Elisp: Match Data (Regex Result)

By Xah Lee. Date: . Last updated: .

What is Match Data

Elisp: Regex Functions store match result in match data. Such as caputured groups, or begin/end positions of the captured pattern, or whole matched string.

Match data can be accessed by several functions.

Get Match String (Captured Groups)

Whenever you call regex functions such as • re-search-forward • string-match the captured text is stored in match-string.

match-string
(match-string NUM &optional STRING)

Return string of text matched by last search.

NUM an is integer.

  • 0 means the whole matched text.
  • 1 means first captured group.
  • 2 means second captured group.
  • etc.

If the last search is buffer text search, STRING should be nil. But if last regex search is done on a string by string-match, it should be the string that is searched. 

;; example of match-string

(re-search-forward "id=\\([0-9]+\\)" )
(message "%s" (match-string 1 ))
;; prints 172

;; id=172

(setq xx "swimming in sea")

(string-match "\\([a-z]+?ing\\) " xx)

(match-string 1 xx)
;; "swimming"
match-string-no-properties
(match-string-no-properties n)

similar to match-string but without Text Properties

Get Begin/End Positions (of Captured Groups)

match-beginning
(match-beginning n)

return the beginning position of the nth captured string of last regex search.

0 means the whole match. 

(let ((case-fold-search nil) p1 p2)
  (re-search-forward "\\([0-9]+\\)")
  (setq p1 (match-beginning 1))
  (setq p2 (match-end 1))
  (buffer-substring-no-properties p1 p2))
;; sample text 123 abc
;; "123"
match-end
(match-end n)

return the end position of the nth captured string of last regex search.

Other Match Data Functions

save-match-data
(save-match-data &rest BODY)

save and restore match data.

Reference

Emacs Lisp, Regex in Lisp Code