It's a replacement for regular expressions in purpose, not according to theoretical equivalence criteria. The main selling points appear to be naming patterns

It's a replacement for regular expressions in purpose, not according to theoretical equivalence criteria. The main selling points appear to be naming patterns in order to reuse them from libraries, functions, and more systematic grouping and lookahead operators: increased expressive power, which probably matters only in complex situations far beyond the limits of appropriate use of grep-like tools, is only a minor benefit.