR-ENTRY
* Lispref: (lispref).           XEmacs Lisp Reference Manual.
END-INFO-DIR-ENTRY

   Edition History:

   GNU Emacs Lisp Reference Manual Second Edition (v2.01), May 1993 GNU
Emacs Lisp Reference Manual Further Revised (v2.02), August 1993 Lucid
Emacs Lisp Reference Manual (for 19.10) First Edition, March 1994
XEmacs Lisp Programmer's Manual (for 19.12) Second Edition, April 1995
GNU Emacs Lisp Reference Manual v2.4, June 1995 XEmacs Lisp
Programmer's Manual (for 19.13) Third Edition, July 1995 XEmacs Lisp
Reference Manual (for 19.14 and 20.0) v3.1, March 1996 XEmacs Lisp
Reference Manual (for 19.15 and 20.1, 20.2, 20.3) v3.2, April, May,
November 1997 XEmacs Lisp Reference Manual (for 21.0) v3.3, April 1998

   Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995 Free Software
Foundation, Inc.  Copyright (C) 1994, 1995 Sun Microsystems, Inc.
Copyright (C) 1995, 1996 Ben Wing.

   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 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 this permission notice may be stated in a
translation approved by the Foundation.

   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.

\1f
File: lispref.info,  Node: Change Hooks,  Next: Transformations,  Prev: Transposition,  Up: Text

Change Hooks
============

   These hook variables let you arrange to take notice of all changes in
all buffers (or in a particular buffer, if you make them buffer-local).

   The functions you use in these hooks should save and restore the
match data if they do anything that uses regular expressions;
otherwise, they will interfere in bizarre ways with the editing
operations that call them.

   Buffer changes made while executing the following hooks don't
themselves cause any change hooks to be invoked.

 - Variable: before-change-functions
     This variable holds a list of a functions to call before any buffer
     modification.  Each function gets two arguments, the beginning and
     end of the region that is about to change, represented as
     integers.  The buffer that is about to change is always the
     current buffer.

 - Variable: after-change-functions
     This variable holds a list of a functions to call after any buffer
     modification.  Each function receives three arguments: the
     beginning and end of the region just changed, and the length of
     the text that existed before the change.  (To get the current
     length, subtract the region beginning from the region end.)  All
     three arguments are integers.  The buffer that's about to change
     is always the current buffer.

 - Variable: before-change-function
     This obsolete variable holds one function to call before any buffer
     modification (or `nil' for no function).  It is called just like
     the functions in `before-change-functions'.

 - Variable: after-change-function
     This obsolete variable holds one function to call after any buffer
     modification (or `nil' for no function).  It is called just like
     the functions in `after-change-functions'.

 - Variable: first-change-hook
     This variable is a normal hook that is run whenever a buffer is
     changed that was previously in the unmodified state.

\1f
File: lispref.info,  Node: Transformations,  Prev: Change Hooks,  Up: Text

Textual transformations--MD5 and base64 support
===============================================

   Some textual operations inherently require examining each character
in turn, and performing arithmetic operations on them.  Such operations
can, of course, be implemented in Emacs Lisp, but tend to be very slow
for large portions of text or data.  This is why some of them are
implemented in C, with an appropriate interface for Lisp programmers.
Examples of algorithms thus provided are MD5 and base64 support.

   MD5 is an algorithm for calculating message digests, as described in
rfc1321.  Given a message of arbitrary length, MD5 produces an 128-bit
"fingerprint" ("message digest") corresponding to that message.  It is
considered computationally infeasible to produce two messages having
the same MD5 digest, or to produce a message having a prespecified
target digest.  MD5 is used heavily by various authentication schemes.

   Emacs Lisp interface to MD5 consists of a single function `md5':

 - Function: md5 object &optional start end coding noerror
     This function returns the MD5 message digest of OBJECT, a buffer
     or string.

     Optional arguments START and END denote positions for computing
     the digest of a portion of OBJECT.

     The optional CODING argument specifies the coding system the text
     is to be represented in while computing the digest.  If
     unspecified, it defaults to the current format of the data, or is
     guessed.

     If NOERROR is non-`nil', silently assume binary coding if the
     guesswork fails.  Normally, an error is signaled in such case.

     CODING and NOERROR arguments are meaningful only in XEmacsen with
     file-coding or Mule support.  Otherwise, they are ignored.  Some
     examples of usage:

          ;; Calculate the digest of the entire buffer
          (md5 (current-buffer))
               => "8842b04362899b1cda8d2d126dc11712"
          
          ;; Calculate the digest of the current line
          (md5 (current-buffer) (point-at-bol) (point-at-eol))
               => "60614d21e9dee27dfdb01fa4e30d6d00"
          
          ;; Calculate the digest of your name and email address
          (md5 (concat (format "%s <%s>" (user-full-name) user-mail-address)))
               => "0a2188c40fd38922d941fe6032fce516"

   Base64 is a portable encoding for arbitrary sequences of octets, in a
form that need not be readable by humans.  It uses a 65-character subset
of US-ASCII, as described in rfc2045.  Base64 is used by MIME to encode
binary bodies, and to encode binary characters in message headers.

   The Lisp interface to base64 consists of four functions:

 - Command: base64-encode-region start end &optional no-line-break
     This function encodes the region between START and END of the
     current buffer to base64 format.  This means that the original
     region is deleted, and replaced with its base64 equivalent.

     Normally, encoded base64 output is multi-line, with 76-character
     lines.  If NO-LINE-BREAK is non-`nil', newlines will not be
     inserted, resulting in single-line output.

     Mule note: you should make sure that you convert the multibyte
     characters (those that do not fit into 0-255 range) to something
     else, because they cannot be meaningfully converted to base64.  If
     the `base64-encode-region' encounters such characters, it will
     signal an error.

     `base64-encode-region' returns the length of the encoded text.

          ;; Encode the whole buffer in base64
          (base64-encode-region (point-min) (point-max))

     The function can also be used interactively, in which case it
     works on the currently active region.

 - Function: base64-encode-string string &optional no-line-break
     This function encodes STRING to base64, and returns the encoded
     string.

     Normally, encoded base64 output is multi-line, with 76-character
     lines.  If NO-LINE-BREAK is non-`nil', newlines will not be
     inserted, resulting in single-line output.

     For Mule, the same considerations apply as for
     `base64-encode-region'.

          (base64-encode-string "fubar")
              => "ZnViYXI="

 - Command: base64-decode-region start end
     This function decodes the region between START and END of the
     current buffer.  The region should be in base64 encoding.

     If the region was decoded correctly, `base64-decode-region' returns
     the length of the decoded region.  If the decoding failed, `nil' is
     returned.

          ;; Decode a base64 buffer, and replace it with the decoded version
          (base64-decode-region (point-min) (point-max))

 - Function: base64-decode-string string
     This function decodes STRING to base64, and returns the decoded
     string.  STRING should be valid base64-encoded text.

     If encoding was not possible, `nil' is returned.

          (base64-decode-string "ZnViYXI=")
              => "fubar"
          
          (base64-decode-string "totally bogus")
              => nil

\1f
File: lispref.info,  Node: Searching and Matching,  Next: Syntax Tables,  Prev: Text,  Up: Top

Searching and Matching
**********************

   XEmacs provides two ways to search through a buffer for specified
text: exact string searches and regular expression searches.  After a
regular expression search, you can examine the "match data" to
determine which text matched the whole regular expression or various
portions of it.

* Menu:

* String Search::         Search for an exact match.
* Regular Expressions::   Describing classes of strings.
* Regexp Search::         Searching for a match for a regexp.
* POSIX Regexps::         Searching POSIX-style for the longest match.
* Search and Replace::    Internals of `query-replace'.
* Match Data::            Finding out which part of the text matched
                            various parts of a regexp, after regexp search.
* Searching and Case::    Case-independent or case-significant searching.
* Standard Regexps::      Useful regexps for finding sentences, pages,...

   The `skip-chars...' functions also perform a kind of searching.
*Note Skipping Characters::.

\1f
File: lispref.info,  Node: String Search,  Next: Regular Expressions,  Up: Searching and Matching

Searching for Strings
=====================

   These are the primitive functions for searching through the text in a
buffer.  They are meant for use in programs, but you may call them
interactively.  If you do so, they prompt for the search string; LIMIT
and NOERROR are set to `nil', and COUNT is set to 1.

 - Command: search-forward string &optional limit noerror count buffer
     This function searches forward from point for an exact match for
     STRING.  If successful, it sets point to the end of the occurrence
     found, and returns the new value of point.  If no match is found,
     the value and side effects depend on NOERROR (see below).

     In the following example, point is initially at the beginning of
     the line.  Then `(search-forward "fox")' moves point after the last
     letter of `fox':

          ---------- Buffer: foo ----------
          -!-The quick brown fox jumped over the lazy dog.
          ---------- Buffer: foo ----------
          
          (search-forward "fox")
               => 20
          
          ---------- Buffer: foo ----------
          The quick brown fox-!- jumped over the lazy dog.
          ---------- Buffer: foo ----------

     The argument LIMIT specifies the upper bound to the search.  (It
     must be a position in the current buffer.)  No match extending
     after that position is accepted.  If LIMIT is omitted or `nil', it
     defaults to the end of the accessible portion of the buffer.

     What happens when the search fails depends on the value of
     NOERROR.  If NOERROR is `nil', a `search-failed' error is
     signaled.  If NOERROR is `t', `search-forward' returns `nil' and
     does nothing.  If NOERROR is neither `nil' nor `t', then
     `search-forward' moves point to the upper bound and returns `nil'.
     (It would be more consistent now to return the new position of
     point in that case, but some programs may depend on a value of
     `nil'.)

     If COUNT is supplied (it must be an integer), then the search is
     repeated that many times (each time starting at the end of the
     previous time's match).  If COUNT is negative, the search
     direction is backward.  If the successive searches succeed, the
     function succeeds, moving point and returning its new value.
     Otherwise the search fails.

     BUFFER is the buffer to search in, and defaults to the current
     buffer.

 - Command: search-backward string &optional limit noerror count buffer
     This function searches backward from point for STRING.  It is just
     like `search-forward' except that it searches backwards and leaves
     point at the beginning of the match.

 - Command: word-search-forward string &optional limit noerror count
          buffer
     This function searches forward from point for a "word" match for
     STRING.  If it finds a match, it sets point to the end of the
     match found, and returns the new value of point.

     Word matching regards STRING as a sequence of words, disregarding
     punctuation that separates them.  It searches the buffer for the
     same sequence of words.  Each word must be distinct in the buffer
     (searching for the word `ball' does not match the word `balls'),
     but the details of punctuation and spacing are ignored (searching
     for `ball boy' does match `ball.  Boy!').

     In this example, point is initially at the beginning of the
     buffer; the search leaves it between the `y' and the `!'.

          ---------- Buffer: foo ----------
          -!-He said "Please!  Find
          the ball boy!"
          ---------- Buffer: foo ----------
          
          (word-search-forward "Please find the ball, boy.")
               => 35
          
          ---------- Buffer: foo ----------
          He said "Please!  Find
          the ball boy-!-!"
          ---------- Buffer: foo ----------

     If LIMIT is non-`nil' (it must be a position in the current
     buffer), then it is the upper bound to the search.  The match
     found must not extend after that position.

     If NOERROR is `nil', then `word-search-forward' signals an error
     if the search fails.  If NOERROR is `t', then it returns `nil'
     instead of signaling an error.  If NOERROR is neither `nil' nor
     `t', it moves point to LIMIT (or the end of the buffer) and
     returns `nil'.

     If COUNT is non-`nil', then the search is repeated that many
     times.  Point is positioned at the end of the last match.

     BUFFER is the buffer to search in, and defaults to the current
     buffer.

 - Command: word-search-backward string &optional limit noerror count
          buffer
     This function searches backward from point for a word match to
     STRING.  This function is just like `word-search-forward' except
     that it searches backward and normally leaves point at the
     beginning of the match.

\1f
File: lispref.info,  Node: Regular Expressions,  Next: Regexp Search,  Prev: String Search,  Up: Searching and Matching

Regular Expressions
===================

   A "regular expression" ("regexp", for short) is a pattern that
denotes a (possibly infinite) set of strings.  Searching for matches for
a regexp is a very powerful operation.  This section explains how to
write regexps; the following section says how to search for them.

   To gain a thorough understanding of regular expressions and how to
use them to best advantage, we recommend that you study `Mastering
Regular Expressions, by Jeffrey E.F. Friedl, O'Reilly and Associates,
1997'. (It's known as the "Hip Owls" book, because of the picture on its
cover.)  You might also read the manuals to *Note (gawk)Top::, *Note
(ed)Top::, `sed', `grep', *Note (perl)Top::, *Note (regex)Top::, *Note
(rx)Top::, `pcre', and *Note (flex)Top::, which also make good use of
regular expressions.

   The XEmacs regular expression syntax most closely resembles that of
`ed', or `grep', the GNU versions of which all utilize the GNU `regex'
library.  XEmacs' version of `regex' has recently been extended with
some Perl-like capabilities, described in the next section.

* Menu:

* Syntax of Regexps::       Rules for writing regular expressions.
* Regexp Example::          Illustrates regular expression syntax.

\1f
File: lispref.info,  Node: Syntax of Regexps,  Next: Regexp Example,  Up: Regular Expressions

Syntax of Regular Expressions
-----------------------------

   Regular expressions have a syntax in which a few characters are
special constructs and the rest are "ordinary".  An ordinary character
is a simple regular expression that matches that character and nothing
else.  The special characters are `.', `*', `+', `?', `[', `]', `^',
`$', and `\'; no new special characters will be defined in the future.
Any other character appearing in a regular expression is ordinary,
unless a `\' precedes it.

   For example, `f' is not a special character, so it is ordinary, and
therefore `f' is a regular expression that matches the string `f' and
no other string.  (It does _not_ match the string `ff'.)  Likewise, `o'
is a regular expression that matches only `o'.

   Any two regular expressions A and B can be concatenated.  The result
is a regular expression that matches a string if A matches some amount
of the beginning of that string and B matches the rest of the string.

   As a simple example, we can concatenate the regular expressions `f'
and `o' to get the regular expression `fo', which matches only the
string `fo'.  Still trivial.  To do something more powerful, you need
to use one of the special characters.  Here is a list of them:

`. (Period)'
     is a special character that matches any single character except a
     newline.  Using concatenation, we can make regular expressions
     like `a.b', which matches any three-character string that begins
     with `a' and ends with `b'.

`*'
     is not a construct by itself; it is a quantifying suffix operator
     that means to repeat the preceding regular expression as many
     times as possible.  In `fo*', the `*' applies to the `o', so `fo*'
     matches one `f' followed by any number of `o's.  The case of zero
     `o's is allowed: `fo*' does match `f'.

     `*' always applies to the _smallest_ possible preceding
     expression.  Thus, `fo*' has a repeating `o', not a repeating `fo'.

     The matcher processes a `*' construct by matching, immediately, as
     many repetitions as can be found; it is "greedy".  Then it
     continues with the rest of the pattern.  If that fails,
     backtracking occurs, discarding some of the matches of the
     `*'-modified construct in case that makes it possible to match the
     rest of the pattern.  For example, in matching `ca*ar' against the
     string `caaar', the `a*' first tries to match all three `a's; but
     the rest of the pattern is `ar' and there is only `r' left to
     match, so this try fails.  The next alternative is for `a*' to
     match only two `a's.  With this choice, the rest of the regexp
     matches successfully.

     Nested repetition operators can be extremely slow if they specify
     backtracking loops.  For example, it could take hours for the
     regular expression `\(x+y*\)*a' to match the sequence
     `xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxz'.  The slowness is because
     Emacs must try each imaginable way of grouping the 35 `x''s before
     concluding that none of them can work.  To make sure your regular
     expressions run fast, check nested repetitions carefully.

`+'
     is a quantifying suffix operator similar to `*' except that the
     preceding expression must match at least once.  It is also
     "greedy".  So, for example, `ca+r' matches the strings `car' and
     `caaaar' but not the string `cr', whereas `ca*r' matches all three
     strings.

`?'
     is a quantifying suffix operator similar to `*', except that the
     preceding expression can match either once or not at all.  For
     example, `ca?r' matches `car' or `cr', but does not match anything
     else.

`*?'
     works just like `*', except that rather than matching the longest
     match, it matches the shortest match.  `*?' is known as a
     "non-greedy" quantifier, a regexp construct borrowed from Perl.

     This construct is very useful for when you want to match the text
     inside a pair of delimiters.  For instance, `/\*.*?\*/' will match
     C comments in a string.  This could not easily be achieved without
     the use of a non-greedy quantifier.

     This construct has not been available prior to XEmacs 20.4.  It is
     not available in FSF Emacs.

`+?'
     is the non-greedy version of `+'.

`??'
     is the non-greedy version of `?'.

`\{n,m\}'
     serves as an interval quantifier, analogous to `*' or `+', but
     specifies that the expression must match at least N times, but no
     more than M times.  This syntax is supported by most Unix regexp
     utilities, and has been introduced to XEmacs for the version 20.3.

     Unfortunately, the non-greedy version of this quantifier does not
     exist currently, although it does in Perl.

`[ ... ]'
     `[' begins a "character set", which is terminated by a `]'.  In
     the simplest case, the characters between the two brackets form
     the set.  Thus, `[ad]' matches either one `a' or one `d', and
     `[ad]*' matches any string composed of just `a's and `d's
     (including the empty string), from which it follows that `c[ad]*r'
     matches `cr', `car', `cdr', `caddaar', etc.

     The usual regular expression special characters are not special
     inside a character set.  A completely different set of special
     characters exists inside character sets: `]', `-' and `^'.

     `-' is used for ranges of characters.  To write a range, write two
     characters with a `-' between them.  Thus, `[a-z]' matches any
     lower case letter.  Ranges may be intermixed freely with individual
     characters, as in `[a-z$%.]', which matches any lower case letter
     or `$', `%', or a period.

     To include a `]' in a character set, make it the first character.
     For example, `[]a]' matches `]' or `a'.  To include a `-', write
     `-' as the first character in the set, or put it immediately after
     a range.  (You can replace one individual character C with the
     range `C-C' to make a place to put the `-'.)  There is no way to
     write a set containing just `-' and `]'.

     To include `^' in a set, put it anywhere but at the beginning of
     the set.

`[^ ... ]'
     `[^' begins a "complement character set", which matches any
     character except the ones specified.  Thus, `[^a-z0-9A-Z]' matches
     all characters _except_ letters and digits.

     `^' is not special in a character set unless it is the first
     character.  The character following the `^' is treated as if it
     were first (thus, `-' and `]' are not special there).

     Note that a complement character set can match a newline, unless
     newline is mentioned as one of the characters not to match.

`^'
     is a special character that matches the empty string, but only at
     the beginning of a line in the text being matched.  Otherwise it
     fails to match anything.  Thus, `^foo' matches a `foo' that occurs
     at the beginning of a line.

     When matching a string instead of a buffer, `^' matches at the
     beginning of the string or after a newline character `\n'.

`$'
     is similar to `^' but matches only at the end of a line.  Thus,
     `x+$' matches a string of one `x' or more at the end of a line.

     When matching a string instead of a buffer, `$' matches at the end
     of the string or before a newline character `\n'.

`\'
     has two functions: it quotes the special characters (including
     `\'), and it introduces additional special constructs.

     Because `\' quotes special characters, `\$' is a regular
     expression that matches only `$', and `\[' is a regular expression
     that matches only `[', and so on.

     Note that `\' also has special meaning in the read syntax of Lisp
     strings (*note String Type::), and must be quoted with `\'.  For
     example, the regular expression that matches the `\' character is
     `\\'.  To write a Lisp string that contains the characters `\\',
     Lisp syntax requires you to quote each `\' with another `\'.
     Therefore, the read syntax for a regular expression matching `\'
     is `"\\\\"'.

   *Please note:* For historical compatibility, special characters are
treated as ordinary ones if they are in contexts where their special
meanings make no sense.  For example, `*foo' treats `*' as ordinary
since there is no preceding expression on which the `*' can act.  It is
poor practice to depend on this behavior; quote the special character
anyway, regardless of where it appears.

   For the most part, `\' followed by any character matches only that
character.  However, there are several exceptions: characters that,
when preceded by `\', are special constructs.  Such characters are
always ordinary when encountered on their own.  Here is a table of `\'
constructs:

`\|'
     specifies an alternative.  Two regular expressions A and B with
     `\|' in between form an expression that matches anything that
     either A or B matches.

     Thus, `foo\|bar' matches either `foo' or `bar' but no other string.

     `\|' applies to the largest possible surrounding expressions.
     Only a surrounding `\( ... \)' grouping can limit the grouping
     power of `\|'.

     Full backtracking capability exists to handle multiple uses of
     `\|'.

`\( ... \)'
     is a grouping construct that serves three purposes:

       1. To enclose a set of `\|' alternatives for other operations.
          Thus, `\(foo\|bar\)x' matches either `foox' or `barx'.

       2. To enclose an expression for a suffix operator such as `*' to
          act on.  Thus, `ba\(na\)*' matches `bananana', etc., with any
          (zero or more) number of `na' strings.

       3. To record a matched substring for future reference.

     This last application is not a consequence of the idea of a
     parenthetical grouping; it is a separate feature that happens to be
     assigned as a second meaning to the same `\( ... \)' construct
     because there is no conflict in practice between the two meanings.
     Here is an explanation of this feature:

`\DIGIT'
     matches the same text that matched the DIGITth occurrence of a `\(
     ... \)' construct.

     In other words, after the end of a `\( ... \)' construct.  the
     matcher remembers the beginning and end of the text matched by that
     construct.  Then, later on in the regular expression, you can use
     `\' followed by DIGIT to match that same text, whatever it may
     have been.

     The strings matching the first nine `\( ... \)' constructs
     appearing in a regular expression are assigned numbers 1 through 9
     in the order that the open parentheses appear in the regular
     expression.  So you can use `\1' through `\9' to refer to the text
     matched by the corresponding `\( ... \)' constructs.

     For example, `\(.*\)\1' matches any newline-free string that is
     composed of two identical halves.  The `\(.*\)' matches the first
     half, which may be anything, but the `\1' that follows must match
     the same exact text.

`\(?: ... \)'
     is called a "shy" grouping operator, and it is used just like `\(
     ... \)', except that it does not cause the matched substring to be
     recorded for future reference.

     This is useful when you need a lot of grouping `\( ... \)'
     constructs, but only want to remember one or two - or if you have
     more than nine groupings and need to use backreferences to refer to
     the groupings at the end.

     Using `\(?: ... \)' rather than `\( ... \)' when you don't need
     the captured substrings ought to speed up your programs some,
     since it shortens the code path followed by the regular expression
     engine, as well as the amount of memory allocation and string
     copying it must do.  The actual performance gain to be observed
     has not been measured or quantified as of this writing.

     The shy grouping operator has been borrowed from Perl, and has not
     been available prior to XEmacs 20.3, nor is it available in FSF
     Emacs.

`\w'
     matches any word-constituent character.  The editor syntax table
     determines which characters these are.  *Note Syntax Tables::.

`\W'
     matches any character that is not a word constituent.

`\sCODE'
     matches any character whose syntax is CODE.  Here CODE is a
     character that represents a syntax code: thus, `w' for word
     constituent, `-' for whitespace, `(' for open parenthesis, etc.
     *Note Syntax Tables::, for a list of syntax codes and the
     characters that stand for them.

`\SCODE'
     matches any character whose syntax is not CODE.

   The following regular expression constructs match the empty
string--that is, they don't use up any characters--but whether they
match depends on the context.

`\`'
     matches the empty string, but only at the beginning of the buffer
     or string being matched against.

`\''
     matches the empty string, but only at the end of the buffer or
     string being matched against.

`\='
     matches the empty string, but only at point.  (This construct is
     not defined when matching against a string.)

`\b'
     matches the empty string, but only at the beginning or end of a
     word.  Thus, `\bfoo\b' matches any occurrence of `foo' as a
     separate word.  `\bballs?\b' matches `ball' or `balls' as a
     separate word.

`\B'
     matches the empty string, but _not_ at the beginning or end of a
     word.

`\<'
     matches the empty string, but only at the beginning of a word.

`\>'
     matches the empty string, but only at the end of a word.

   Not every string is a valid regular expression.  For example, a
string with unbalanced square brackets is invalid (with a few
exceptions, such as `[]]'), and so is a string that ends with a single
`\'.  If an invalid regular expression is passed to any of the search
functions, an `invalid-regexp' error is signaled.

 - Function: regexp-quote string
     This function returns a regular expression string that matches
     exactly STRING and nothing else.  This allows you to request an
     exact string match when calling a function that wants a regular
     expression.

          (regexp-quote "^The cat$")
               => "\\^The cat\\$"

     One use of `regexp-quote' is to combine an exact string match with
     context described as a regular expression.  For example, this
     searches for the string that is the value of `string', surrounded
     by whitespace:

          (re-search-forward
           (concat "\\s-" (regexp-quote string) "\\s-"))

\1f
File: lispref.info,  Node: Regexp Example,  Prev: Syntax of Regexps,  Up: Regular Expressions

Complex Regexp Example
----------------------

   Here is a complicated regexp, used by XEmacs to recognize the end of
a sentence together with any whitespace that follows.  It is the value
of the variable `sentence-end'.

   First, we show the regexp as a string in Lisp syntax to distinguish
spaces from tab characters.  The string constant begins and ends with a
double-quote.  `\"' stands for a double-quote as part of the string,
`\\' for a backslash as part of the string, `\t' for a tab and `\n' for
a newline.

     "[.?!][]\"')}]*\\($\\| $\\|\t\\|  \\)[ \t\n]*"

   In contrast, if you evaluate the variable `sentence-end', you will
see the following:

     sentence-end
     =>
     "[.?!][]\"')}]*\\($\\| $\\|  \\|  \\)[
     ]*"

In this output, tab and newline appear as themselves.

   This regular expression contains four parts in succession and can be
deciphered as follows:

`[.?!]'
     The first part of the pattern is a character set that matches any
     one of three characters: period, question mark, and exclamation
     mark.  The match must begin with one of these three characters.

`[]\"')}]*'
     The second part of the pattern matches any closing braces and
     quotation marks, zero or more of them, that may follow the period,
     question mark or exclamation mark.  The `\"' is Lisp syntax for a
     double-quote in a string.  The `*' at the end indicates that the
     immediately preceding regular expression (a character set, in this
     case) may be repeated zero or more times.

`\\($\\| $\\|\t\\|  \\)'
     The third part of the pattern matches the whitespace that follows
     the end of a sentence: the end of a line, or a tab, or two spaces.
     The double backslashes mark the parentheses and vertical bars as
     regular expression syntax; the parentheses delimit a group and the
     vertical bars separate alternatives.  The dollar sign is used to
     match the end of a line.

`[ \t\n]*'
     Finally, the last part of the pattern matches any additional
     whitespace beyond the minimum needed to end a sentence.

\1f
File: lispref.info,  Node: Regexp Search,  Next: POSIX Regexps,  Prev: Regular Expressions,  Up: Searching and Matching

Regular Expression Searching
============================

   In XEmacs, you can search for the next match for a regexp either
incrementally or not.  Incremental search commands are described in the
`The XEmacs Lisp Reference Manual'.  *Note Regular Expression Search:
(xemacs)Regexp Search.  Here we describe only the search functions
useful in programs.  The principal one is `re-search-forward'.

 - Command: re-search-forward regexp &optional limit noerror count
          buffer
     This function searches forward in the current buffer for a string
     of text that is matched by the regular expression REGEXP.  The
     function skips over any amount of text that is not matched by
     REGEXP, and leaves point at the end of the first match found.  It
     returns the new value of point.

     If LIMIT is non-`nil' (it must be a position in the current
     buffer), then it is the upper bound to the search.  No match
     extending after that position is accepted.

     What happens when the search fails depends on the value of
     NOERROR.  If NOERROR is `nil', a `search-failed' error is
     signaled.  If NOERROR is `t', `re-search-forward' does nothing and
     returns `nil'.  If NOERROR is neither `nil' nor `t', then
     `re-search-forward' moves point to LIMIT (or the end of the
     buffer) and returns `nil'.

     If COUNT is supplied (it must be a positive number), then the
     search is repeated that many times (each time starting at the end
     of the previous time's match).  If these successive searches
     succeed, the function succeeds, moving point and returning its new
     value.  Otherwise the search fails.

     In the following example, point is initially before the `T'.
     Evaluating the search call moves point to the end of that line
     (between the `t' of `hat' and the newline).

          ---------- Buffer: foo ----------
          I read "-!-The cat in the hat
          comes back" twice.
          ---------- Buffer: foo ----------
          
          (re-search-forward "[a-z]+" nil t 5)
               => 27
          
          ---------- Buffer: foo ----------
          I read "The cat in the hat-!-
          comes back" twice.
          ---------- Buffer: foo ----------

 - Command: re-search-backward regexp &optional limit noerror count
          buffer
     This function searches backward in the current buffer for a string
     of text that is matched by the regular expression REGEXP, leaving
     point at the beginning of the first text found.

     This function is analogous to `re-search-forward', but they are not
     simple mirror images.  `re-search-forward' finds the match whose
     beginning is as close as possible to the starting point.  If
     `re-search-backward' were a perfect mirror image, it would find the
     match whose end is as close as possible.  However, in fact it
     finds the match whose beginning is as close as possible.  The
     reason is that matching a regular expression at a given spot
     always works from beginning to end, and starts at a specified
     beginning position.

     A true mirror-image of `re-search-forward' would require a special
     feature for matching regexps from end to beginning.  It's not
     worth the trouble of implementing that.

 - Function: string-match regexp string &optional start buffer
     This function returns the index of the start of the first match for
     the regular expression REGEXP in STRING, or `nil' if there is no
     match.  If START is non-`nil', the search starts at that index in
     STRING.

     Optional arg BUFFER controls how case folding is done (according
     to the value of `case-fold-search' in BUFFER and BUFFER's case
     tables) and defaults to the current buffer.

     For example,

          (string-match
           "quick" "The quick brown fox jumped quickly.")
               => 4
          (string-match
           "quick" "The quick brown fox jumped quickly." 8)
               => 27

     The index of the first character of the string is 0, the index of
     the second character is 1, and so on.

     After this function returns, the index of the first character
     beyond the match is available as `(match-end 0)'.  *Note Match
     Data::.

          (string-match
           "quick" "The quick brown fox jumped quickly." 8)
               => 27
          
          (match-end 0)
               => 32

 - Function: split-string string &optional pattern
     This function splits STRING to substrings delimited by PATTERN,
     and returns a list of substrings.  If PATTERN is omitted, it
     defaults to `[ \f\t\n\r\v]+', which means that it splits STRING by
     white-space.

          (split-string "foo bar")
               => ("foo" "bar")
          
          (split-string "something")
               => ("something")
          
          (split-string "a:b:c" ":")
               => ("a" "b" "c")
          
          (split-string ":a::b:c" ":")
               => ("" "a" "" "b" "c")

 - Function: split-path path
     This function splits a search path into a list of strings.  The
     path components are separated with the characters specified with
     `path-separator'.  Under Unix, `path-separator' will normally be
     `:', while under Windows, it will be `;'.

 - Function: looking-at regexp &optional buffer
     This function determines whether the text in the current buffer
     directly following point matches the regular expression REGEXP.
     "Directly following" means precisely that: the search is
     "anchored" and it can succeed only starting with the first
     character following point.  The result is `t' if so, `nil'
     otherwise.

     This function does not move point, but it updates the match data,
     which you can access using `match-beginning' and `match-end'.
     *Note Match Data::.

     In this example, point is located directly before the `T'.  If it
     were anywhere else, the result would be `nil'.

          ---------- Buffer: foo ----------
          I read "-!-The cat in the hat
          comes back" twice.
          ---------- Buffer: foo ----------
          
          (looking-at "The cat in the hat$")
               => t

\1f
File: lispref.info,  Node: POSIX Regexps,  Next: Search and Replace,  Prev: Regexp Search,  Up: Searching and Matching

POSIX Regular Expression Searching
==================================

   The usual regular expression functions do backtracking when necessary
to handle the `\|' and repetition constructs, but they continue this
only until they find _some_ match.  Then they succeed and report the
first match found.

   This section describes alternative search functions which perform the
full backtracking specified by the POSIX standard for regular expression
matching.  They continue backtracking until they have tried all
possibilities and found all matches, so they can report the longest
match, as required by POSIX.  This is much slower, so use these
functions only when you really need the longest match.

   In Emacs versions prior to 19.29, these functions did not exist, and
the functions described above implemented full POSIX backtracking.

 - Command: posix-search-forward regexp &optional limit noerror count
          buffer
     This is like `re-search-forward' except that it performs the full
     backtracking specified by the POSIX standard for regular expression
     matching.

 - Command: posix-search-backward regexp &optional limit noerror count
          buffer
     This is like `re-search-backward' except that it performs the full
     backtracking specified by the POSIX standard for regular expression
     matching.

 - Function: posix-looking-at regexp &optional buffer
     This is like `looking-at' except that it performs the full
     backtracking specified by the POSIX standard for regular expression
     matching.

 - Function: posix-string-match regexp string &optional start buffer
     This is like `string-match' except that it performs the full
     backtracking specified by the POSIX standard for regular expression
     matching.

     Optional arg BUFFER controls how case folding is done (according
     to the value of `case-fold-search' in BUFFER and BUFFER's case
     tables) and defaults to the current buffer.

\1f
File: lispref.info,  Node: Search and Replace,  Next: Match Data,  Prev: POSIX Regexps,  Up: Searching and Matching

Search and Replace
==================

 - Function: perform-replace from-string replacements query-flag
          regexp-flag delimited-flag &optional repeat-count map
     This function is the guts of `query-replace' and related commands.
     It searches for occurrences of FROM-STRING and replaces some or
     all of them.  If QUERY-FLAG is `nil', it replaces all occurrences;
     otherwise, it asks the user what to do about each one.

     If REGEXP-FLAG is non-`nil', then FROM-STRING is considered a
     regular expression; otherwise, it must match literally.  If
     DELIMITED-FLAG is non-`nil', then only replacements surrounded by
     word boundaries are considered.

     The argument REPLACEMENTS specifies what to replace occurrences
     with.  If it is a string, that string is used.  It can also be a
     list of strings, to be used in cyclic order.

     If REPEAT-COUNT is non-`nil', it should be an integer.  Then it
     specifies how many times to use each of the strings in the
     REPLACEMENTS list before advancing cyclicly to the next one.

     Normally, the keymap `query-replace-map' defines the possible user
     responses for queries.  The argument MAP, if non-`nil', is a
     keymap to use instead of `query-replace-map'.

 - Variable: query-replace-map
     This variable holds a special keymap that defines the valid user
     responses for `query-replace' and related functions, as well as
     `y-or-n-p' and `map-y-or-n-p'.  It is unusual in two ways:

        * The "key bindings" are not commands, just symbols that are
          meaningful to the functions that use this map.

        * Prefix keys are not supported; each key binding must be for a
          single event key sequence.  This is because the functions
          don't use read key sequence to get the input; instead, they
          read a single event and look it up "by hand."

   Here are the meaningful "bindings" for `query-replace-map'.  Several
of them are meaningful only for `query-replace' and friends.

`act'
     Do take the action being considered--in other words, "yes."

`skip'
     Do not take action for this question--in other words, "no."

`exit'
     Answer this question "no," and give up on the entire series of
     questions, assuming that the answers will be "no."

`act-and-exit'
     Answer this question "yes," and give up on the entire series of
     questions, assuming that subsequent answers will be "no."

`act-and-show'
     Answer this question "yes," but show the results--don't advance yet
     to the next question.

`automatic'
     Answer this question and all subsequent questions in the series
     with "yes," without further user interaction.

`backup'
     Move back to the previous place that a question was asked about.

`edit'
     Enter a recursive edit to deal with this question--instead of any
     other action that would normally be taken.

`delete-and-edit'
     Delete the text being considered, then enter a recursive edit to
     replace it.

`recenter'
     Redisplay and center the window, then ask the same question again.

`quit'
     Perform a quit right away.  Only `y-or-n-p' and related functions
     use this answer.

`help'
     Display some help, then ask again.

\1f
File: lispref.info,  Node: Match Data,  Next: Searching and Case,  Prev: Search and Replace,  Up: Searching and Matching

The Match Data
==============

   XEmacs keeps track of the positions of the start and end of segments
of text found during a regular expression search.  This means, for
example, that you can search for a complex pattern, such as a date in
an Rmail message, and then extract parts of the match under control of
the pattern.

   Because the match data normally describe the most recent search only,
you must be careful not to do another search inadvertently between the
search you wish to refer back to and the use of the match data.  If you
can't avoid another intervening search, you must save and restore the
match data around it, to prevent it from being overwritten.

* Menu:

* Simple Match Data::     Accessing single items of match data,
                            such as where a particular subexpression started.
* Replacing Match::       Replacing a substring that was matched.
* Entire Match Data::     Accessing the entire match data at once, as a list.
* Saving Match Data::     Saving and restoring the match data.

\1f
File: lispref.info,  Node: Simple Match Data,  Next: Replacing Match,  Up: Match Data

Simple Match Data Access
------------------------

   This section explains how to use the match data to find out what was
matched by the last search or match operation.

   You can ask about the entire matching text, or about a particular
parenthetical subexpression of a regular expression.  The COUNT
argument in the functions below specifies which.  If COUNT is zero, you
are asking about the entire match.  If COUNT is positive, it specifies
which subexpression you want.

   Recall that the subexpressions of a regular expression are those
expressions grouped with escaped parentheses, `\(...\)'.  The COUNTth
subexpression is found by counting occurrences of `\(' from the
beginning of the whole regular expression.  The first subexpression is
numbered 1, the second 2, and so on.  Only regular expressions can have
subexpressions--after a simple string search, the only information
available is about the entire match.

 - Function: match-string count &optional in-string
     This function returns, as a string, the text matched in the last
     search or match operation.  It returns the entire text if COUNT is
     zero, or just the portion corresponding to the COUNTth
     parenthetical subexpression, if COUNT is positive.  If COUNT is
     out of range, or if that subexpression didn't match anything, the
     value is `nil'.

     If the last such operation was done against a string with
     `string-match', then you should pass the same string as the
     argument IN-STRING.  Otherwise, after a buffer search or match,
     you should omit IN-STRING or pass `nil' for it; but you should
     make sure that the current buffer when you call `match-string' is
     the one in which you did the searching or matching.

 - Function: match-beginning count
     This function returns the position of the start of text matched by
     the last regular expression searched for, or a subexpression of it.

     If COUNT is zero, then the value is the position of the start of
     the entire match.  Otherwise, COUNT specifies a subexpression in
     the regular expression, and the value of the function is the
     starting position of the match for that subexpression.

     The value is `nil' for a subexpression inside a `\|' alternative
     that wasn't used in the match.

 - Function: match-end count
     This function is like `match-beginning' except that it returns the
     position of the end of the match, rather than the position of the
     beginning.

   Here is an example of using the match data, with a comment showing
the positions within the text:

     (string-match "\\(qu\\)\\(ick\\)"
                   "The quick fox jumped quickly.")
                   ;0123456789
          => 4
     
     (match-string 0 "The quick fox jumped quickly.")
          => "quick"
     (match-string 1 "The quick fox jumped quickly.")
          => "qu"
     (match-string 2 "The quick fox jumped quickly.")
          => "ick"
     
     (match-beginning 1)       ; The beginning of the match
          => 4                 ;   with `qu' is at index 4.
     
     (match-beginning 2)       ; The beginning of the match
          => 6                 ;   with `ick' is at index 6.
     
     (match-end 1)             ; The end of the match
          => 6                 ;   with `qu' is at index 6.
     
     (match-end 2)             ; The end of the match
          => 9                 ;   with `ick' is at index 9.

   Here is another example.  Point is initially located at the beginning
of the line.  Searching moves point to between the space and the word
`in'.  The beginning of the entire match is at the 9th character of the
buffer (`T'), and the beginning of the match for the first
subexpression is at the 13th character (`c').

     (list
       (re-search-forward "The \\(cat \\)")
       (match-beginning 0)
       (match-beginning 1))
         => (9 9 13)
     
     ---------- Buffer: foo ----------
     I read "The cat -!-in the hat comes back" twice.
             ^   ^
             9  13
     ---------- Buffer: foo ----------

(In this case, the index returned is a buffer position; the first
character of the buffer counts as 1.)