…I bet you can find some input that would work differently on PIRE and PCRE. Just use an escape sequence that PCRE recognise, and PIRE interprets as ordinary characters.
Failing that, we may be able to find some input that is accepted by PCRE and rejected by PIRE; then some input that is accepted by PIRE and rejected by PCRE.
If any of those two hypothesis turn out to be true, then PIRE is neither forward nor backward compatible with PCRE. So it would truly be incompatible. I expect the provided example works on both simply because most simple inputs would work on both. Showing incompatibilities was probably not the goal of this example.
…I bet you can find some input that would work differently on PIRE and PCRE. Just use an escape sequence that PCRE recognise, and PIRE interprets as ordinary characters.
Failing that, we may be able to find some input that is accepted by PCRE and rejected by PIRE; then some input that is accepted by PIRE and rejected by PCRE.
If any of those two hypothesis turn out to be true, then PIRE is neither forward nor backward compatible with PCRE. So it would truly be incompatible. I expect the provided example works on both simply because most simple inputs would work on both. Showing incompatibilities was probably not the goal of this example.