Go to the first, previous, next, last section, table of contents.
Regular expressions have a syntax in which a few characters are
special constructs and the rest are ordinary. An ordinary
character is a simple regular expression that matches that character and
nothing else. The special characters are `.', `*', `+',
`?', `[', `]', `^', `$', and `\'; no new
special characters will be defined in the future. Any other character
appearing in a regular expression is ordinary, unless a `\'
precedes it.
For example, `f' is not a special character, so it is ordinary, and
therefore `f' is a regular expression that matches the string
`f' and no other string. (It does not match the string
`ff'.) Likewise, `o' is a regular expression that matches
only `o'.
Any two regular expressions a and b can be concatenated. The
result is a regular expression that matches a string if a matches
some amount of the beginning of that string and b matches the rest of
the string.
As a simple example, we can concatenate the regular expressions `f'
and `o' to get the regular expression `fo', which matches only
the string `fo'. Still trivial. To do something more powerful, you
need to use one of the special characters. Here is a list of them:
- . (Period)
-
is a special character that matches any single character except a newline.
Using concatenation, we can make regular expressions like `a.b', which
matches any three-character string that begins with `a' and ends with
`b'.
- *
-
is not a construct by itself; it is a suffix operator that means to
repeat the preceding regular expression as many times as possible. In
`fo*', the `*' applies to the `o', so `fo*' matches
one `f' followed by any number of `o's. The case of zero
`o's is allowed: `fo*' does match `f'.
`*' always applies to the smallest possible preceding
expression. Thus, `fo*' has a repeating `o', not a
repeating `fo'.
The matcher processes a `*' construct by matching, immediately,
as many repetitions as can be found. Then it continues with the rest
of the pattern. If that fails, backtracking occurs, discarding some
of the matches of the `*'-modified construct in case that makes
it possible to match the rest of the pattern. For example, in matching
`ca*ar' against the string `caaar', the `a*' first
tries to match all three `a's; but the rest of the pattern is
`ar' and there is only `r' left to match, so this try fails.
The next alternative is for `a*' to match only two `a's.
With this choice, the rest of the regexp matches successfully.
Nested repetition operators can be extremely slow if they specify
backtracking loops. For example, it could take hours for the regular
expression `\(x+y*\)*a' to match the sequence
`xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxz'. The slowness is because
Emacs must try each imaginable way of grouping the 35 `x''s before
concluding that none of them can work. To make sure your regular
expressions run fast, check nested repetitions carefully.
- +
-
is a suffix operator similar to `*' except that the preceding
expression must match 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 suffix operator similar to `*' except that the preceding
expression can match either once or not at all. For example,
`ca?r' matches `car' or `cr', but does not match anyhing
else.
- [ ... ]
-
`[' begins a character set, which is terminated by a
`]'. In the simplest case, the characters between the two brackets
form the set. Thus, `[ad]' matches either one `a' or one
`d', and `[ad]*' matches any string composed of just `a's
and `d's (including the empty string), from which it follows that
`c[ad]*r' matches `cr', `car', `cdr',
`caddaar', etc.
The usual regular expression special characters are not special inside a
character set. A completely different set of special characters exists
inside character sets: `]', `-' and `^'.
`-' is used for ranges of characters. To write a range, write two
characters with a `-' between them. Thus, `[a-z]' matches any
lower case letter. Ranges may be intermixed freely with individual
characters, as in `[a-z$%.]', which matches any lower case letter
or `$', `%', or a period.
To include a `]' in a character set, make it the first character.
For example, `[]a]' matches `]' or `a'. To include a
`-', write `-' as the first character in the set, or put it
immediately after a range. (You can replace one individual character
c with the range `c-c' to make a place to put the
`-'.) There is no way to write a set containing just `-' and
`]'.
To include `^' in a set, put it anywhere but at the beginning of
the set.
- [^ ... ]
-
`[^' begins a complement character set, which matches any
character except the ones specified. Thus, `[^a-z0-9A-Z]'
matches all characters except letters and digits.
`^' is not special in a character set unless it is the first
character. The character following the `^' is treated as if it
were first (thus, `-' and `]' are not special there).
Note that a complement character set can match a newline, unless
newline is mentioned as one of the characters not to match.
- ^
-
is a special character that matches the empty string, but only at the
beginning of a line in the text being matched. Otherwise it fails to
match anything. Thus, `^foo' matches a `foo' that occurs at
the beginning of a line.
When matching a string instead of a buffer, `^' matches at the
beginning of the string or after a newline character `\n'.
- $
-
is similar to `^' but matches only at the end of a line. Thus,
`x+$' matches a string of one `x' or more at the end of a line.
When matching a string instead of a buffer, `$' matches at the end
of the string or before a newline character `\n'.
- \
-
has two functions: it quotes the special characters (including
`\'), and it introduces additional special constructs.
Because `\' quotes special characters, `\$' is a regular
expression that matches only `$', and `\[' is a regular
expression that matches only `[', and so on.
Note that `\' also has special meaning in the read syntax of Lisp
strings (see section String Type), and must be quoted with `\'. For
example, the regular expression that matches the `\' character is
`\\'. To write a Lisp string that contains the characters
`\\', Lisp syntax requires you to quote each `\' with another
`\'. Therefore, the read syntax for a regular expression matching
`\' is
"\\\\"
.
Please note: For historical compatibility, special characters
are treated as ordinary ones if they are in contexts where their special
meanings make no sense. For example, `*foo' treats `*' as
ordinary since there is no preceding expression on which the `*'
can act. It is poor practice to depend on this behavior; quote the
special character anyway, regardless of where it appears.
For the most part, `\' followed by any character matches only
that character. However, there are several exceptions: characters
that, when preceded by `\', are special constructs. Such
characters are always ordinary when encountered on their own. Here
is a table of `\' constructs:
- \|
-
specifies an alternative.
Two regular expressions a and b with `\|' in
between form an expression that matches anything that either a or
b matches.
Thus, `foo\|bar' matches either `foo' or `bar'
but no other string.
`\|' applies to the largest possible surrounding expressions. Only a
surrounding `\( ... \)' grouping can limit the grouping power of
`\|'.
Full backtracking capability exists to handle multiple uses of `\|'.
- \( ... \)
-
is a grouping construct that serves three purposes:
-
To enclose a set of `\|' alternatives for other operations. Thus,
the regular expression `\(foo\|bar\)x' matches either `foox'
or `barx'.
-
To enclose an expression for a suffix operator such as `*' to act
on. Thus, `ba\(na\)*' matches `bananana', etc., with any
(zero or more) number of `na' strings.
-
To record a matched substring for future reference.
This last application is not a consequence of the idea of a
parenthetical grouping; it is a separate feature that happens to be
assigned as a second meaning to the same `\( ... \)' construct
because there is no conflict in practice between the two meanings.
Here is an explanation of this feature:
- \digit
-
matches the same text that matched the digitth occurrence of a
`\( ... \)' construct.
In other words, after the end of a `\( ... \)' construct. the
matcher remembers the beginning and end of the text matched by that
construct. Then, later on in the regular expression, you can use
`\' followed by digit to match that same text, whatever it
may have been.
The strings matching the first nine `\( ... \)' constructs
appearing in a regular expression are assigned numbers 1 through 9 in
the order that the open parentheses appear in the regular expression.
So you can use `\1' through `\9' to refer to the text matched
by the corresponding `\( ... \)' constructs.
For example, `\(.*\)\1' matches any newline-free string that is
composed of two identical halves. The `\(.*\)' matches the first
half, which may be anything, but the `\1' that follows must match
the same exact text.
- \w
-
matches any word-constituent character. The editor syntax table
determines which characters these are. See section Syntax Tables.
- \W
-
matches any character that is not a word constituent.
- \scode
-
matches any character whose syntax is code. Here code is a
character that represents a syntax code: thus, `w' for word
constituent, `-' for whitespace, `(' for open parenthesis,
etc. See section Syntax Tables, for a list of syntax codes and the
characters that stand for them.
- \Scode
-
matches any character whose syntax is not code.
The following regular expression constructs match the empty string--that is,
they don't use up any characters--but whether they match depends on the
context.
- \`
-
matches the empty string, but only at the beginning
of the buffer or string being matched against.
- \'
-
matches the empty string, but only at the end of
the buffer or string being matched against.
- \=
-
matches the empty string, but only at point.
(This construct is not defined when matching against a string.)
- \b
-
matches the empty string, but only at the beginning or
end of a word. Thus, `\bfoo\b' matches any occurrence of
`foo' as a separate word. `\bballs?\b' matches
`ball' or `balls' as a separate word.
- \B
-
matches the empty string, but not at the beginning or
end of a word.
- \<
-
matches the empty string, but only at the beginning of a word.
- \>
-
matches the empty string, but only at the end of a word.
Not every string is a valid regular expression. For example, a string
with unbalanced square brackets is invalid (with a few exceptions, such
as `[]]'), and so is a string that ends with a single `\'. If
an invalid regular expression is passed to any of the search functions,
an invalid-regexp
error is signaled.
- Function: regexp-quote string
-
This function returns a regular expression string that matches exactly
string and nothing else. This allows you to request an exact
string match when calling a function that wants a regular expression.
(regexp-quote "^The cat$")
=> "\\^The cat\\$"
One use of regexp-quote
is to combine an exact string match with
context described as a regular expression. For example, this searches
for the string that is the value of string
, surrounded by
whitespace:
(re-search-forward
(concat "\\s-" (regexp-quote string) "\\s-"))
Go to the first, previous, next, last section, table of contents.