Implement reference to non-existent group regex rule

[diokey]

This PR adds a new lint rule that catches references to a non-existent capture group.

To achieve this, I had to add a new AST node RegExpNumericBackReference and take into account octal escapes while parsing.

The backreference will match the group capture if it exists, otherwise, it will be treated as an octal escape char.

For example

    String.raw`let foo = /([abc]+)=\1/;foo;`, // matches first capture group
    String.raw`let foo = /([abc]+)=\2/;foo;`, // matches \2 escaped
    String.raw`let foo = /([abc]+)=\8/;foo;`, // matches '8'
    String.raw`let foo = /([abc]+)=\9/;foo;`, // matches '9'
    String.raw`let foo = /([abc]+)=\119/;foo;`, // matches '\t' followd by '9'
    String.raw`let foo = /([abc]+)=\338/;foo;`, // matches \33 (char code 255) followed by '8'
    String.raw`let foo = /([abc]+)=\377/;foo;`, // matches \377 (char code 255)
    String.raw`let foo = /([abc]+)=\777/;foo;`, // matches \77  (char code 63) followed by '7'

The PR catches cases where the backreference is not an octal escape and does not refer to any existing group capture.

    String.raw`let foo = /([abc]+)=\18/;foo;`, 
    String.raw`let foo = /([abc]+)=\49/;foo;`,
    String.raw`let foo = /([abc]+)=\78/;foo;`,
    String.raw`let foo = /([abc]+)=\99/;foo;`,
    String.raw`let foo = /(([abc])\19)+=\28/;foo;`,

I am no expert in either compilers or regex, so let me know if this is not the right approach. I used https://www.regular-expressions.info/backref.html and tested those example on https://regexr.com/

Thank you

[diokey]

@sebmck this is not related, but it looks like a wrong index. The condition below is always false. Maybe it should be get0(index) + 2

[diokey]

remove that change, but wanted to bring it to your attention

[sebmck]

Aha yeah it should be. Feel free to leave that in change in! Thanks for flagging.

[diokey]

Great, will do

[diokey]

Done

[sebmck]

Is there a way we could put this into tokenize instead?

[diokey]

Sure, will do

[diokey]

Done

[diokey]

@sebmck the CI is failing, but it doesn't look like it's related to this PR. Any ideas?

[sebmck]

This is awesome! Thank you so much!