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Fix similar_escape() so that SIMILAR TO works properly for patterns involving
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.