alternatives ("|" symbol).  The original coding allowed the added ^ and $
constraints to be absorbed into the first and last alternatives, producing
a pattern that would match more than it should.  Per report from Eric Noriega.

I also changed the pattern to add an ARE director ("***:"), ensuring that
SIMILAR TO patterns do not change behavior if regex_flavor is changed.  This
is necessary to make the non-capturing parentheses work, and seems like a
good idea on general principles.

Back-patched as far as 7.4.  7.3 also has the bug, but a fix seems impractical
because that version's regex engine doesn't have non-capturing parens.