13.2.10 Non-capturing clusters

It is often required to specify a cluster (typically for quantification) but without triggering the capture of submatch information. Such clusters are called non-capturing. In such cases, use (?: instead of ( as the cluster opener. In the following example, the non-capturing cluster eliminates the “directory” portion of a given pathname, and the capturing cluster identifies the basename.

(pregexp-match "^(?:[a-z]*/)*([a-z]+)$" 
  "/usr/local/bin/mzscheme")
 ⇒ ("/usr/local/bin/mzscheme" "mzscheme")