1. What does (?<!a) do?

    Zero-length negative lookbehind. Ensures that the string of a's is not preceded by another a.

  2. What does (?!b) do?

    Zero-length negative lookahead. Ensures the matched string of b's is not followed by another b.

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In reply to Re^3: Integrating match counts into regex matching by BrowserUk
in thread Integrating match counts into regex matching by pat_mc

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