I have tried hard, but have utterly failed to figure out why you have chosen the semantics for ^ you mention, tried to come up with a plausible use case, and I have utterly failed.

I find it distinctly counter intuitive.

I think the Principle of Least Astonishment compliant definition of ^ and $ would be that they match the start and end of the string offered for matching, ie: taking start+end into account.

The real use-case behind this is searching through a mmap'ed database file, for a particular regexp in a particular field of the records, with the minimum amount of copying.

The semantics you mention, makes ^ and $ useless in this, and as far as I can tell, any other scenario involving start+end arguments.