[chise/xemacs-chise.git.1] / man / lispref / searching.texi

@c -*-texinfo-*-
@c This is part of the XEmacs Lisp Reference Manual.
@c Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc.
@c See the file lispref.texi for copying conditions.
@setfilename ../../info/searching.info
@node Searching and Matching, Syntax Tables, Text, Top
@chapter Searching and Matching
@cindex searching

  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 @dfn{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 @code{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,...
@end menu

  The @samp{skip-chars@dots{}} functions also perform a kind of searching.
@xref{Skipping Characters}.

@node String Search
@section Searching for Strings
@cindex string search

  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;
@var{limit} and @var{noerror} are set to @code{nil}, and @var{count}
is set to 1.

@deffn Command search-forward string &optional limit noerror count buffer
  This function searches forward from point for an exact match for
@var{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 @var{noerror} (see below).

  In the following example, point is initially at the beginning of the
line.  Then @code{(search-forward "fox")} moves point after the last
letter of @samp{fox}:

@example
@group
---------- Buffer: foo ----------
@point{}The quick brown fox jumped over the lazy dog.
---------- Buffer: foo ----------
@end group

@group
(search-forward "fox")
     @result{} 20

---------- Buffer: foo ----------
The quick brown fox@point{} jumped over the lazy dog.
---------- Buffer: foo ----------
@end group
@end example

  The argument @var{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 @var{limit} is omitted or @code{nil}, it
defaults to the end of the accessible portion of the buffer.

@kindex search-failed
  What happens when the search fails depends on the value of
@var{noerror}.  If @var{noerror} is @code{nil}, a @code{search-failed}
error is signaled.  If @var{noerror} is @code{t}, @code{search-forward}
returns @code{nil} and does nothing.  If @var{noerror} is neither
@code{nil} nor @code{t}, then @code{search-forward} moves point to the
upper bound and returns @code{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 @code{nil}.)

If @var{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 @var{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.

@var{buffer} is the buffer to search in, and defaults to the current buffer.
@end deffn

@deffn Command search-backward string &optional limit noerror count buffer
This function searches backward from point for @var{string}.  It is
just like @code{search-forward} except that it searches backwards and
leaves point at the beginning of the match.
@end deffn

@deffn Command word-search-forward string &optional limit noerror count buffer
@cindex word search
This function searches forward from point for a ``word'' match for
@var{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 @var{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 @samp{ball} does not match the word @samp{balls}), but the
details of punctuation and spacing are ignored (searching for @samp{ball
boy} does match @samp{ball.  Boy!}).

In this example, point is initially at the beginning of the buffer; the
search leaves it between the @samp{y} and the @samp{!}.

@example
@group
---------- Buffer: foo ----------
@point{}He said "Please!  Find
the ball boy!"
---------- Buffer: foo ----------
@end group

@group
(word-search-forward "Please find the ball, boy.")
     @result{} 35

---------- Buffer: foo ----------
He said "Please!  Find
the ball boy@point{}!"
---------- Buffer: foo ----------
@end group
@end example

If @var{limit} is non-@code{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 @var{noerror} is @code{nil}, then @code{word-search-forward} signals
an error if the search fails.  If @var{noerror} is @code{t}, then it
returns @code{nil} instead of signaling an error.  If @var{noerror} is
neither @code{nil} nor @code{t}, it moves point to @var{limit} (or the
end of the buffer) and returns @code{nil}.

If @var{count} is non-@code{nil}, then the search is repeated that many
times.  Point is positioned at the end of the last match.

@var{buffer} is the buffer to search in, and defaults to the current buffer.
@end deffn

@deffn Command word-search-backward string &optional limit noerror count buffer
This function searches backward from point for a word match to
@var{string}.  This function is just like @code{word-search-forward}
except that it searches backward and normally leaves point at the
beginning of the match.
@end deffn

@node Regular Expressions
@section Regular Expressions
@cindex regular expression
@cindex regexp

  A @dfn{regular expression} (@dfn{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 @cite{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 @ref{(gawk)Top},
@ref{(ed)Top}, @cite{sed}, @cite{grep}, @ref{(perl)Top},
@ref{(regex)Top}, @ref{(rx)Top}, @cite{pcre}, and @ref{(flex)Top}, which
also make good use of regular expressions.

 The XEmacs regular expression syntax most closely resembles that of
@cite{ed}, or @cite{grep}, the GNU versions of which all utilize the GNU
@cite{regex} library.  XEmacs' version of @cite{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.
@end menu

@node Syntax of Regexps
@subsection Syntax of Regular Expressions

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

For example, @samp{f} is not a special character, so it is ordinary, and
therefore @samp{f} is a regular expression that matches the string
@samp{f} and no other string.  (It does @emph{not} match the string
@samp{ff}.)  Likewise, @samp{o} is a regular expression that matches
only @samp{o}.@refill

Any two regular expressions @var{a} and @var{b} can be concatenated.  The
result is a regular expression that matches a string if @var{a} matches
some amount of the beginning of that string and @var{b} matches the rest of
the string.@refill

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

@need 1200
@table @kbd
@item .@: @r{(Period)}
@cindex @samp{.} in regexp
is a special character that matches any single character except a newline.
Using concatenation, we can make regular expressions like @samp{a.b}, which
matches any three-character string that begins with @samp{a} and ends with
@samp{b}.@refill

@item *
@cindex @samp{*} in regexp
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 @samp{fo*}, the @samp{*} applies to the @samp{o}, so
@samp{fo*} matches one @samp{f} followed by any number of @samp{o}s.
The case of zero @samp{o}s is allowed: @samp{fo*} does match
@samp{f}.@refill

@samp{*} always applies to the @emph{smallest} possible preceding
expression.  Thus, @samp{fo*} has a repeating @samp{o}, not a
repeating @samp{fo}.@refill

The matcher processes a @samp{*} 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 @samp{*}-modified construct in
case that makes it possible to match the rest of the pattern.  For
example, in matching @samp{ca*ar} against the string @samp{caaar}, the
@samp{a*} first tries to match all three @samp{a}s; but the rest of the
pattern is @samp{ar} and there is only @samp{r} left to match, so this
try fails.  The next alternative is for @samp{a*} to match only two
@samp{a}s.  With this choice, the rest of the regexp matches
successfully.@refill

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

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

@item ?
@cindex @samp{?} in regexp
is a quantifying suffix operator similar to @samp{*}, except that the
preceding expression can match either once or not at all.  For example,
@samp{ca?r} matches @samp{car} or @samp{cr}, but does not match anything
else.

@item *?
@cindex @samp{*?} in regexp
works just like @samp{*}, except that rather than matching the longest
match, it matches the shortest match.  @samp{*?} is known as a
@dfn{non-greedy} quantifier, a regexp construct borrowed from Perl.
@c Did perl get this from somewhere?  What's the real history of *? ?

This construct is very useful for when you want to match the text inside
a pair of delimiters.  For instance, @samp{/\*.*?\*/} 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.

@item +?
@cindex @samp{+?} in regexp
is the non-greedy version of @samp{+}.

@item ??
@cindex @samp{??} in regexp
is the non-greedy version of @samp{?}.

@item \@{n,m\@}
@c Note the spacing after the close brace is deliberate.
@cindex @samp{\@{n,m\@} }in regexp
serves as an interval quantifier, analogous to @samp{*} or @samp{+}, but
specifies that the expression must match at least @var{n} times, but no
more than @var{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.

@item [ @dots{} ]
@cindex character set (in regexp)
@cindex @samp{[} in regexp
@cindex @samp{]} in regexp
@samp{[} begins a @dfn{character set}, which is terminated by a
@samp{]}.  In the simplest case, the characters between the two brackets
form the set.  Thus, @samp{[ad]} matches either one @samp{a} or one
@samp{d}, and @samp{[ad]*} matches any string composed of just @samp{a}s
and @samp{d}s (including the empty string), from which it follows that
@samp{c[ad]*r} matches @samp{cr}, @samp{car}, @samp{cdr},
@samp{caddaar}, etc.@refill

The usual regular expression special characters are not special inside a
character set.  A completely different set of special characters exists
inside character sets: @samp{]}, @samp{-} and @samp{^}.@refill

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

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

To include @samp{^} in a set, put it anywhere but at the beginning of
the set.

@item [^ @dots{} ]
@cindex @samp{^} in regexp
@samp{[^} begins a @dfn{complement character set}, which matches any
character except the ones specified.  Thus, @samp{[^a-z0-9A-Z]}
matches all characters @emph{except} letters and digits.@refill

@samp{^} is not special in a character set unless it is the first
character.  The character following the @samp{^} is treated as if it
were first (thus, @samp{-} and @samp{]} 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.

@item ^
@cindex @samp{^} in regexp
@cindex beginning of line in regexp
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, @samp{^foo} matches a @samp{foo} that occurs at
the beginning of a line.

When matching a string instead of a buffer, @samp{^} matches at the
beginning of the string or after a newline character @samp{\n}.

@item $
@cindex @samp{$} in regexp
is similar to @samp{^} but matches only at the end of a line.  Thus,
@samp{x+$} matches a string of one @samp{x} or more at the end of a line.

When matching a string instead of a buffer, @samp{$} matches at the end
of the string or before a newline character @samp{\n}.

@item \
@cindex @samp{\} in regexp
has two functions: it quotes the special characters (including
@samp{\}), and it introduces additional special constructs.

Because @samp{\} quotes special characters, @samp{\$} is a regular
expression that matches only @samp{$}, and @samp{\[} is a regular
expression that matches only @samp{[}, and so on.

Note that @samp{\} also has special meaning in the read syntax of Lisp
strings (@pxref{String Type}), and must be quoted with @samp{\}.  For
example, the regular expression that matches the @samp{\} character is
@samp{\\}.  To write a Lisp string that contains the characters
@samp{\\}, Lisp syntax requires you to quote each @samp{\} with another
@samp{\}.  Therefore, the read syntax for a regular expression matching
@samp{\} is @code{"\\\\"}.@refill
@end table

@strong{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, @samp{*foo} treats @samp{*} as
ordinary since there is no preceding expression on which the @samp{*}
can act.  It is poor practice to depend on this behavior; quote the
special character anyway, regardless of where it appears.@refill

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

@table @kbd
@item \|
@cindex @samp{|} in regexp
@cindex regexp alternative
specifies an alternative.
Two regular expressions @var{a} and @var{b} with @samp{\|} in
between form an expression that matches anything that either @var{a} or
@var{b} matches.@refill

Thus, @samp{foo\|bar} matches either @samp{foo} or @samp{bar}
but no other string.@refill

@samp{\|} applies to the largest possible surrounding expressions.  Only a
surrounding @samp{\( @dots{} \)} grouping can limit the grouping power of
@samp{\|}.@refill

Full backtracking capability exists to handle multiple uses of @samp{\|}.

@item \( @dots{} \)
@cindex @samp{(} in regexp
@cindex @samp{)} in regexp
@cindex regexp grouping
is a grouping construct that serves three purposes:

@enumerate
@item
To enclose a set of @samp{\|} alternatives for other operations.
Thus, @samp{\(foo\|bar\)x} matches either @samp{foox} or @samp{barx}.

@item
To enclose an expression for a suffix operator such as @samp{*} to act
on.  Thus, @samp{ba\(na\)*} matches @samp{bananana}, etc., with any
(zero or more) number of @samp{na} strings.@refill

@item
To record a matched substring for future reference.
@end enumerate

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 @samp{\( @dots{} \)} construct
because there is no conflict in practice between the two meanings.
Here is an explanation of this feature:

@item \@var{digit}
matches the same text that matched the @var{digit}th occurrence of a
@samp{\( @dots{} \)} construct.

In other words, after the end of a @samp{\( @dots{} \)} 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
@samp{\} followed by @var{digit} to match that same text, whatever it
may have been.

The strings matching the first nine @samp{\( @dots{} \)} 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 @samp{\1} through @samp{\9} to refer to the text matched
by the corresponding @samp{\( @dots{} \)} constructs.

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

@item \(?: @dots{} \)
@cindex @samp{\(?:} in regexp
@cindex regexp grouping
is called a @dfn{shy} grouping operator, and it is used just like
@samp{\( @dots{} \)}, 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 @samp{\( @dots{} \)}
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.  It also allows construction of regular
expressions from variable subexpressions that contain varying numbers of
non-capturing subexpressions, without disturbing the group counts for
the main expression.  For example

@example
(let ((sre (if foo "\\(?:bar\\|baz\\)" "quux")))
  (re-search-forward (format "a\\(b+ %s c+\\) d" sre) nil t)
  (match-string 1))
@end example

It is very tedious to write this kind of code without shy groups, even
if you know what all the alternative subexpressions will look like.

Using @samp{\(?: @dots{} \)} rather than @samp{\( @dots{} \)} should
give little performance gain, as the start of each group must be
recorded for the purpose of back-tracking in any case, and no string
copying is done until @code{match-string} is called.

The shy grouping operator has been borrowed from Perl, and was not
available prior to XEmacs 20.3, and has only been available in GNU Emacs
since version 21.

@item \w
@cindex @samp{\w} in regexp
matches any word-constituent character.  The editor syntax table
determines which characters these are.  @xref{Syntax Tables}.

@item \W
@cindex @samp{\W} in regexp
matches any character that is not a word constituent.

@item \s@var{code}
@cindex @samp{\s} in regexp
matches any character whose syntax is @var{code}.  Here @var{code} is a
character that represents a syntax code: thus, @samp{w} for word
constituent, @samp{-} for whitespace, @samp{(} for open parenthesis,
etc.  @xref{Syntax Tables}, for a list of syntax codes and the
characters that stand for them.

@item \S@var{code}
@cindex @samp{\S} in regexp
matches any character whose syntax is not @var{code}.
@end table

  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.

@table @kbd
@item \`
@cindex @samp{\`} in regexp
matches the empty string, but only at the beginning
of the buffer or string being matched against.

@item \'
@cindex @samp{\'} in regexp
matches the empty string, but only at the end of
the buffer or string being matched against.

@item \=
@cindex @samp{\=} in regexp
matches the empty string, but only at point.
(This construct is not defined when matching against a string.)

@item \b
@cindex @samp{\b} in regexp
matches the empty string, but only at the beginning or
end of a word.  Thus, @samp{\bfoo\b} matches any occurrence of
@samp{foo} as a separate word.  @samp{\bballs?\b} matches
@samp{ball} or @samp{balls} as a separate word.@refill

@item \B
@cindex @samp{\B} in regexp
matches the empty string, but @emph{not} at the beginning or
end of a word.

@item \<
@cindex @samp{\<} in regexp
matches the empty string, but only at the beginning of a word.

@item \>
@cindex @samp{\>} in regexp
matches the empty string, but only at the end of a word.
@end table

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

@defun regexp-quote string
This function returns a regular expression string that matches exactly
@var{string} and nothing else.  This allows you to request an exact
string match when calling a function that wants a regular expression.

@example
@group
(regexp-quote "^The cat$")
     @result{} "\\^The cat\\$"
@end group
@end example

One use of @code{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 @code{string}, surrounded by
whitespace:

@example
@group
(re-search-forward
 (concat "\\s-" (regexp-quote string) "\\s-"))
@end group
@end example
@end defun

@node Regexp Example
@subsection 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 @code{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.  @samp{\"} stands for a double-quote as part of the
string, @samp{\\} for a backslash as part of the string, @samp{\t} for a
tab and @samp{\n} for a newline.

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

  In contrast, if you evaluate the variable @code{sentence-end}, you
will see the following:

@example
@group
sentence-end
@result{}
"[.?!][]\"')@}]*\\($\\| $\\|  \\|  \\)[
]*"
@end group
@end example

@noindent
In this output, tab and newline appear as themselves.

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

@table @code
@item [.?!]
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.

@item []\"')@}]*
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 @code{\"} is Lisp syntax for a double-quote in
a string.  The @samp{*} at the end indicates that the immediately
preceding regular expression (a character set, in this case) may be
repeated zero or more times.

@item \\($\\|@ $\\|\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.

@item [ \t\n]*
Finally, the last part of the pattern matches any additional whitespace
beyond the minimum needed to end a sentence.
@end table

@node Regexp Search
@section Regular Expression Searching
@cindex regular expression searching
@cindex regexp searching
@cindex searching for regexp

  In XEmacs, you can search for the next match for a regexp either
incrementally or not.  Incremental search commands are described in the
@cite{The XEmacs Lisp Reference Manual}.  @xref{Regexp Search, , Regular Expression
Search, xemacs, The XEmacs Lisp Reference Manual}.  Here we describe only the search
functions useful in programs.  The principal one is
@code{re-search-forward}.

@deffn 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 @var{regexp}.  The
function skips over any amount of text that is not matched by
@var{regexp}, and leaves point at the end of the first match found.
It returns the new value of point.

If @var{limit} is non-@code{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
@var{noerror}.  If @var{noerror} is @code{nil}, a @code{search-failed}
error is signaled.  If @var{noerror} is @code{t},
@code{re-search-forward} does nothing and returns @code{nil}.  If
@var{noerror} is neither @code{nil} nor @code{t}, then
@code{re-search-forward} moves point to @var{limit} (or the end of the
buffer) and returns @code{nil}.

If @var{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 @samp{T}.
Evaluating the search call moves point to the end of that line (between
the @samp{t} of @samp{hat} and the newline).

@example
@group
---------- Buffer: foo ----------
I read "@point{}The cat in the hat
comes back" twice.
---------- Buffer: foo ----------
@end group

@group
(re-search-forward "[a-z]+" nil t 5)
     @result{} 27

---------- Buffer: foo ----------
I read "The cat in the hat@point{}
comes back" twice.
---------- Buffer: foo ----------
@end group
@end example
@end deffn

@deffn 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 @var{regexp}, leaving
point at the beginning of the first text found.

This function is analogous to @code{re-search-forward}, but they are not
simple mirror images.  @code{re-search-forward} finds the match whose
beginning is as close as possible to the starting point.  If
@code{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 @code{re-search-forward} would require a special
feature for matching regexps from end to beginning.  It's not worth the
trouble of implementing that.
@end deffn

@defun string-match regexp string &optional start buffer
This function returns the index of the start of the first match for
the regular expression @var{regexp} in @var{string}, or @code{nil} if
there is no match.  If @var{start} is non-@code{nil}, the search starts
at that index in @var{string}.


Optional arg @var{buffer} controls how case folding is done (according
to the value of @code{case-fold-search} in @var{buffer} and
@var{buffer}'s case tables) and defaults to the current buffer.

For example,

@example
@group
(string-match
 "quick" "The quick brown fox jumped quickly.")
     @result{} 4
@end group
@group
(string-match
 "quick" "The quick brown fox jumped quickly." 8)
     @result{} 27
@end group
@end example

@noindent
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 @code{(match-end 0)}.  @xref{Match Data}.

@example
@group
(string-match
 "quick" "The quick brown fox jumped quickly." 8)
     @result{} 27
@end group

@group
(match-end 0)
     @result{} 32
@end group
@end example
@end defun

@defun split-string string &optional pattern
This function splits @var{string} to substrings delimited by
@var{pattern}, and returns a list of substrings.  If @var{pattern} is
omitted, it defaults to @samp{[ \f\t\n\r\v]+}, which means that it
splits @var{string} by white--space.

@example
@group
(split-string "foo bar")
     @result{} ("foo" "bar")
@end group

@group
(split-string "something")
     @result{} ("something")
@end group

@group
(split-string "a:b:c" ":")
     @result{} ("a" "b" "c")
@end group

@group
(split-string ":a::b:c" ":")
     @result{} ("" "a" "" "b" "c")
@end group
@end example
@end defun

@defun split-path path
This function splits a search path into a list of strings.  The path
components are separated with the characters specified with
@code{path-separator}.  Under Unix, @code{path-separator} will normally
be @samp{:}, while under Windows, it will be @samp{;}.
@end defun

@defun looking-at regexp &optional buffer
This function determines whether the text in the current buffer directly
following point matches the regular expression @var{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 @code{t} if so, @code{nil} otherwise.

This function does not move point, but it updates the match data, which
you can access using @code{match-beginning} and @code{match-end}.
@xref{Match Data}.

In this example, point is located directly before the @samp{T}.  If it
were anywhere else, the result would be @code{nil}.

@example
@group
---------- Buffer: foo ----------
I read "@point{}The cat in the hat
comes back" twice.
---------- Buffer: foo ----------

(looking-at "The cat in the hat$")
     @result{} t
@end group
@end example
@end defun

@node POSIX Regexps
@section POSIX Regular Expression Searching

  The usual regular expression functions do backtracking when necessary
to handle the @samp{\|} and repetition constructs, but they continue
this only until they find @emph{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.

@deffn Command posix-search-forward regexp &optional limit noerror count buffer
This is like @code{re-search-forward} except that it performs the full
backtracking specified by the POSIX standard for regular expression
matching.
@end deffn

@deffn Command posix-search-backward regexp &optional limit noerror count buffer
This is like @code{re-search-backward} except that it performs the full
backtracking specified by the POSIX standard for regular expression
matching.
@end deffn

@defun posix-looking-at regexp &optional buffer
This is like @code{looking-at} except that it performs the full
backtracking specified by the POSIX standard for regular expression
matching.
@end defun

@defun posix-string-match regexp string &optional start buffer
This is like @code{string-match} except that it performs the full
backtracking specified by the POSIX standard for regular expression
matching.

Optional arg @var{buffer} controls how case folding is done (according
to the value of @code{case-fold-search} in @var{buffer} and
@var{buffer}'s case tables) and defaults to the current buffer.
@end defun

@ignore
@deffn Command delete-matching-lines regexp
This function is identical to @code{delete-non-matching-lines}, save
that it deletes what @code{delete-non-matching-lines} keeps.

In the example below, point is located on the first line of text.

@example
@group
---------- Buffer: foo ----------
We hold these truths
to be self-evident,
that all men are created
equal, and that they are
---------- Buffer: foo ----------
@end group

@group
(delete-matching-lines "the")
     @result{} nil

---------- Buffer: foo ----------
to be self-evident,
that all men are created
---------- Buffer: foo ----------
@end group
@end example
@end deffn

@deffn Command flush-lines regexp
This function is an alias of @code{delete-matching-lines}.
@end deffn

@deffn Command delete-non-matching-lines regexp
This function deletes all lines following point which don't
contain a match for the regular expression @var{regexp}.
@end deffn

@deffn Command keep-lines regexp
This function is the same as @code{delete-non-matching-lines}.
@end deffn

@deffn Command count-matches regexp
This function counts the number of matches for @var{regexp} there are in
the current buffer following point.  It prints this number in
the echo area, returning the string printed.
@end deffn

@deffn Command how-many regexp
This function is an alias of @code{count-matches}.
@end deffn

@deffn Command list-matching-lines regexp &optional nlines
This function is a synonym of @code{occur}.
Show all lines following point containing a match for @var{regexp}.
Display each line with @var{nlines} lines before and after,
or @code{-}@var{nlines} before if @var{nlines} is negative.
@var{nlines} defaults to @code{list-matching-lines-default-context-lines}.
Interactively it is the prefix arg.

The lines are shown in a buffer named @samp{*Occur*}.
It serves as a menu to find any of the occurrences in this buffer.
@kbd{C-h m} (@code{describe-mode} in that buffer gives help.
@end deffn

@defopt list-matching-lines-default-context-lines
Default value is 0.
Default number of context lines to include around a @code{list-matching-lines}
match.  A negative number means to include that many lines before the match.
A positive number means to include that many lines both before and after.
@end defopt
@end ignore

@node Search and Replace
@section Search and Replace
@cindex replacement

@defun perform-replace from-string replacements query-flag regexp-flag delimited-flag &optional repeat-count map
This function is the guts of @code{query-replace} and related commands.
It searches for occurrences of @var{from-string} and replaces some or
all of them.  If @var{query-flag} is @code{nil}, it replaces all
occurrences; otherwise, it asks the user what to do about each one.

If @var{regexp-flag} is non-@code{nil}, then @var{from-string} is
considered a regular expression; otherwise, it must match literally.  If
@var{delimited-flag} is non-@code{nil}, then only replacements
surrounded by word boundaries are considered.

The argument @var{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 @var{repeat-count} is non-@code{nil}, it should be an integer.  Then
it specifies how many times to use each of the strings in the
@var{replacements} list before advancing cyclicly to the next one.

Normally, the keymap @code{query-replace-map} defines the possible user
responses for queries.  The argument @var{map}, if non-@code{nil}, is a
keymap to use instead of @code{query-replace-map}.
@end defun

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

@itemize @bullet
@item
The ``key bindings'' are not commands, just symbols that are meaningful
to the functions that use this map.

@item
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.''
@end itemize
@end defvar

Here are the meaningful ``bindings'' for @code{query-replace-map}.
Several of them are meaningful only for @code{query-replace} and
friends.

@table @code
@item act
Do take the action being considered---in other words, ``yes.''

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

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

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

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

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

@item backup
Move back to the previous place that a question was asked about.

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

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

@item recenter
Redisplay and center the window, then ask the same question again.

@item quit
Perform a quit right away.  Only @code{y-or-n-p} and related functions
use this answer.

@item help
Display some help, then ask again.
@end table

@node Match Data
@section The Match Data
@cindex 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.
@end menu

@node Simple Match Data
@subsection 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 @var{count}
argument in the functions below specifies which.  If @var{count} is
zero, you are asking about the entire match.  If @var{count} is
positive, it specifies which subexpression you want.

  Recall that the subexpressions of a regular expression are those
expressions grouped with escaped parentheses, @samp{\(@dots{}\)}.  The
@var{count}th subexpression is found by counting occurrences of
@samp{\(} 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.

@defun 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 @var{count} is zero,
or just the portion corresponding to the @var{count}th parenthetical
subexpression, if @var{count} is positive.  If @var{count} is out of
range, or if that subexpression didn't match anything, the value is
@code{nil}.

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

@defun 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 @var{count} is zero, then the value is the position of the start of
the entire match.  Otherwise, @var{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 @code{nil} for a subexpression inside a @samp{\|}
alternative that wasn't used in the match.
@end defun

@defun match-end count
This function is like @code{match-beginning} except that it returns the
position of the end of the match, rather than the position of the
beginning.
@end defun

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

@example
@group
(string-match "\\(qu\\)\\(ick\\)"
              "The quick fox jumped quickly.")
              ;0123456789
     @result{} 4
@end group

@group
(match-string 0 "The quick fox jumped quickly.")
     @result{} "quick"
(match-string 1 "The quick fox jumped quickly.")
     @result{} "qu"
(match-string 2 "The quick fox jumped quickly.")
     @result{} "ick"
@end group

@group
(match-beginning 1)       ; @r{The beginning of the match}
     @result{} 4                 ;   @r{with @samp{qu} is at index 4.}
@end group

@group
(match-beginning 2)       ; @r{The beginning of the match}
     @result{} 6                 ;   @r{with @samp{ick} is at index 6.}
@end group

@group
(match-end 1)             ; @r{The end of the match}
     @result{} 6                 ;   @r{with @samp{qu} is at index 6.}

(match-end 2)             ; @r{The end of the match}
     @result{} 9                 ;   @r{with @samp{ick} is at index 9.}
@end group
@end example

  Here is another example.  Point is initially located at the beginning
of the line.  Searching moves point to between the space and the word
@samp{in}.  The beginning of the entire match is at the 9th character of
the buffer (@samp{T}), and the beginning of the match for the first
subexpression is at the 13th character (@samp{c}).

@example
@group
(list
  (re-search-forward "The \\(cat \\)")
  (match-beginning 0)
  (match-beginning 1))
    @result{} (9 9 13)
@end group

@group
---------- Buffer: foo ----------
I read "The cat @point{}in the hat comes back" twice.
        ^   ^
        9  13
---------- Buffer: foo ----------
@end group
@end example

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

@node Replacing Match
@subsection Replacing the Text That Matched

  This function replaces the text matched by the last search with
@var{replacement}.

@cindex case in replacements
@defun replace-match replacement &optional fixedcase literal string strbuffer
This function replaces the text in the buffer (or in @var{string}) that
was matched by the last search.  It replaces that text with
@var{replacement}.

If you did the last search in a buffer, you should specify @code{nil}
for @var{string}.  Then @code{replace-match} does the replacement by
editing the buffer; it leaves point at the end of the replacement text,
and returns @code{t}.

If you did the search in a string, pass the same string as @var{string}.
Then @code{replace-match} does the replacement by constructing and
returning a new string.

If the fourth argument @var{string} is a string, fifth argument
@var{strbuffer} specifies the buffer to be used for syntax-table and
case-table lookup and defaults to the current buffer.  When @var{string}
is not a string, the buffer that the match occurred in has automatically
been remembered and you do not need to specify it.

If @var{fixedcase} is non-@code{nil}, then the case of the replacement
text is not changed; otherwise, the replacement text is converted to a
different case depending upon the capitalization of the text to be
replaced.  If the original text is all upper case, the replacement text
is converted to upper case.  If the first word of the original text is
capitalized, then the first word of the replacement text is capitalized.
If the original text contains just one word, and that word is a capital
letter, @code{replace-match} considers this a capitalized first word
rather than all upper case.

If @code{case-replace} is @code{nil}, then case conversion is not done,
regardless of the value of @var{fixedcase}.  @xref{Searching and Case}.

If @var{literal} is non-@code{nil}, then @var{replacement} is inserted
exactly as it is, the only alterations being case changes as needed.
If it is @code{nil} (the default), then the character @samp{\} is treated
specially.  If a @samp{\} appears in @var{replacement}, then it must be
part of one of the following sequences:

@table @asis
@item @samp{\&}
@cindex @samp{&} in replacement
@samp{\&} stands for the entire text being replaced.

@item @samp{\@var{n}}
@cindex @samp{\@var{n}} in replacement
@samp{\@var{n}}, where @var{n} is a digit, stands for the text that
matched the @var{n}th subexpression in the original regexp.
Subexpressions are those expressions grouped inside @samp{\(@dots{}\)}.

@item @samp{\\}
@cindex @samp{\} in replacement
@samp{\\} stands for a single @samp{\} in the replacement text.
@end table
@end defun

@node Entire Match Data
@subsection Accessing the Entire Match Data

  The functions @code{match-data} and @code{set-match-data} read or
write the entire match data, all at once.

@defun match-data &optional integers reuse
This function returns a newly constructed list containing all the
information on what text the last search matched.  Element zero is the
position of the beginning of the match for the whole expression; element
one is the position of the end of the match for the expression.  The
next two elements are the positions of the beginning and end of the
match for the first subexpression, and so on.  In general, element
@ifinfo
number 2@var{n}
@end ifinfo
@tex
number {\mathsurround=0pt $2n$}
@end tex
corresponds to @code{(match-beginning @var{n})}; and
element
@ifinfo
number 2@var{n} + 1
@end ifinfo
@tex
number {\mathsurround=0pt $2n+1$}
@end tex
corresponds to @code{(match-end @var{n})}.

All the elements are markers or @code{nil} if matching was done on a
buffer, and all are integers or @code{nil} if matching was done on a
string with @code{string-match}.  However, if the optional first
argument @var{integers} is non-@code{nil}, always use integers (rather
than markers) to represent buffer positions.

If the optional second argument @var{reuse} is a list, reuse it as part
of the value.  If @var{reuse} is long enough to hold all the values, and if
@var{integers} is non-@code{nil}, no new lisp objects are created.

As always, there must be no possibility of intervening searches between
the call to a search function and the call to @code{match-data} that is
intended to access the match data for that search.

@example
@group
(match-data)
     @result{}  (#<marker at 9 in foo>
          #<marker at 17 in foo>
          #<marker at 13 in foo>
          #<marker at 17 in foo>)
@end group
@end example
@end defun

@defun set-match-data match-list
This function sets the match data from the elements of @var{match-list},
which should be a list that was the value of a previous call to
@code{match-data}.

If @var{match-list} refers to a buffer that doesn't exist, you don't get
an error; that sets the match data in a meaningless but harmless way.

@findex store-match-data
@code{store-match-data} is an alias for @code{set-match-data}.
@end defun

@node Saving Match Data
@subsection Saving and Restoring the Match Data

  When you call a function that may do a search, you may need to save
and restore the match data around that call, if you want to preserve the
match data from an earlier search for later use.  Here is an example
that shows the problem that arises if you fail to save the match data:

@example
@group
(re-search-forward "The \\(cat \\)")
     @result{} 48
(foo)                   ; @r{Perhaps @code{foo} does}
                        ;   @r{more searching.}
(match-end 0)
     @result{} 61              ; @r{Unexpected result---not 48!}
@end group
@end example

  You can save and restore the match data with @code{save-match-data}:

@defspec save-match-data body@dots{}
This special form executes @var{body}, saving and restoring the match
data around it.
@end defspec

  You can use @code{set-match-data} together with @code{match-data} to
imitate the effect of the special form @code{save-match-data}.  This is
useful for writing code that can run in Emacs 18.  Here is how:

@example
@group
(let ((data (match-data)))
  (unwind-protect
      @dots{}   ; @r{May change the original match data.}
    (set-match-data data)))
@end group
@end example

  Emacs automatically saves and restores the match data when it runs
process filter functions (@pxref{Filter Functions}) and process
sentinels (@pxref{Sentinels}).

@ignore
  Here is a function which restores the match data provided the buffer
associated with it still exists.

@smallexample
@group
(defun restore-match-data (data)
@c It is incorrect to split the first line of a doc string.
@c If there's a problem here, it should be solved in some other way.
  "Restore the match data DATA unless the buffer is missing."
  (catch 'foo
    (let ((d data))
@end group
      (while d
        (and (car d)
             (null (marker-buffer (car d)))
@group
             ;; @file{match-data} @r{buffer is deleted.}
             (throw 'foo nil))
        (setq d (cdr d)))
      (set-match-data data))))
@end group
@end smallexample
@end ignore

@node Searching and Case
@section Searching and Case
@cindex searching and case

  By default, searches in Emacs ignore the case of the text they are
searching through; if you specify searching for @samp{FOO}, then
@samp{Foo} or @samp{foo} is also considered a match.  Regexps, and in
particular character sets, are included: thus, @samp{[aB]} would match
@samp{a} or @samp{A} or @samp{b} or @samp{B}.

  If you do not want this feature, set the variable
@code{case-fold-search} to @code{nil}.  Then all letters must match
exactly, including case.  This is a buffer-local variable; altering the
variable affects only the current buffer.  (@xref{Intro to
Buffer-Local}.)  Alternatively, you may change the value of
@code{default-case-fold-search}, which is the default value of
@code{case-fold-search} for buffers that do not override it.

  Note that the user-level incremental search feature handles case
distinctions differently.  When given a lower case letter, it looks for
a match of either case, but when given an upper case letter, it looks
for an upper case letter only.  But this has nothing to do with the
searching functions Lisp functions use.

@defopt case-replace
This variable determines whether the replacement functions should
preserve case.  If the variable is @code{nil}, that means to use the
replacement text verbatim.  A non-@code{nil} value means to convert the
case of the replacement text according to the text being replaced.

The function @code{replace-match} is where this variable actually has
its effect.  @xref{Replacing Match}.
@end defopt

@defopt case-fold-search
This buffer-local variable determines whether searches should ignore
case.  If the variable is @code{nil} they do not ignore case; otherwise
they do ignore case.
@end defopt

@defvar default-case-fold-search
The value of this variable is the default value for
@code{case-fold-search} in buffers that do not override it.  This is the
same as @code{(default-value 'case-fold-search)}.
@end defvar

@node Standard Regexps
@section Standard Regular Expressions Used in Editing
@cindex regexps used standardly in editing
@cindex standard regexps used in editing

  This section describes some variables that hold regular expressions
used for certain purposes in editing:

@defvar page-delimiter
This is the regexp describing line-beginnings that separate pages.  The
default value is @code{"^\014"} (i.e., @code{"^^L"} or @code{"^\C-l"});
this matches a line that starts with a formfeed character.
@end defvar

  The following two regular expressions should @emph{not} assume the
match always starts at the beginning of a line; they should not use
@samp{^} to anchor the match.  Most often, the paragraph commands do
check for a match only at the beginning of a line, which means that
@samp{^} would be superfluous.  When there is a nonzero left margin,
they accept matches that start after the left margin.  In that case, a
@samp{^} would be incorrect.  However, a @samp{^} is harmless in modes
where a left margin is never used.

@defvar paragraph-separate
This is the regular expression for recognizing the beginning of a line
that separates paragraphs.  (If you change this, you may have to
change @code{paragraph-start} also.)  The default value is
@w{@code{"[@ \t\f]*$"}}, which matches a line that consists entirely of
spaces, tabs, and form feeds (after its left margin).
@end defvar

@defvar paragraph-start
This is the regular expression for recognizing the beginning of a line
that starts @emph{or} separates paragraphs.  The default value is
@w{@code{"[@ \t\n\f]"}}, which matches a line starting with a space, tab,
newline, or form feed (after its left margin).
@end defvar

@defvar sentence-end
This is the regular expression describing the end of a sentence.  (All
paragraph boundaries also end sentences, regardless.)  The default value
is:

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

This means a period, question mark or exclamation mark, followed
optionally by a closing parenthetical character, followed by tabs,
spaces or new lines.

For a detailed explanation of this regular expression, see @ref{Regexp
Example}.
@end defvar