For the specific case (phone numbers) the only real need i could ever see i prefix-matching being needed. Do you see any other use-cases at all? While i do think people should care about the technology they use, i don’t see demanding people learn regex as reasonable.
Regex as a backend however is good, since it is well-defined and extensible, should any other use-cases pop up (though i doubt it).
Then there is the case of input - the phone keyboard already has *. Though this being the only reasonable use-case, there could even be a dedicated tick-box for it.