While I have no objection to including LPeg with the standard Lua distribution, I would strongly object to having it replace the current patterns. The beauty of the current patterns is their simplicity and easy learning curve. I find neither to be true about LPeg. I've tried to use LPeg a few times, and I find it to be confusing and not at all intuitive. In part, the use of single letters for function names leaves me scratching my head, and I can never remember what operators mean what. LPeg is powerful, yes, so I can see how it is useful. But if added to the stock Lua distribution, it should be its own standard library module (preferably with longer and more descriptive function names), without replacing patterns.
Also, unless I misunderstood something or something has changed since I last looked at it, LPeg cannot do anything requiring backtracking, so there are things matchable by patterns but unmatchable by LPeg. Possible example being "capture the three characters between a question mark and a dollar sign, immediately before the dollar sign", for which the (untested) pattern "%?.-([^$][^$][^$])%$" should match easily, but AFAICT, there is no equivalent LPeg pattern since the only way is to match up to the $ and then back up three characters.
If I am misunderstanding something, I invite somebody to correct and educate me.