As noted on their Phabricator recently, Miraheze is considering retirement of the RegexFunctions extension from their list during their transition to MW 1.40. As Agent Isai (talk · contribs) has pointed out, "This extension has some very long standing security issues which led to be disabled globally over a year ago. Glancing at the git repo...nothing has changed. [It] should thus be undeployed." While porting conlang-dictionary entries from Referata to my MH site, yours truly found out the hard way back then (as documented at length in T8866).
In case you've caught this message now or in the next few days, maybe you could try upgrading it to a level that meets MH's security threshold, or take up the matter to a developer who knows their way around regex. Perhaps Wikimedia's own Phab might be informed next?
To @Bawolff: From here, I'll leave further commentary/ideas/suggestions to you. --Slgrandson (talk) 20:14, 26 May 2023 (UTC)