Regex C Engine Warning
Using `pd.read_csv(..., sep=", ", ...)` I'm now getting a warning about falling back to the C engine because regex parsing isn't supported in the C engine. That's fine, but this isn't actually using regex.

I don't have an idea for a good transition strategy, and maybe the ship has sailed, but perhaps there should be a separate `read_regex` or a `regex` keyword instead of emitting this warning for any string greater than length 1.

Pandas 0.16.0.
