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Vertical bar characters are used to separate alternative patterns. For example, the pattern
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.
Note: This topic was taken from the PCRE library manual. The PCRE library is open source software, written by Philip Hazel <email@example.com>, and copyright by the University of Cambridge, England.