Oddly, lookbehinds are evil only in a specific backtracking world. We never got around to implementing arbitrary lookarounds in Hyperscan (https://github.com/intel/hyperscan) but if we had done something in the automata world to handle lookaround, lookbehinds are way easier than lookaheads.

To handle a lookbehind, you really only need to occasionally 'AND' together some states (not an operation you would normally do in a standard NFA whether Glushkov or Thompson). To handle lookaheads... well, it gets ugly.

Even for non-fixed length expressions?

Variable-length backward asserts are fine in automata-land, which is a bit peculiar.

It's something I really like about .NET's regular expressions. Lookbehind has no limitations and will just match backwards with all features you can use in other parts.

So depending on the language or flavor you're working in, running away isn't really necessary.

Lookbehinds stay behind.