POSIX and PCRE are arguably redundant. They both support backreferences, which puts very significant constraints on their implementations. PCRE is at least func

POSIX and PCRE are arguably redundant. They both support backreferences, which puts very significant constraints on their implementations. PCRE is at least functionally a superset of POSIX, whether or not there's some quirky thing POSIX supports that PCRE does not. re2 adds a legitimate option to the menu of using NDFAs, which have the disadvantage of not supporting backreferences, but have the advantage of having constrained complexity of scanning a string. This does not come for free; you can conceivably end up with a compiled regexp of very large size with an NDFA approach, but most of the time you won't. The result may be generally slower than a PCRE-type approach, but it can also end up safer because you can be confident that there isn't a pathological input string for a given regexp that will go exponential. This is one of those cases where ~99% of the time, it doesn't really matter which you choose, but at the scale of the Entire Programming World, both options need to be available. I've got some security applications where I legitimately prefer the re2 implementation in Go because it is advantageous to be confident that the REs I write have no pathological cases in the arbitrary input they face. PCRE can be necessary in certain high-performance cases, as long as you can be sure you're not going to get that pathological input. RE engines don't quite engender the same emotions as programming languages as a whole, but this is not cheerleading, this is a sober engineering assessment. I use both styles in my code. I've even got one unlucky exe I've been working with lately that has both, because it rather irreducibly has the requirements for both. Professionally annoying, but not actually a problem.