A lack of `^` is equivalent to prepending `(.*)`, then trimming the match span to the end of that capture. And similarly for a lack of `$` (but suddenly I remember how nasty Python was before `.fullmatch` was added ...).

More interesting is word boundaries:

`\b` is just `\<|\>` though that should be bubbled up and usually only one side will actually produce a matchable regex.

`A\<B` is just `(A&\W)(\w&B)`, and similar for `\>`.