Ag supports the same regexes as Ack. I use the PCRE library. I only call pcre_study once, and I use the new PCRE-JIT[1] on systems where it's available. These tweaks add up to a 3-5x speedup over Ack when regex-matching.
"Perl Compatible" isn't really Perl compatible, see http://en.wikipedia.org/wiki/PCRE for details.