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21.5.2 Match Structures

A match structure is the object returned by string-match and regexp-exec. It describes which portion of a string, if any, matched the given regular expression. Match structures include: a reference to the string that was checked for matches; the starting and ending positions of the regexp match; and, if the regexp included any parenthesized subexpressions, the starting and ending positions of each submatch.

In each of the regexp match functions described below, the match argument must be a match structure returned by a previous call to string-match or regexp-exec. Most of these functions return some information about the original target string that was matched against a regular expression; we will call that string target for easy reference.

regexp-match? obj Scheme Procedure
Return #t if obj is a match structure returned by a previous call to regexp-exec, or #f otherwise.

match:substring match [n] Scheme Procedure
Return the portion of target matched by subexpression number n. Submatch 0 (the default) represents the entire regexp match. If the regular expression as a whole matched, but the subexpression number n did not match, return #f. 

(define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
(match:substring s)
=> "2002"

;; match starting at offset 6 in the string
(match:substring
  (string-match "[0-9][0-9][0-9][0-9]" "blah987654" 6))
=> "7654"

match:start match [n] Scheme Procedure
Return the starting position of submatch number n.

In the following example, the result is 4, since the match starts at character index 4: 

(define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
(match:start s)
=> 4

match:end match [n] Scheme Procedure
Return the ending position of submatch number n.

In the following example, the result is 8, since the match runs between characters 4 and 8 (i.e. the "2002"). 

(define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
(match:end s)
=> 8

match:prefix match Scheme Procedure
Return the unmatched portion of target preceding the regexp match. 
(define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
(match:prefix s)
=> "blah"

match:suffix match Scheme Procedure
Return the unmatched portion of target following the regexp match. 

(define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
(match:suffix s)
=> "foo"

match:count match Scheme Procedure
Return the number of parenthesized subexpressions from match. Note that the entire regular expression match itself counts as a subexpression, and failed submatches are included in the count.

match:string match Scheme Procedure
Return the original target string. 

(define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
(match:string s)
=> "blah2002foo"