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Well, not all regex engines support variable-length lookbehinds. But I know for a fact that calibre's Editor does, because it was calibre that I tested it with!
Furthermore, calibre does not have a "RegEx builder", it has a single input box and it doesn't go red when there is an error -- it pops up an error dialog.
So clearly you did not try this regex using calibre's Editor, and given that this is the subforum for the calibre Editor, I cannot be expected to read your mind and know which piece of software you decided to use instead (without telling me).
So... try using calibre's Editor.
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