What ECMAScript implementations extend the RegExp syntax?

Question

So I know in JavaScript an implementation is allowed to extend the regular expressions grammar:

An implementation may extend the ECMAScript Regular Expression grammar defined in 21.2.1, but it must not extend the RegularExpressionBody and RegularExpressionFlags productions defined below or the productions used by these productions.

Was this ability ever used? Do any existing JavaScript implementations extend the regular expressions grammar?

Answer 1

Yes, Mozilla's Gecko engine did support the sticky y flag, which was not part of ES5. It did eventually became part of ES6.

This ability may be utilised again when engines start implementing look-behind (I hope they start experimenting before it will get specced).

_{This is not an exhaustive list, just what first came to my mind. There may be other examples.}