Thanks, the eg I tried before works now, I must have misinterpreted the output. Nice, a language can be recognized, even if a different parse tree. I vaguely

Thanks, the eg I tried before works now, I must have misinterpreted the output. Nice, a language can be recognized, even if a different parse tree. I vaguely recall the shunting yard algorithm... am I right in characterising your solution as a second level of parsing? It's not "pure", but from a practical point of view, it's no worse than back-references in PCRE. The practical issue is just of ui usability.