The attached patch adds and documents this option:
Use an alternative regular expression engine when searching. Pass the option,
e.g., "RE2" to load and use L<re::engine::RE2>.
provide a re-engine option
Why would someone want to do this? I don't understand the benefit, and there's nothing in the docs that explains the benefit.
Perl's built-in regular expression engine is quite slow, for one. There are a couple re::engine modules that use NFA algorithms (RE2, Plan9), performing far better on large bodies of text.
And someone else pointed this out to me, but using re::engine::PCRE would let one use ack to emulate grep -P.
If either of those are persuasive, I can write a follow-up patch to better-document the option.
This has been moved to the ack2 queue: petdance/ack2#161