MINOR: regex: Use native PCRE API.

MINOR: regex: Use native PCRE API.

The pcreposix layer (in the pcre projetc) execute strlen to find
thlength of the string. When we are using the function "regex_exex*2",
the length is used to add a final \0, when pcreposix is executed a
strlen is executed to compute the length.

If we are using a native PCRE api, the length is provided as an
argument, and these operations disappear.

This is useful because PCRE regex are more used than POSIC regex.