There is a very simple response to this: if ripgrep were a 100% drop-in replacement for something like GNU grep, then there would be basically no point for it to exist in the first place. The whole point is that it is very similar but does some things differently that a lot of people consider "better." I imagine this is truish for the other tools mentioned in this thread as well. (And still other tools, like xsv, have no pre-existing standard. Standard UNIX tooling doesn't work on CSV.)
Have you considered making a "grep mode" for ripgrep? If it's triggered by the name of the binary, it could provide both the traditional flags as well as its own.
A couple of people have asked for it, but the ROI isn't worth it. Think about all the various POSIX compatibilities, locale support, subtle differences in flag meanings, supporting the different flavors of regex (BREs, EREs) and of course, all the various differences in the regex engine itself, such as leftmost-first vs leftmost-longest matching, and subcapture match locations as well.
I feel like people significantly underestimate the difficulty of being 100% compatible with another tool. Not even GNU grep is strictly POSIX compatible, for example. To get POSIX compatibility, you need to set the POSIXLY_CORRECT environment variable.
And then what do I gain from this? Do you think distros are going to start throwing away GNU grep for ripgrep? No, I don't think so. So what then? A few people will have the pleasure of using ripgrep with grep's flags, even though they are already significantly similar? Not. Worth. It.
Now... If someone does think it's worth it, then they are more than welcome to start that journey. Most of ripgrep is factored out into libraries, so in theory there is a lot of reuse possible. But if you want to support compatibility all the way down into the regex engine, then you might be hosed.
RMS' original convention for requesting GNU tools be compatible with questionable legacy behaviour was POSIX_ME_HARDER, but he toned it down as a gesture of goodwill to the other members of the POSIX committee.
> if ripgrep were a 100% drop-in replacement for something like GNU grep, then there would be basically no point for it to exist in the first place.
That seems to miss a huge lesson: backward compatibility eases transition, new features retain users.
Therefore, your assertion makes only sense if there would be no point in attracting existing users to use a tool whose added value is entirely irrelevant.
It doesn't miss any lesson. It's a trade off. The headline feature involves a default mode of operation that's fundamentally incompatible with GNU grep. Full stop.
ripgrep is an evolution on ag, which in turn is an evolution on ack. All three tools have similar defaults, so in fact, ripgrep preserves some amount of backward compatibility with previously established tools in terms of the default mode of operation. Just not GNU grep.
This goes further in that the intersection between ripgrep's features/flags and GNU grep's features is quite large---certainly much larger than the differences between them, which is just another form of preserving backward compatibility. This was done on purpose for exactly the lesson you're claiming I missed: backward compatibility eases transition.
(The context of this conversation was a 100% backward compatible version of ripgrep with GNU grep. See my other comments on ROI. Just because I can argue against 100% backward compatibility doesn't mean I've missed the importance of backward compatibility.)
