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Regular Expression Functions

These functions operate on regular expressions.

Function: regexp-quote string

This function returns a regular expression whose only exact match is string. Using this regular expression in looking-at will succeed only if the next characters in the buffer are string; using it in a search function will succeed if the text being searched contains string. See Regexp Search.

This allows you to request an exact string match or search 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-"))

The returned string may be string itself if it does not contain any special characters.

Function: regexp-opt strings &optional paren

This function returns an efficient regular expression that will match any of the strings in the list strings. This is useful when you need to make matching or searching as fast as possible—for example, for Font Lock mode²⁰.

If strings is the empty list, the return value is a regexp that never matches anything.

The optional argument paren can be any of the following:

a string

The resulting regexp is preceded by paren and followed by ‘\)’, e.g. use ‘"\\(?1:"’ to produce an explicitly numbered group.

words

The resulting regexp is surrounded by ‘\<\(’ and ‘\)\>’.

symbols

The resulting regexp is surrounded by ‘\_<\(’ and ‘\)\_>’ (this is often appropriate when matching programming-language keywords and the like).

non-nil

The resulting regexp is surrounded by ‘\(’ and ‘\)’.

nil

The resulting regexp is surrounded by ‘\(?:’ and ‘\)’, if it is necessary to ensure that a postfix operator appended to it will apply to the whole expression.

The returned regexp is ordered in such a way that it will always match the longest string possible.

Up to reordering, the resulting regexp of regexp-opt is equivalent to but usually more efficient than that of a simplified version:

(defun simplified-regexp-opt (strings &optional paren)
 (let ((parens
        (cond
         ((stringp paren)       (cons paren "\\)"))
         ((eq paren 'words)    '("\\<\\(" . "\\)\\>"))
         ((eq paren 'symbols) '("\\_<\\(" . "\\)\\_>"))
         ((null paren)          '("\\(?:" . "\\)"))
         (t                       '("\\(" . "\\)")))))
   (concat (car parens)
           (mapconcat 'regexp-quote strings "\\|")
           (cdr parens))))

Function: regexp-opt-depth regexp

This function returns the total number of grouping constructs (parenthesized expressions) in regexp. This does not include shy groups (see Regexp Backslash).

Function: regexp-opt-charset chars

This function returns a regular expression matching a character in the list of characters chars.

(regexp-opt-charset '(?a ?b ?c ?d ?e))
     ⇒ "[a-e]"

Variable: regexp-unmatchable

This variable contains a regexp that is guaranteed not to match any string at all. It is particularly useful as default value for variables that may be set to a pattern that actually matches something.