Regular expressions are not my favourite either, but one thing I will say is that normal code becomes difficult to read over time as features are bolted on and

Regular expressions are not my favourite either, but one thing I will say is that normal code becomes difficult to read over time as features are bolted on and optimizations are added to address performance concerns. Whereas, some form of executable DSL can serve as self-testing documentation indefinitely. DSLs can provide a separation of concerns, where the syntax is designed for readability, and the engine is designed for performance. Modern Regexen are a dumpster fire, of course, but you asked why someone might want to, not whether this is something Everyone might want to do.