Support for UTF-8 inputs. Required.

Support for ‘⁠\p{xx}⁠’ and ‘⁠\P{xx}⁠’ in regular expressions. Desirable and used by some CRAN packages. As of PCRE2, always present with support for UTF-8.

Support for just-in-time compilation. Desirable for speed (but only available as a compile-time option on certain architectures, and may be unused as unreliable on some of those, e.g. arm64).

Does match recursion use a stack (TRUE, the default for PCRE1 and PCRE2 older than 10.30) or a heap? See the discussion at https://www.pcre.org/original/doc/html/pcrestack.html (Added in R 3.4.0.). No longer relevant and always FALSE in PCRE2 since version 10.30 which no longer uses function recursion to remember backtracking positions.