correctly detect lookaround for \K reporting

[tonycoz]

fixes #18123

[hvds]

I note that pod/perlre.pod still states:

            The use of "\K" inside of another lookaround assertion is allowed,
            but the behaviour is currently not well defined.

[xsawyerx]

I note that pod/perlre.pod still states:

            The use of "\K" inside of another lookaround assertion is allowed,
            but the behaviour is currently not well defined.

Any suggestion for how to rephrase this entry? Do we want to keep this "not well defined"?

[xsawyerx]

I could not replicate the podcheck.t test failure. @hvds, any feedback on this before merging?

[hvds]

@hvds, any feedback on this before merging?

I think the pod should explicitly say "The use of "\K" inside of another lookaround assertion is currently not allowed." Optionally it could also add a reference to #18134.

I haven't traced the code to verify if this actually correctly limits the scope of the disallowance, but the tests look like a reasonable selection. I'd be tempted to add some more though:

  qr{(?=(?=x)x\K)x}
  qr{(?=(?!y)x\K)x}
  qr{(?=(?!y\K)x)x}
  qr{(?!(?=x\K)y)x}

... and maybe a couple similarly for lookbehind and for lookahead/behind mixes.