Parse escaped backreferences and subpatterns #88
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hamishknight
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Updated to disambiguate octal vs backreferences in the parser, and also extended the |
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| num <= numberOfPriorGroups { | ||
| src.advance(digits.count) | ||
| return .backreference(.absolute(num)) | ||
| } |
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Should we just return the octal literal?
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We can do, though we'd need to call into expectUnicodeScalar as we can only read up to 3 digits of octal, and it wouldn't cover the \0nn case unless we also add a condition for that here
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Where is that logic when we fall through?
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In expectUnicodeScalar, called from expectEscaped
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It seems more straight-forward to make the explicit call, especially if we have context-local constraints on the number of digits or other aspects of interpretation. Otherwise, would the general case have to reconstruct this logic?
Fall-through also means that any intermediary code may have to be concerned with or reason about this possibility. In general I'm a fan of local reasoning. But this can be done after this PR, and it's not a huge deal.
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Otherwise, would the general case have to reconstruct this logic?
I might not be following what you mean by this, but I don't think so. Both the backreference and unicode scalar cases have differences in what they accept, but there is a syntactic ambiguity between them. In general, we just need to make sure to try lex as a backreference before a unicode scalar. IMO if we change this logic to produce a scalar, we should also change the scalar logic to be able to produce a backreference so we no longer need to reason about which is called first. I'm happy to do that in a follow-up, but going to merge this for now to cleanup the option parsing PR
Missed this when implementing the rest of the group kinds. Because we don't backtrack, we should throw an error here after consuming a `*` in a group.
Throw an error if we reach the end of input before we encounter the closing delimiter we expect. Also add an overload of `lexUntil(eating:)` that takes a character.
Parse the escaped syntaxes for backreferences and subpatterns (the latter are so syntactically similar, it made sense to also parse them). This doesn't yet handle the non-escaped syntax for either, in particular Python-style backreferences `(?P=...)`. These are syntactically similar to groups (despite being atoms), so will require some more thought on how to parse.
Implement octal disambiguation for the `\nnn` syntax where a backreference is only formed if there have been that many prior groups, or it begins with 8 or 9, or is less than 10. In addition, generalize the \0nn syntax to support arbitrary \nnn octal sequences inside and outside character classes.
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Parse the escaped syntaxes for backreferences and subpatterns (the latter are so syntactically similar, it made sense to also parse them).
This doesn't yet handle the non-escaped syntax for either, in particular Python-style backreferences
(?P=...). These are syntactically similar to groups (despite being atoms), so will require some more thought on how to parse.