@cindex syntax of regexps
This manual describes regular expression features that users
-typically want to use. There are additional features that are
-mainly used in Lisp programs; see @ref{Regular Expressions,,,
-elisp, The Emacs Lisp Reference Manual}.
+typically use. @xref{Regular Expressions,,, elisp, The Emacs Lisp
+Reference Manual}, for additional features used mainly in Lisp
+programs.
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 which matches that same
-character and nothing else. The special characters are @samp{$},
-@samp{^}, @samp{.}, @samp{*}, @samp{+}, @samp{?}, @samp{[}, and
-@samp{\}. The character @samp{]} is special if it ends a character
-alternative (see later). The character @samp{-} is special inside a
-character alternative. Any other character appearing in a regular
-expression is ordinary, unless a @samp{\} precedes it. (When you use
-regular expressions in a Lisp program, each @samp{\} must be doubled,
-see the example near the end of this section.)
+character matches that same character and nothing else. The special
+characters are @samp{$^.*+?[\}. The character @samp{]} is special if
+it ends a character alternative (see later). The character @samp{-}
+is special inside a character alternative. Any other character
+appearing in a regular expression is ordinary, unless a @samp{\}
+precedes it. (When you use regular expressions in a Lisp program,
+each @samp{\} must be doubled, see the example near the end of this
+section.)
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
also match @samp{F} and @samp{O}, but we consider this a generalization
of ``the same string,'' rather than an exception.)
- Any two regular expressions @var{a} and @var{b} can be concatenated. The
-result is a regular expression which 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 nontrivial, you
-need to use one of the special characters. Here is a list of them.
+ Any two regular expressions @var{a} and @var{b} can be concatenated.
+The result is a regular expression which matches a string if @var{a}
+matches some amount of the beginning of that string and @var{b}
+matches the rest of the string. For example, concatenating the
+regular expressions @samp{f} and @samp{o} gives the regular expression
+@samp{fo}, which matches only the string @samp{fo}. Still trivial.
+To do something nontrivial, you need to use one of the special
+characters. Here is a list of them.
@table @asis
@item @kbd{.}@: @r{(Period)}
-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
+is a special character that matches any single character except a
+newline. For example, the regular expressions @samp{a.b} matches any
+three-character string that begins with @samp{a} and ends with
+@samp{b}.
@item @kbd{*}
is not a construct by itself; it is a postfix operator that means to
-match the preceding regular expression repetitively as many times as
-possible. Thus, @samp{o*} matches any number of @samp{o}s (including no
-@samp{o}s).
+match the preceding regular expression repetitively any number of
+times, as many times as possible. Thus, @samp{o*} matches any number
+of @samp{o}s, including no @samp{o}s.
@samp{*} always applies to the @emph{smallest} possible preceding
expression. Thus, @samp{fo*} has a repeating @samp{o}, not a repeating
@item @kbd{+}
is a postfix operator, similar to @samp{*} except that it must match
-the preceding expression at least once. 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.
+the preceding expression at least once. Thus, @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 @kbd{?}
-is a postfix operator, similar to @samp{*} except that it can match the
-preceding expression either once or not at all. For example,
-@samp{ca?r} matches @samp{car} or @samp{cr}; nothing else.
+is a postfix operator, similar to @samp{*} except that it can match
+the preceding expression either once or not at all. Thus, @samp{ca?r}
+matches @samp{car} or @samp{cr}, and nothing else.
@item @kbd{*?}, @kbd{+?}, @kbd{??}
@cindex non-greedy regexp matching
-are non-greedy variants of the operators above. The normal operators
-@samp{*}, @samp{+}, @samp{?} are @dfn{greedy} in that they match as
-much as they can, as long as the overall regexp can still match. With
-a following @samp{?}, they are non-greedy: they will match as little
-as possible.
+are non-@dfn{greedy} variants of the operators above. The normal
+operators @samp{*}, @samp{+}, @samp{?} match as much as they can, as
+long as the overall regexp can still match. With a following
+@samp{?}, they will match as little as possible.
Thus, both @samp{ab*} and @samp{ab*?} can match the string @samp{a}
and the string @samp{abbbb}; but if you try to match them both against
starting at the first @samp{a}, it does.
@item @kbd{\@{@var{n}\@}}
-is a postfix operator that specifies repetition @var{n} times---that
-is, the preceding regular expression must match exactly @var{n} times
-in a row. For example, @samp{x\@{4\@}} matches the string @samp{xxxx}
-and nothing else.
+is a postfix operator specifying @var{n} repetitions---that is, the
+preceding regular expression must match exactly @var{n} times in a
+row. For example, @samp{x\@{4\@}} matches the string @samp{xxxx} and
+nothing else.
@item @kbd{\@{@var{n},@var{m}\@}}
-is a postfix operator that specifies repetition between @var{n} and
-@var{m} times---that is, the preceding regular expression must match
-at least @var{n} times, but no more than @var{m} times. If @var{m} is
+is a postfix operator specifying between @var{n} and @var{m}
+repetitions---that is, the preceding regular expression must match at
+least @var{n} times, but no more than @var{m} times. If @var{m} is
omitted, then there is no upper limit, but the preceding regular
expression must match at least @var{n} times.@* @samp{\@{0,1\@}} is
equivalent to @samp{?}. @* @samp{\@{0,\@}} is equivalent to
@samp{*}. @* @samp{\@{1,\@}} is equivalent to @samp{+}.
@item @kbd{[ @dots{} ]}
-is a @dfn{character set}, which begins with @samp{[} and is terminated
-by @samp{]}. In the simplest case, the characters between the two
-brackets are what this set can match.
+is a @dfn{character set}, beginning with @samp{[} and terminated by
+@samp{]}.
-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.
+In the simplest case, the characters between the two brackets are what
+this set can match. 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). It follows that
+@samp{c[ad]*r} matches @samp{cr}, @samp{car}, @samp{cdr},
+@samp{caddaar}, etc.
You can also include character ranges in a character set, by writing the
starting and ending characters with a @samp{-} between them. Thus,
which matches any lower-case @acronym{ASCII} letter or @samp{$}, @samp{%} or
period.
-Note that the usual regexp special characters are not special inside a
-character set. A completely different set of special characters exists
-inside character sets: @samp{]}, @samp{-} and @samp{^}.
+You can also include certain special @dfn{character classes} in a
+character set. A @samp{[:} and balancing @samp{:]} enclose a
+character class inside a character alternative. For instance,
+@samp{[[:alnum:]]} matches any letter or digit. @xref{Char Classes,,,
+elisp, The Emacs Lisp Reference Manual}, for a list of character
+classes.
To include a @samp{]} in a character set, you must make it the first
character. For example, @samp{[]a]} matches @samp{]} or @samp{a}. To
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