In addition to the repetition operators there are an `or' and an `if' operator. These are given by
//a|"bc"/
//a%b/
The symbol | indicates the `or'. It indicates that either the
character a or the string bc will cause a match. The % sign is a kind of
`if'. The patterns matches to an a if it is followed by a b, but the
b isn't consumed yet. The difference with //ab/ would become very
clear in a replace statement as the last pattern would also take out the
b.