3 ms·

BRE vs ERE is really a non-issue for me: the way they're done in GNU's regular expression implementation, BRE and ERE are the same except for small differences

by hvdijk 4y ago

BRE vs ERE is really a non-issue for me: the way they're done in GNU's regular expression implementation, BRE and ERE are the same except for small differences in when you do and don't put \ in front of certain characters. GNU supports \< and \> in BRE and in ERE mode exactly the same way, and I do use ERE mode a lot as well. I had a stab at implementing this to see how easy or difficult it would be. Coming from someone who doesn't write Rust: it seems to be pretty much trivial. A quick and dirty implementation is at <https://github.com/hvdijk/regex/commit/511c6485e49c22c79b47a3663a97624348343e6d https://github.com/hvdijk/regex/commit/511c6485e49c22c79b47a...>. I guess I can use ripgrep with this patch for now to stop my own complaining.

burntsushi 4y ago
> BRE and ERE are the same except for small differences in when you do and don't put \ in front of certain characters I guess I'd argue that EREs and the regex crate syntax are the same except for small differences too. :P And I'd absolutely categorize "use \b instead of \< or \>" as a pretty small difference. In any case, yes, I agree that adding \< and \> to the regex crate is not a difficult implementation challenge. To me, it's not "can we" but "should we." Another question is whether we need to also add the negation of each of \< and \> too, to match \b and \B. (I don't know of any commonly accepted syntax for the negation of \< or \>.)
- hvdijk 4y ago
  > And I'd absolutely categorize "use \b instead of \< or \>" as a pretty small difference. I don't know, to me \b always felt like a bad idea. Start of string (^) and end of string ($) make sense as different symbols, I'm not aware of people asking for a single "boundary of string" symbol that acts as (^|$), I'm not aware of any realistic use case for it that isn't better written using plain old ^ or $. I feel the same way about \< and \>: in any place where I would otherwise have to write \b I already know whether I'll want "start of word" or "end of word" and it won't make sense to allow the other, but \b forces that to be accepted too. That said... > Another question is whether we need to also add the negation of each of \< and \> too, to match \b and \B. (I don't know of any commonly accepted syntax for the negation of \< or \>.) ...the negation of word boundaries clearly has uses, but the negation of \< or \> would be a new invention that would add more incompatibilities between ripgrep and other implementations. As much as I'd be in favour of a negation of \< and \>, I'd be more in favour of something that also works in other implementations, which is the \B that you already have. Just my personal views. I'm served either way by my locally patched version of ripgrep, I'll assume you know better what the rest of your users want.

by hvdijk 4y ago

burntsushi 4y ago

> BRE and ERE are the same except for small differences in when you do and don't put \ in front of certain characters I guess I'd argue that EREs and the regex crate syntax are the same except for small differences too. :P And I'd absolutely categorize "use \b instead of \< or \>" as a pretty small difference. In any case, yes, I agree that adding \< and \> to the regex crate is not a difficult implementation challenge. To me, it's not "can we" but "should we." Another question is whether we need to also add the negation of each of \< and \> too, to match \b and \B. (I don't know of any commonly accepted syntax for the negation of \< or \>.)

hvdijk 4y ago
> And I'd absolutely categorize "use \b instead of \< or \>" as a pretty small difference. I don't know, to me \b always felt like a bad idea. Start of string (^) and end of string ($) make sense as different symbols, I'm not aware of people asking for a single "boundary of string" symbol that acts as (^|$), I'm not aware of any realistic use case for it that isn't better written using plain old ^ or $. I feel the same way about \< and \>: in any place where I would otherwise have to write \b I already know whether I'll want "start of word" or "end of word" and it won't make sense to allow the other, but \b forces that to be accepted too. That said... > Another question is whether we need to also add the negation of each of \< and \> too, to match \b and \B. (I don't know of any commonly accepted syntax for the negation of \< or \>.) ...the negation of word boundaries clearly has uses, but the negation of \< or \> would be a new invention that would add more incompatibilities between ripgrep and other implementations. As much as I'd be in favour of a negation of \< and \>, I'd be more in favour of something that also works in other implementations, which is the \B that you already have. Just my personal views. I'm served either way by my locally patched version of ripgrep, I'll assume you know better what the rest of your users want.