Return a regexp to match a string in the list STRINGS.
Each string should be unique in STRINGS and should not contain any regexps,
quoted or not. If optional PAREN is non-nil, ensure that the returned regexp
is enclosed by at least one regexp grouping construct.
The returned regexp is typically more efficient than the equivalent regexp:
(let ((open (if PAREN "\\(" "")) (close (if PAREN "\\)" "")))
(concat open (mapconcat 'regexp-quote STRINGS "\\|") close))
If PAREN is `words', then the resulting regexp is additionally surrounded
by \< and \>.
If PAREN is `symbols', then the resulting regexp is additionally surrounded by \_< and \_>.