Is “regex” in modern programming languages really “context sensitive grammar”?

Question

Over the years, \"regex\" pattern matching has been getting more and more powerful to the point where I wonder: is it really just context-sensitive-grammar matching? Is it a

Answer 1

There are features in modern regular expression implementations that break the rules of the classic regular expression definition.

For example Microsoft’s .NET Balancing Group (? … ):

^(?:0(?)|1(?<-L>))*(?(L)(?!))$

This does match the language L₀₁ = {ε, 01, 0011, 000111, … }. But this language is not regular according to the Pumping Lemma.