Variable-length RegExp lookbehind does not work

[littledan]

The TC39 proposal for RegExp lookbehind specifies that variable-length lookbehinds are supposed to be supported. V8 implements this, and it's tested in these test262 tests, but it doesn't seem to be supported in the RegExp implementation in Moddable here.

A test case:

/(?<=a*)/

This produces the following error message:

/home/littledan/moddable/examples/helloworld/main.js:15: error: invalid regular expression: lookbehind assertion is not fixed length!

Did you consider supporting variable-length lookbehind? Maybe it doesn't work out on an embedded profile.

If Moddable implements lookbehind per spec, this will be helpful for the TC39 proposal reaching Stage 4, as well as limiting compatibility issues with non-Moddable JS.

[phoddie]

Hey @littledan, thanks for trying this out. RegExp Lookbehind is at Stage 3. We have generally waited until Stage 4 to implement new features.

We do plan to take a look at implementing Lookbehind sooner than later. RegExp features have often proven challenging especially when modifications to PCRE are required (as they are here... the error message you note above is from PCRE).

[littledan]

Thanks for responding. Good to understand where you are here. The change in label seems appropriate.

[phoddie]

Good news. We replaced PCRE with our own (brand new!) regular expression engine. The Stage 3 RegExp Lookbehind proposal now works. See our blog post, "A smaller, more compatible regular expression engine for XS", for further details.

[littledan]

Amazing!