Lookbehinds are annoying to implement, because they require driving the character iteration backwards. Now every regex construct has to be parametrized on wheth

Lookbehinds are annoying to implement, because they require driving the character iteration backwards. Now every regex construct has to be parametrized on whether it is tracking forwards or backwards. It is not something which can be easily tacked on to an existing engine. I will plug my own Rust regex engine "regress", which supports arbitrary lookbehinds. Note fancy-regex only supports fixed-width lookbehinds. https://crates.io/crates/regress https://crates.io/crates/regress