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\input texinfo
@setfilename regexp.info
@comment @newindex {fn}


@comment From the `ed' documentation.

@chapter Regular expressions

Regular expressions are patterns used in selecting text.

In addition to a specifying string literals, regular expressions can
represent classes of strings.  Strings thus represented are said to be
matched by the corresponding regular expression.  If it is possible for
a regular expression to match several strings in a line, then the
left-most longest match is the one selected.

The following symbols are used in constructing regular expressions:

@table @code

@item @var{c}
Any character @var{c} not listed below, including @samp{@{}, @samp{@}},
@samp{(}, @samp{)}, @samp{<} and @samp{>}, matches itself.

@item \@var{c}
Any backslash-escaped character @var{c}, other than @samp{@{},
`@samp{@}}, @samp{(}, @samp{)}, @samp{<}, @samp{>}, @samp{b}, @samp{B},
@samp{w}, @samp{W}, @samp{+} and @samp{?}, matches itself.

Note that @samp{\} also has special meaning in the read syntax of Lisp
strings, 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

@item .
Matches any single character.

@item [@var{char-class}]
Matches any single character in @var{char-class}.  To include a @samp{]}
in @var{char-class}, it must be the first character.  A range of
characters may be specified by separating the end characters of the
range with a @samp{-}, e.g., @samp{a-z} specifies the lower case
characters.  The following literal expressions can also be used in
@var{char-class} to specify sets of characters:

@example
[:alnum:] [:cntrl:] [:lower:] [:space:]
[:alpha:] [:digit:] [:print:] [:upper:]
[:blank:] [:graph:] [:punct:] [:xdigit:]
@end example

If @samp{-} appears as the first or last character of @var{char-class},
then it matches itself.  All other characters in @var{char-class} match
themselves.

Patterns in
@var{char-class}
of the form:
@example
[.@var{col-elm}.]
[=@var{col-elm}=]
@end example

@noindent
where @var{col-elm} is a @dfn{collating element} are interpreted
according to @code{locale (5)} (not currently supported).  See
@code{regex (3)} for an explanation of these constructs.

@item [^@var{char-class}]
Matches any single character, other than newline, not in
@var{char-class}.  @var{char-class} is defined as above.

@item ^
If @samp{^} is the first character of a regular expression, then it
anchors the regular expression to the beginning of a line.  Otherwise,
it matches itself.

@item $
If @samp{$} is the last character of a regular expression, it anchors
the regular expression to the end of a line.  Otherwise, it matches
itself.

@item \(@var{re}\)
Defines a (possibly null) subexpression @var{re}.
Subexpressions may be nested.  A
subsequent backreference of the form @samp{\@var{n}}, where @var{n} is a
number in the range [1,9], expands to the text matched by the @var{n}th
subexpression. For example, the regular expression @samp{\(a.c\)\1} matches
the string @samp{abcabc}, but not @samp{abcadc}.
Subexpressions are ordered relative to their left delimiter.

@item *
Matches the single character regular expression or subexpression
immediately preceding it zero or more times.  If @samp{*} is the first
character of a regular expression or subexpression, then it matches
itself.  The @samp{*} operator sometimes yields unexpected results.  For
example, the regular expression @samp{b*} matches the beginning of the
string @samp{abbb}, as opposed to the substring @samp{bbb}, since a
null match is the only left-most match.

@item \@{@var{n,m}\@}
@itemx \@{@var{n,}\@}
@itemx \@{@var{n}\@}
Matches the single character regular expression or subexpression
immediately preceding it at least @var{n} and at most @var{m} times.  If
@var{m} is omitted, then it matches at least @var{n} times.  If the
comma is also omitted, then it matches exactly @var{n} times.
If any of these forms occurs first in a regular expression or subexpression,
then it is interpreted literally (i.e., the regular expression @samp{\@{2\@}}
matches the string @samp{@{2@}}, and so on).

@item \<
@itemx \>
Anchors the single character regular expression or subexpression
immediately following it to the beginning (in the case of @samp{\<})
or ending (in the case of @samp{\>}) of
a @dfn{word}, i.e., in ASCII, a maximal string of alphanumeric characters,
including the underscore (_).

@end table

The following extended operators are preceded by a backslash @samp{\} to
distinguish them from traditional @code{ed} syntax.

@table @code

@item  \`
@itemx \'
Unconditionally matches the beginning @samp{\`} or ending @samp{\'} of a line.

@item \?
Optionally matches the single character regular expression or subexpression
immediately preceding it.  For example, the regular expression @samp{a[bd]\?c}
matches the strings @samp{abc}, @samp{adc} and @samp{ac}.
If @samp{\?} occurs at the beginning
of a regular expressions or subexpression, then it matches a literal @samp{?}.

@item \+
Matches the single character regular expression or subexpression
immediately preceding it one or more times.  So the regular expression
@samp{a+} is shorthand for @samp{aa*}.  If @samp{\+} occurs at the
beginning of a regular expression or subexpression, then it matches a
literal @samp{+}.

@item \b
Matches the beginning or ending (null string) of a word.  Thus the regular
expression @samp{\bhello\b} is equivalent to @samp{\<hello\>}.
However, @samp{\b\b}
is a valid regular expression whereas @samp{\<\>} is not.

@item \B
Matches (a null string) inside a word.

@item \w
Matches any character in a word.

@item \W
Matches any character not in a word.

@end table


@comment From the `emacs' documentation.
@comment The RE_SYNTAX_POSIX_BASIC syntax we use differs from the Emacs syntax
@comment in the following bits:
@comment RE_CHAR_CLASSES     ->  character classes are supported
@comment RE_DOT_NEWLINE      ->  . matches newline
@comment RE_DOT_NOT_NULL     ->  . doesn't match NUL
@comment RE_INTERVALS        ->
@comment RE_NO_EMPTY_RANGES  ->  [z-a] is invalid
@comment RE_BK_PLUS_QM       ->  + and ? are operators, \+ and \? are literals

@node Regular Expressions
@chapter 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.

@menu
* Syntax of Regexps::       Rules for writing regular expressions.
* Regexp Examples::         Illustrates regular expression syntax.
@end menu

@node Syntax of Regexps
@section 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 which matches that character and nothing else.
The special characters are @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 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 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.
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 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.  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

@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

The following literal expressions can also be used in
@var{char-class} to specify sets of characters:

@example
[:alnum:] [:cntrl:] [:lower:] [:space:]
[:alpha:] [:digit:] [:print:] [:upper:]
[:blank:] [:graph:] [:punct:] [:xdigit:]
@end example

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
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} which occurs
at the beginning of a line.

When matching a string, @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, @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 which matches only @samp{$}, and @samp{\[} is a regular
expression which matches only @samp{[}, and so on.

Note that @samp{\} also has special meaning in the read syntax of Lisp
strings, 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

For the most part, @samp{\} followed by any character matches only
that character.  However, there are several exceptions: characters
which, 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
is a suffix operator similar to @samp{*} except that the preceding
expression must match 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.

@item \?
@cindex @samp{\?} in regexp
is a 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 anyhing
else.

@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 which 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 which 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 \w
@cindex @samp{\w} in regexp
matches any word-constituent character.

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

  These 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 \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.

@node Regexp Examples
@chapter Examples
@section Complex Regexp Example

  Here is a complicated regexp, used by Emacs 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 C 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, in Lisp, you have to type the tab as Ctrl-V Ctrl-I, producing
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 consists of three characters, a period, a
question mark and an exclamation mark, within square brackets.  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 C or 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 mark the 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 Common Regexps
@section Common Regular Expressions Used in Editing
@cindex regexps used standardly in editing
@cindex standard regexps used in editing

  This section describes some common regular expressions
used for certain purposes in editing:

Page delimiter:
This is the regexp describing line-beginnings that separate pages.  A good
value is @code{(string #\Page)}.

Paragraph separator:
This is the regular expression for recognizing the beginning of a line
that separates paragraphs.  A good value is (in C syntax) @code{"^[
\t\f]*$"}, which is a line that consists entirely of spaces, tabs, and
form feeds.

Paragraph start:
This is the regular expression for recognizing the beginning of a line
that starts @emph{or} separates paragraphs.  A good value is (in C syntax)
@code{"^[ \t\n\f]"}, which matches a line starting with a space, tab,
newline, or form feed.

Sentence end:
This is the regular expression describing the end of a sentence.  (All
paragraph boundaries also end sentences, regardless.)  A good value
is (in C syntax, again):

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

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

@bye