DB<7> p 'okoK'=~ /(o|k)(?{print "#1 matched"})(k|i)(O|K)(K)/
#1 matched

##</code><code>##

  DB<12> p 'okoK'=~ /(o|k)(?{print "#1 matched"})(k|i)X(O|K)(K)/     # no "X", why bother?

  DB<13>