How about, instead of specific regex matching, you ignore all signs that have non-survival colors, aka the base colors that are available with the paragraph sign.

That won't help people on servers with Chest shops, those signs are generally default black.

But this is an idea for an additional option, or several additional options: "ignore signs that have formatting", and "ignore signs that have formatting that can't be obtained in vanilla", which would include bold, italic, and all that as well.