compute endpoint fails to match all capture group results

Created by: CristinaBirkel

Repro:

1. On sourcegraph.com, open up the API console.
2. Run the following query:

query {
  compute(query:"patterntype:regexp background-color:#([0-9]) count:all repo:^github\\.com/sourcegraph/sourcegraph$@87f96d3608db4f6c535897b90f34256cc5b093c2") {
    ... on ComputeMatchContext{
        matches {
            value
	}
    }
  }
}

Note that this matches groups with values 4, 3, and 1. 3. Now modify that query slightly by changing the capture group from [0-9] to [0-9a-z] and re-run the query.

Expected: The initial matches should all still be there, with any additions (in this case f) from the newly added a-z

Actual: 3 is missing from the results