These functions operate on regular expressions.
This function returns a regular expression whose only exact match is string. Using this regular expression in
looking-atwill 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-quoteis 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-"))
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 mode1.
If the optional argument paren is non-
nil, then the returned regular expression is always enclosed by at least one parentheses-grouping construct. If paren is
words, then that construct is additionally surrounded by ‘\<’ and ‘\>’; alternatively, if paren is
symbols, then that construct is additionally surrounded by ‘\_<’ and ‘\_>’ (
symbolsis often appropriate when matching programming-language keywords and the like).
This simplified definition of
regexp-optproduces a regular expression which is equivalent to the actual value (but not as efficient):(defun regexp-opt (strings &optional paren) (let ((open-paren (if paren "\\(" "")) (close-paren (if paren "\\)" ""))) (concat open-paren (mapconcat 'regexp-quote strings "\\|") close-paren)))
This function returns the total number of grouping constructs (parenthesized expressions) in regexp. This does not include shy groups (see Regexp Backslash).
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]"
 Note that
regexp-opt does not
guarantee that its result is absolutely the most efficient form
possible. A hand-tuned regular expression can sometimes be slightly
more efficient, but is almost never worth the effort.