The wgmeeting-github-ircbot should support the syntax:

topic: GitHubURL

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.