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(emacs)Regexps
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 which matches that same character and
nothing else. The special characters are `$', `^', `.', `*', `+', `?',
`[', `]' and `\'. Any other character appearing in a regular
expression is ordinary, unless a `\' precedes it. (When you use regular
expressions in a Lisp program, each `\' must be doubled, see the
example near the end of this section.)
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'. (When case distinctions
are being ignored, these regexps also match `F' and `O', but we
consider this a generalization of "the same string," rather than an
exception.)
Any two regular expressions A and B can be concatenated. The result
is a regular expression which 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 nontrivial, 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 postfix operator that means
to match the preceding regular expression repetitively as many
times as possible. Thus, `o*' matches any number of `o's
(including no `o's).
`*' always applies to the _smallest_ possible preceding
expression. Thus, `fo*' has a repeating `o', not a repeating
`fo'. It matches `f', `fo', `foo', and so on.
The matcher processes a `*' 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 `*'-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.
`+'
is a postfix operator, similar to `*' except that it must match
the preceding expression at least once. 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 postfix operator, similar to `*' except that it can match the
preceding expression either once or not at all. For example,
`ca?r' matches `car' or `cr'; nothing else.
`*?, +?, ??'
are non-greedy variants of the operators above. The normal
operators `*', `+', `?' are "greedy" in that they match as much as
they can, as long as the overall regexp can still match. With a
following `?', they are non-greedy: they will match as little as
possible.
Thus, both `ab*' and `ab*?' can match the string `a' and the
string `abbbb'; but if you try to match them both against the text
`abbb', `ab*' will match it all (the longest valid match), while
`ab*?' will match just `a' (the shortest valid match).
`\{N\}'
is a postfix operator that specifies repetition N times--that is,
the preceding regular expression must match exactly N times in a
row. For example, `x\{4\}' matches the string `xxxx' and nothing
else.
`\{N,M\}'
is a postfix operator that specifies repetition between N and M
times--that is, the preceding regular expression must match at
least N times, but no more than M times. If M is omitted, then
there is no upper limit, but the preceding regular expression must
match at least N times.
`\{0,1\}' is equivalent to `?'.
`\{0,\}' is equivalent to `*'.
`\{1,\}' is equivalent to `+'.
`[ ... ]'
is a "character set", which begins with `[' and is terminated by
`]'. In the simplest case, the characters between the two
brackets are what this set can match.
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.
You can also include character ranges in a character set, by
writing the starting and ending characters with a `-' between
them. Thus, `[a-z]' matches any lower-case ASCII letter. Ranges
may be intermixed freely with individual characters, as in
`[a-z$%.]', which matches any lower-case ASCII letter or `$', `%'
or period.
Note that the usual regexp special characters are not special
inside a character set. A completely different set of special
characters exists inside character sets: `]', `-' and `^'.
To include a `]' in a character set, you must make it the first
character. For example, `[]a]' matches `]' or `a'. To include a
`-', write `-' as the first or last character of the set, or put
it after a range. Thus, `[]-]' matches both `]' and `-'.
To include `^' in a set, put it anywhere but at the beginning of
the set. (At the beginning, it complements the set--see below.)
When you use a range in case-insensitive search, you should write
both ends of the range in upper case, or both in lower case, or
both should be non-letters. The behavior of a mixed-case range
such as `A-z' is somewhat ill-defined, and it may change in future
Emacs versions.
`[^ ... ]'
`[^' begins a "complemented character set", which matches any
character except the ones specified. Thus, `[^a-z0-9A-Z]' matches
all characters _except_ ASCII letters and digits.
`^' is not special in a character set unless it is the first
character. The character following the `^' is treated as if it
were first (in other words, `-' and `]' are not special there).
A complemented character set can match a newline, unless newline is
mentioned as one of the characters not to match. This is in
contrast to the handling of regexps in programs such as `grep'.
`^'
is a special character that matches the empty string, but only at
the beginning of a line in the text being matched. Otherwise it
fails to match anything. Thus, `^foo' matches a `foo' that occurs
at the beginning of a line.
`$'
is similar to `^' but matches only at the end of a line. Thus,
`x+$' matches a string of one `x' or more at the end of a line.
`\'
has two functions: it quotes the special characters (including
`\'), and it introduces additional special constructs.
Because `\' quotes special characters, `\$' is a regular
expression that matches only `$', and `\[' is a regular expression
that matches only `[', and so on.
Note: 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; it is better to 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: two-character
sequences starting with `\' that have special meanings. The second
character in the sequence is always an ordinary character when used on
its own. Here is a table of `\' constructs.
`\|'
specifies an alternative. Two regular expressions A and B with
`\|' in between form an expression that matches some text if
either A matches it or B matches it. It works by trying to match
A, and if that fails, by trying to match B.
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 a complicated expression for the postfix operators
`*', `+' and `?' to operate 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 is assigned
as a second meaning to the same `\( ... \)' construct. In practice
there is usually no conflict between the two meanings; when there
is a conflict, you can use a "shy" group.
`\(?: ... \)'
specifies a "shy" group that does not record the matched substring;
you can't refer back to it with `\D'. This is useful in
mechanically combining regular expressions, so that you can add
groups for syntactic purposes without interfering with the
numbering of the groups that were written by the user.
`\D'
matches the same text that matched the Dth occurrence of a `\( ...
\)' construct.
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 the
digit D to mean "match the same text matched the Dth time by the
`\( ... \)' construct."
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.
If a particular `\( ... \)' construct matches more than once
(which can easily happen if it is followed by `*'), only the last
match is recorded.
`\`'
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.
`\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 at the beginning or end of the buffer regardless of
what text appears next to it.
`\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 at the beginning of the buffer only if a
word-constituent character follows.
`\>'
matches the empty string, but only at the end of a word. `\>'
matches at the end of the buffer only if the contents end with a
word-constituent character.
`\w'
matches any word-constituent character. The syntax table
determines which characters these are. Note: Syntax.
`\W'
matches any character that is not a word-constituent.
`\sC'
matches any character whose syntax is C. Here C is a character
that designates a particular syntax class: thus, `w' for word
constituent, `-' or ` ' for whitespace, `.' for ordinary
punctuation, etc. Note: Syntax.
`\SC'
matches any character whose syntax is not C.
`\cC'
matches any character that belongs to the category C. For
example, `\cc' matches Chinese characters, `\cg' matches Greek
characters, etc. For the description of the known categories,
type `M-x describe-categories <RET>'.
`\CC'
matches any character that does _not_ belong to category C.
The constructs that pertain to words and syntax are controlled by the
setting of the syntax table (Note: Syntax).
Here is a complicated regexp, stored in `sentence-end' and used by
Emacs to recognize the end of a sentence together with any whitespace
that follows. We show its Lisp syntax to distinguish the spaces from
the tab characters. In Lisp syntax, the string constant begins and
ends with a double-quote. `\"' stands for a double-quote as part of
the regexp, `\\' for a backslash as part of the regexp, `\t' for a tab,
and `\n' for a newline.
"[.?!][]\"')]*\\($\\| $\\|\t\\| \\)[ \t\n]*"
This contains four parts in succession: a character set matching
period, `?', or `!'; a character set matching close-brackets, quotes,
or parentheses, repeated zero or more times; a set of alternatives
within backslash-parentheses that matches either end-of-line, a space
at the end of a line, a tab, or two spaces; and a character set
matching whitespace characters, repeated any number of times.
To enter the same regexp interactively, you would type <TAB> to
enter a tab, and `C-j' to enter a newline. (When typed interactively,
`C-j' should be preceded by a `C-q', to prevent Emacs from running the
command bound to a newline.) You would also type single backslashes as
themselves, instead of doubling them for Lisp syntax.
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