$string = 'my name is mister smith';

if($string =~ /(mr\.|mister (\w+))/){
  #  if the string contains either 'mr.' or
  #  'mister', then a space, then some word chars
  #  matching for instance 'mister jones' but not
  #  'hey mister!'
  print "surname is $2";
  #  $1 contains the whole thing, $2 just the surname
}