Port to PCRE2

As I'm sure you know, PCRE is no longer under development. Using PCRE2 instead would help ensure your users are less vulnerable to security problems. Moreover, older software tends to get trickier to build on newer systems as time goes by, so switching will probably make maintenance simpler in the long run.

This might be helpful: https://github.com/PCRE2Project/pcre2/issues/51