If original build_regex can return expression with "start of string" or "end of string" markers, these should be replaced, for my approach, with lookarounds for $sep, of course.

Huge regex with many alternations is different to what I suggested, and may suit better your data/expected output.

In reply to Re^3: Multi-thread combining the results together by vr
in thread Multi-thread combining the results together by Marshall

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