Regex inconsistencies

[PgBiel]

Hello,
I've observed several inconsistencies between the regex pandoc uses when reading Typst documents and the regex Typst uses.

Here are a few of them:

1. Not all flags are supported. Typst regex supports the flags i, m, s, u, x. Of those, only i appears to be supported by Pandoc.
   For example, #(regex("(?m)a") in "A") compiles in Typst, but doesn't in Pandoc (3.1.11.1 via try.pandoc.org), with the error (line 1, column 2): parseRegex for Text.Regex.TDFA.Text failed:"({0,1}m)a" (line 1, column 4): unexpected '0' expecting an atom.
   • I especially miss the m (multiline) flag in order to be able to match the start of a line with ^ and the end of a line with $.
2. Unnamed capture groups are not supported: #(regex("(?:x)") in "x") compiles in Typst, but not in pandoc ((line 1, column 2): parseRegex for Text.Regex.TDFA.Text failed:"({0,1}:x)" (line 1, column 4): unexpected '0' expecting an atom).
   • This is needed to avoid unnecessary capture groups in the output, and is frequently used across my packages.
3. Explicitly named capture groups are not supported: #(regex("(?P<a>x)") in "x") compiles in Typst, but not in Pandoc ((line 1, column 2): parseRegex for Text.Regex.TDFA.Text failed:"({0,1}P<a>x)" (line 1, column 4): unexpected '0' expecting an atom).

Besides non-compilation, there are inconsistencies in the results of regex matching as well.

1. #(regex("[\s\S]+") in "x") returns true in Typst, but false in Pandoc.
2. #("a \n b" == "a \n b".match(regex("[^.]+")).text) returns true in Typst, but false in Pandoc. In general, [ ] seems to unable to accept newlines, when it should.

There are probably inconsistencies I haven't found yet as well, but they could be added to this issue as they are found.

[jgm]

Yes. Problem is that we can't just use the regex engine typst uses. We are limited to the Haskell ecosystem. So what I do is use the regex-tdfa package for the basics, and try to supplement it when possible for things it is missing. E.g. it is missing \d \w \s, ?, and +, so I just replace these with equivalents. Of course, this isn't 100% reliable, and we can already see a place where it produces bad results in your #1 -- (?m) is a special construction; ? here doesn't mean "0 or 1", but my hack just replaces the ? with {0,1} with terrible results.

I could switch to using another regex engine. Hackage has regex-pcre-builtin, which comes with the C sources so that an external dependency isn't introduced. I've tried to avoid using wrapped C libraries in pandoc, but maybe could reconcsider in this case. I imagine pcre would be pretty close.

I also reimplemented as much as I needed of the regex engine used by KDE for my skylighting library. This isn't currently published as a separate package, though.

[jgm]

Oh, I see there is now https://hackage.haskell.org/package/regex-rure
But this would make pandoc depend on an installation of librure.so/dylib somewhere; I want to avoid that and have a perfectly self-contained static binary.