According to RFC 7230, the reason-phrase section of a HTTP response can be empty. This regex expects a space before an optional reason-phrase. This diff patches it so the space is optional as well.
I discovered recently when integrating a JIRA instance with a self hosted version of Phabricator that the JIRA OAUTH endpoint would respond with just HTTP/1.1 200 instead of the much more common HTTP/1.1 200 OK message. This caused this regex to fail to match the successful response and JIRA account linking to fail, disappointing my corporate overlords.