It was an old TRE that had plenty of bugs and no performance gain over
regex(3). I disabled it by default in r323615, and there was some confusion
about what the knob does- likely due to poor naming on my part- to the tune
of "well, it sounds like it should speed things up" (mentioned by multiple
people).
To compound this, I have no intention of maintaining a second regex
implementation. If someone would like to step up and volunteer to maintain a
lean-and-mean implementation for grep, this is OK, but we have very few
volunteers to maintain even our primary regex implementation.
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