Add the “negated lookahead” match

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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 “negated lookahead”, written x(?!y), to match 'x' only if 'x' is not followed by 'y'. More info at https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions#special-negated-look-ahead.