Currently insource:// allows to do regular expression only on the wikitext content. This content does include the title string. We could think of adding the possibility to do regex search on the title as well.
It would allow users to do more advanced searches on the title text.
It can be useful in cases where the wiki uses certain kind of naming conventions that are to find with the way we tokenize the title text. Allowing regex on title would help to workaround these kind of limitations.
Of course we would have to evaluate the performance implications regarding the overhead of adding a new trigram field but I think it's worth investigating this possibility while working on more advanced syntax support in Q4 goals (2017).
See T156460 for an example usecase where it could have been useful.