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### 33.4 Regular Expression Searching

In GNU Emacs, you can search for the next match for a regular expression (see Syntax of Regexps) either incrementally or not. For incremental search commands, see Regular Expression Search in The GNU Emacs Manual. Here we describe only the search functions useful in programs. The principal one is `re-search-forward`.

These search functions convert the regular expression to multibyte if the buffer is multibyte; they convert the regular expression to unibyte if the buffer is unibyte. See Text Representations.

Command: re-search-forward regexp &optional limit noerror count

  This function searches forward in the current buffer for a string of text that is matched by the regular expression regexp. The function skips over any amount of text that is not matched by regexp, and leaves point at the end of the first match found. It returns the new value of point.

  If limit is non-`nil`, it must be a position in the current buffer. It specifies the upper bound to the search. No match extending after that position is accepted. If limit is omitted or `nil`, it defaults to the end of the accessible portion of the buffer.

  What `re-search-forward` does when the search fails depends on the value of noerror:

  `nil`

    Signal a `search-failed` error.

  `t`

    Do nothing and return `nil`.

  anything else

    Move point to limit (or the end of the accessible portion of the buffer) and return `nil`.

  The argument noerror only affects valid searches which fail to find a match. Invalid arguments cause errors regardless of noerror.

  If count is a positive number n, the search is done n times; each successive search starts at the end of the previous match. If all these successive searches succeed, the function call succeeds, moving point and returning its new value. Otherwise the function call fails, with results depending on the value of noerror, as described above. If count is a negative number -n, the search is done n times in the opposite (backward) direction.

  In the following example, point is initially before the ‘`T`’. Evaluating the search call moves point to the end of that line (between the ‘`t`’ of ‘`hat`’ and the newline).

      ---------- Buffer: foo ----------
      I read "∗The cat in the hat
      comes back" twice.
      ---------- Buffer: foo ----------

      (re-search-forward "[a-z]+" nil t 5)
           ⇒ 27

      ---------- Buffer: foo ----------
      I read "The cat in the hat∗
      comes back" twice.
      ---------- Buffer: foo ----------

Command: re-search-backward regexp &optional limit noerror count

  This function searches backward in the current buffer for a string of text that is matched by the regular expression regexp, leaving point at the beginning of the first text found.

  This function is analogous to `re-search-forward`, but they are not simple mirror images. `re-search-forward` finds the match whose beginning is as close as possible to the starting point. If `re-search-backward` were a perfect mirror image, it would find the match whose end is as close as possible. However, in fact it finds the match whose beginning is as close as possible (and yet ends before the starting point). The reason for this is that matching a regular expression at a given spot always works from beginning to end, and starts at a specified beginning position.

  A true mirror-image of `re-search-forward` would require a special feature for matching regular expressions from end to beginning. It’s not worth the trouble of implementing that.

Function: string-match regexp string &optional start

  This function returns the index of the start of the first match for the regular expression regexp in string, or `nil` if there is no match. If start is non-`nil`, the search starts at that index in string.

  For example,

      (string-match
       "quick" "The quick brown fox jumped quickly.")
           ⇒ 4

      (string-match
       "quick" "The quick brown fox jumped quickly." 8)
           ⇒ 27

  The index of the first character of the string is 0, the index of the second character is 1, and so on.

  If this function finds a match, the index of the first character beyond the match is available as `(match-end 0)`. See Match Data.

      (string-match
       "quick" "The quick brown fox jumped quickly." 8)
           ⇒ 27

      (match-end 0)
           ⇒ 32

Function: string-match-p regexp string &optional start

  This predicate function does what `string-match` does, but it avoids modifying the match data.

Function: looking-at regexp

  This function determines whether the text in the current buffer directly following point matches the regular expression regexp. “Directly following” means precisely that: the search is “anchored” and it can succeed only starting with the first character following point. The result is `t` if so, `nil` otherwise.

  This function does not move point, but it does update the match data. See Match Data. If you need to test for a match without modifying the match data, use `looking-at-p`, described below.

  In this example, point is located directly before the ‘`T`’. If it were anywhere else, the result would be `nil`.

      ---------- Buffer: foo ----------
      I read "∗The cat in the hat
      comes back" twice.
      ---------- Buffer: foo ----------

      (looking-at "The cat in the hat$")
           ⇒ t

Function: looking-back regexp limit &optional greedy

  This function returns `t` if regexp matches the text immediately before point (i.e., ending at point), and `nil` otherwise.

  Because regular expression matching works only going forward, this is implemented by searching backwards from point for a match that ends at point. That can be quite slow if it has to search a long distance. You can bound the time required by specifying a non-`nil` value for limit, which says not to search before limit. In this case, the match that is found must begin at or after limit. Here’s an example:

      ---------- Buffer: foo ----------
      I read "∗The cat in the hat
      comes back" twice.
      ---------- Buffer: foo ----------

      (looking-back "read \"" 3)
           ⇒ t
      (looking-back "read \"" 4)
           ⇒ nil

  If greedy is non-`nil`, this function extends the match backwards as far as possible, stopping when a single additional previous character cannot be part of a match for regexp. When the match is extended, its starting position is allowed to occur before limit.

  As a general recommendation, try to avoid using `looking-back` wherever possible, since it is slow. For this reason, there are no plans to add a `looking-back-p` function.

Function: looking-at-p regexp

  This predicate function works like `looking-at`, but without updating the match data.

Variable: search-spaces-regexp

  If this variable is non-`nil`, it should be a regular expression that says how to search for whitespace. In that case, any group of spaces in a regular expression being searched for stands for use of this regular expression. However, spaces inside of constructs such as ‘`[…]`’ and ‘`*`’, ‘`+`’, ‘`?`’ are not affected by `search-spaces-regexp`.

  Since this variable affects all regular expression search and match constructs, you should bind it temporarily for as small as possible a part of the code.

Next: POSIX Regexps, Previous: Regular Expressions, Up: Searching and Matching [Contents][Index]