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 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 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.

`[ ... ]'

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. 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 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, in both multibyte and unibyte representations, because only the ASCII characters are excluded. The beginning and end of a range must be in the same character set (see section Character Sets). Thus, `[a-\x8e0]' is invalid because `a' is in the ASCII character set but the character 0x8e0 (`a' with grave accent) is in the Emacs character set for Latin-1. Note that the usual regexp special characters are not special inside a character alternative. A completely different set of characters are 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.

`[^ ... ]'

`[^' 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 `\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: two-character sequences starting with `\' which have special meanings. (The second character in such a sequence is always ordinary 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 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:

1. To enclose a set of `\|' alternatives for other operations. Thus, the regular expression `\(foo\|bar\)x' matches either `foox' or `barx'.
2. To enclose a complicated expression for the postfix operators `*', `+' and `?' to operate on. Thus, `ba\(na\)*' matches `ba', `bana', `banana', `bananana', etc., with any number (zero or more) 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 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. To represent whitespace syntax, use either `-' or a space character. See section Table of Syntax Classes, 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 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.

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-"))

Function: regexp-opt strings &optional paren

This function returns an efficient regular expression that will match any of the strings strings. This is useful when you need to make matching or searching as fast as possible--for example, for Font Lock mode.

If the optional argument paren is non-nil, then the returned regular expression is always enclosed by at least one parentheses-grouping construct.

This simplified definition of regexp-opt produces a regular expression which is equivalent to the actual value (but not as efficient):

(defun regexp-opt (strings paren)
  (let ((open-paren (if paren "\\(" ""))
        (close-paren (if paren "\\)" "")))
    (concat open-paren
            (mapconcat 'regexp-quote strings "\\|")
            close-paren)))

Function: regexp-opt-depth regexp

This function returns the total number of grouping constructs (parenthesized expressions) in regexp.

Go to the first, previous, next, last section, table of contents.