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Avoid escaping literal string twice

Fix failure in
    ("(" | ")")("(")
inside an instance of RegexParsers.
The problem is the implementation of the `|` operator in RegexParsers.

When running the above code, "(" is escaped twice. Printing the above
parser results in:

/(\\\()|(\))/

and using a debugger confirms the result.

With this fix, the existing regular expression is not escaped again.
Note that escapeRegex escapes a normal string to produce a regex,
unlike the name may suggest.

A complete example reproducing the issue is available at:
https://gist.github.com/Blaisorblade/4743188
Add testcase for previous bugfix

I verified that this test fails without the bugfix.