Add the “lookbehind” match

[pacdiv]

Do you want to request a feature or report a bug?
Feature.

What is the expected behavior?
As a user, I want to be able to use the “lookbehind”, written (?<=y)x, to match 'x' only if 'x' is preceded by 'y'. More info at https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions#special-lookbehind.