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34.6.2 Simple Match Data Access

This section explains how to use the match data to find out what was matched by the last search or match operation, if it succeeded.

You can ask about the entire matching text, or about a particular parenthetical subexpression of a regular expression. The count argument in the functions below specifies which. If count is zero, you are asking about the entire match. If count is positive, it specifies which subexpression you want.

Recall that the subexpressions of a regular expression are those expressions grouped with escaped parentheses, ‘\(...\)’. The countth subexpression is found by counting occurrences of ‘\(’ from the beginning of the whole regular expression. The first subexpression is numbered 1, the second 2, and so on. Only regular expressions can have subexpressions—after a simple string search, the only information available is about the entire match.

Every successful search sets the match data. Therefore, you should query the match data immediately after searching, before calling any other function that might perform another search. Alternatively, you may save and restore the match data (see Saving Match Data) around the call to functions that could perform another search. Or use the functions that explicitly do not modify the match data; e.g., string-match-p.

A search which fails may or may not alter the match data. In the current implementation, it does not, but we may change it in the future. Don't try to rely on the value of the match data after a failing search.

— Function: match-string count &optional in-string

This function returns, as a string, the text matched in the last search or match operation. It returns the entire text if count is zero, or just the portion corresponding to the countth parenthetical subexpression, if count is positive.

If the last such operation was done against a string with string-match, then you should pass the same string as the argument in-string. After a buffer search or match, you should omit in-string or pass nil for it; but you should make sure that the current buffer when you call match-string is the one in which you did the searching or matching. Failure to follow this advice will lead to incorrect results.

The value is nil if count is out of range, or for a subexpression inside a ‘\|’ alternative that wasn't used or a repetition that repeated zero times.

— Function: match-string-no-properties count &optional in-string

This function is like match-string except that the result has no text properties.

— Function: match-beginning count

This function returns the position of the start of the text matched by the last regular expression searched for, or a subexpression of it.

If count is zero, then the value is the position of the start of the entire match. Otherwise, count specifies a subexpression in the regular expression, and the value of the function is the starting position of the match for that subexpression.

The value is nil for a subexpression inside a ‘\|’ alternative that wasn't used or a repetition that repeated zero times.

— Function: match-end count

This function is like match-beginning except that it returns the position of the end of the match, rather than the position of the beginning.

Here is an example of using the match data, with a comment showing the positions within the text:

     (string-match "\\(qu\\)\\(ick\\)"
                   "The quick fox jumped quickly.")
                   ;0123456789
          ⇒ 4
     
     (match-string 0 "The quick fox jumped quickly.")
          ⇒ "quick"
     (match-string 1 "The quick fox jumped quickly.")
          ⇒ "qu"
     (match-string 2 "The quick fox jumped quickly.")
          ⇒ "ick"
     
     (match-beginning 1)       ; The beginning of the match
          ⇒ 4                 ;   with ‘qu’ is at index 4.
     
     (match-beginning 2)       ; The beginning of the match
          ⇒ 6                 ;   with ‘ick’ is at index 6.
     
     (match-end 1)             ; The end of the match
          ⇒ 6                 ;   with ‘qu’ is at index 6.
     
     (match-end 2)             ; The end of the match
          ⇒ 9                 ;   with ‘ick’ is at index 9.

Here is another example. Point is initially located at the beginning of the line. Searching moves point to between the space and the word ‘in’. The beginning of the entire match is at the 9th character of the buffer (‘T’), and the beginning of the match for the first subexpression is at the 13th character (‘c’).

     (list
       (re-search-forward "The \\(cat \\)")
       (match-beginning 0)
       (match-beginning 1))
         ⇒ (17 9 13)
     
     ---------- Buffer: foo ----------
     I read "The cat -!-in the hat comes back" twice.
             ^   ^
             9  13
     ---------- Buffer: foo ----------

(In this case, the index returned is a buffer position; the first character of the buffer counts as 1.)