Vertical bar characters are used to separate alternative
patterns. For example, the pattern
gilbert|sullivan
matches either "gilbert" or "sullivan". Any number of alternatives
may appear, and an empty alternative is permitted
(matching the empty string). The matching process tries
each alternative in turn, from left to right, and the first
one that succeeds is used. If the alternatives are within a
subpattern (defined below), "succeeds" means matching the
rest of the main pattern as well as the alternative in the
subpattern.
It is possible to record which alternative was matched by using
(*MARK:NAME) or (*:NAME).
Any number of such (*MARK) verbs may appear and their
names do not have to be unique. When a match succeeds, the name of the
last-encountered (*MARK:NAME) will be placed among
matches as if it were a capture group called MARK so it
can be read from the matches of
preg_match() and will be passed to the
callback of preg_replace_callback() etc.