This regexp has many parallel alternations

Change-Id: I8044f460aa7d18f20cb0452e9470557b87facd6d
Reviewed-on: https://go-review.googlesource.com/22471Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>