search error containing `@`: parsing regex: "missing closing )"

Created by: CristinaBirkel

Repro:

1. Run this query on sourcegraph.com:

query {
  compute(query:"patterntype:regexp TODO @([\\w]+) count:all repo:^github\\.com/sourcegraph/handbook") {
	... on ComputeMatchContext{
          matches {
              value
	  }
    }
  }
}

Expected: Should return results. Note that if you change the repo to be sourcegraph/sourcegraph rather than sourcegraph/handbook it does return results.

Actual: Error message:

"1 error occurred:\n\t* Repo search failed: bad request: error parsing regexp: missing closing ): `(TODO).*?(`\n\n"