in reply to RFC: Parsing with perl - Regexes and beyond
The first two definitions are recursive, so they are not "regular" any more.
You can argue that the first production is "tail recursive" which does not pose a problem with respect to regularity. Like a tail recursive function can be transformed into a loop.
The non-regularity comes with the 2nd production.
term -> '(' term ')'
The point is that term can grow to arbitrary length and we still should keep the correspondence between the two parentheses, which is not possible with (CS)-Regexes ("pumping lemma").
This last observation is the border where you need a CFL-parser and would be lost with Regexes alone.