The Alternation Operator (`|' or `\|')
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  If the syntax bit `RE_LIMITED_OPS' is set, then Regex doesn't
recognize this operator.  Otherwise, if the syntax bit `RE_NO_BK_VBAR'
is set, then `|' represents this operator; otherwise, `\|' does.

  Alternatives match one of a choice of regular expressions: if you put
the character(s) representing the alternation operator between any two
regular expressions A and B, the result matches the union of the
strings that A and B match.  For example, supposing that `|' is the
alternation operator, then `foo|bar|quux' would match any of `foo',
`bar' or `quux'.

  The alternation operator operates on the *largest* possible
surrounding regular expressions.  (Put another way, it has the lowest
precedence of any regular expression operator.) Thus, the only way you
can delimit its arguments is to use grouping.  For example, if `(' and
`)' are the open and close-group operators, then `fo(o|b)ar' would
match either `fooar' or `fobar'.  (`foo|bar' would match `foo' or
`bar'.)

  The matcher usually tries all combinations of alternatives so as to
match the longest possible string.  For example, when matching
`(fooq|foo)*(qbarquux|bar)' against `fooqbarquux', it cannot take, say,
the first ("depth-first") combination it could match, since then it
would be content to match just `fooqbar'.