34.2.1.1 Special Characters in Regular Expressions

Here is a list of the characters that are special in a regular expression.

`.' (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 the hope that that will make 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 try to match the sequence `xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxz', before it ultimately fails. 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 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 must match the preceding expression either once or not at all. For example, `ca?r' matches `car' or `cr'; nothing else.

`*?', `+?', `??'

These are "non-greedy" variants of the operators `*', `+' and `?'. Where those operators match the largest possible substring (consistent with matching the entire containing expression), the non-greedy variants match the smallest possible substring (consistent with matching the entire containing expression).

For example, the regular expression `c[ad]*a' when applied to the string `cdaaada' matches the whole string; but the regular expression `c[ad]*?a', applied to that same string, matches just `cda'. (The smallest possible match here for `[ad]*?' that permits the whole expression to match is `d'.)

`[ ... ]'

is a character alternative, which begins with `[' and is terminated by `]'. In the simplest case, the characters between the two brackets are what this character alternative 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 alternative, 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 alternative. A completely different set of characters is special inside character alternatives: `]', `-' and `^'.

To include a `]' in a character alternative, 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 character alternative, or put it after a range. Thus, `[]-]' matches both `]' and `-'.

To include `^' in a character alternative, put it anywhere but at the beginning.

The beginning and end of a range of multibyte characters must be in the same character set (see section 33.5 Character Sets). Thus, "[\x8e0-\x97c]" is invalid because character 0x8e0 (`a' with grave accent) is in the Emacs character set for Latin-1 but the character 0x97c (`u' with diaeresis) is in the Emacs character set for Latin-2. (We use Lisp string syntax to write that example, and a few others in the next few paragraphs, in order to include hex escape sequences in them.)

If a range starts with a unibyte character c and ends with a multibyte character c2, the range is divided into two parts: one is `c..?\377', the other is `c1..c2', where c1 is the first character of the charset to which c2 belongs. You cannot always match all non-ASCII characters with the regular expression "[\200-\377]". This works when searching a unibyte buffer or string (see section 33.1 Text Representations), but not in a multibyte buffer or string, because many non-ASCII characters have codes above octal 0377. However, the regular expression "[^\000-\177]" does match all non-ASCII characters (see below regarding `^'), in both multibyte and unibyte representations, because only the ASCII characters are excluded.

Starting in Emacs 21, a character alternative can also specify named character classes (see section 34.2.1.2 Character Classes). This is a POSIX feature whose syntax is `[:class:]'. Using a character class is equivalent to mentioning each of the characters in that class; but the latter is not feasible in practice, since some classes include thousands of different characters.

`[^ ... ]'

`[^' begins a complemented character alternative, 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 alternative 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 alternative 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.

When matching a string instead of a buffer, `^' matches at the beginning of the string or after a newline character.

For historical compatibility reasons, `^' can be used only at the beginning of the regular expression, or after `\(' or `\|'.

`$'

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.

For historical compatibility reasons, `$' can be used only at the end of the regular expression, or before `\)' or `\|'.

`\'

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 2.3.8 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.