13.2.8 Clusters

Clustering, ie, enclosure within parens (...), identifies the enclosed subpattern as a single entity. It causes the matcher to capture the submatch, or the portion of the string matching the subpattern, in addition to the overall match.

(pregexp-match "([a-z]+) ([0-9]+), ([0-9]+)" "jan 1, 1970")
 ⇒ ("jan 1, 1970" "jan" "1" "1970")

Clustering also causes a following quantifier to treat the entire enclosed subpattern as an entity.

(pregexp-match "(poo )*" "poo poo platter") ⇒ ("poo poo " "poo ")

The number of submatches returned is always equal to the number of subpatterns specified in the regexp, even if a particular subpattern happens to match more than one substring or no substring at all.

(pregexp-match "([a-z ]+;)*" "lather; rinse; repeat;")
 ⇒ ("lather; rinse; repeat;" " repeat;")

Here the *-quantified subpattern matches three times, but it is the last submatch that is returned.

It is also possible for a quantified subpattern to fail to match, even if the overall pattern matches. In such cases, the failing submatch is represented by #f.

(define date-re
  ;match `month year' or `month day, year'.
  ;subpattern matches day, if present 
  (pregexp "([a-z]+) +([0-9]+,)? *([0-9]+)"))

(pregexp-match date-re "jan 1, 1970")
 ⇒ ("jan 1, 1970" "jan" "1," "1970")

(pregexp-match date-re "jan 1970")
 ⇒ ("jan 1970" "jan" #f "1970")

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