I would rather go with:

symfony5_is_great

@Kolyunya I think the case is wrong in your example.

It should be:
symfony5IsGreat → symfony5_is_great.

@Seb33300 my idea was that sequences of letters should be separated from sequences of numbers disregarding the capitalization of letters. So the example I provided is indeed intentional: symfony5isGreat → symfony_5_is_great.

@OskarStark please let me know if this is the final decision regarding the patch, I will update the tests and the production code accordingly.

Although, I believe that

1. We should be separating parts of the string that are not semantically coherent. The separation should work word-wise. Symfony 5 are two words and they don't seem to be a single coherent lexical unit.
2. In natural writing numbers never stick to the preceding letters. Example:

I agree, your example makes sense, but this sounds more like a bugfix to me, also I cannot see a BC break here.

@nicolas-grekas WDYT?

I don't think this is worth it as this is a BC break.

Thanks for this proposal. I also think that we shouldn't do this change. In addition to the BC break, there are many legit cases (think of commercial product names, brands, etc.) where splitting by the numbers would be wrong.