# The special named assertions include:

    / <before pattern> /    # was /(?=pattern)/
    / <after pattern> /     # was /(?<pattern)/ 

    / <ws> /                # match whitespace by :w rules

    / <sp> /                # match a space char

The after assertion implements lookbehind by reversing the syntax tree and looking for things in the opposite order going to the left. It is illegal to do lookbehind on a pattern that cannot be reversed.