The wgmeeting-github-ircbot should support the syntax:
where GitHubURL is a URL that starts with https://github.com/ ... and ends with a number segment (optional # fragment), just as it currently supports the "github topic: GitHubURL" syntax, because it is easier to remember, and 100% of past uses of "topic: GitHubURL" were clearly intended to do just that, and had to be unnecessarily followed up by a redundant "github topic: GitHubURL" command.
E.g. from the #css IRC log search results:
2017-04-20 # shane Topic: https://github.com/w3c/fxtf-drafts/issues/73 22:46:43 PDT # shane Github Topic: https://github.com/w3c/fxtf-drafts/issues/73 22:46:57 PDT # shane Topic: https://github.com/w3c/fxtf-drafts/issues/69 23:05:39 PDT # shane Github Topic: https://github.com/w3c/fxtf-drafts/issues/69 23:05:44 PDT # shane Topic: https://github.com/w3c/fxtf-drafts/issues/51 23:16:00 PDT # shane Github topic: https://github.com/w3c/fxtf-drafts/issues/51 23:16:06 PDT 2017-05-24 # myles TOPIC: https://github.com/w3c/csswg-drafts/issues/1345 09:54:12 PDT # myles Github topic: https://github.com/w3c/csswg-drafts/issues/1345 09:54:25 PDT
Note that this is a very precise request for what to allow, with the text after "Topic: " immediately being an https://github.com/ URL without anything else preceding, and ending with a URL segment that is a number, to avoid errant use of other GitHub URLs.
Given that level of precision, this reflects existing attempted use in the logs 100%, and is sufficiently guarded against false positives, at least from the past ~4 years of history in the logs.