Trying to Understand PEG

Parsing Expression Grammar (PEG) encodes a recursivedescent parser with limited backtracking. Its properties are useful in many applications. In its appearance, PEG is almost identical to a grammar in the Extended Backus-Naur Form (EBNF), but may define a different language. Recent research formulated some conditions under which PEG is equivalent to its interpretation as EBNF. However, PEG has a useful feature, namely syntactic predicate, that is alien to EBNF. The equivalence results apply thus only to PEG without predicates. The paper considers PEG with predicates. Not being able to investigate equivalence, the paper turns to the limited backtracking that is the main source of difficulty in understanding PEG. It is shown that the limitation of backtracking has no effect under conditions similar to those for PEG without predicates. There is, in general, no mechanical way to check these conditions, but they can be often checked by inspection. The paper outlines an experimental tool to facilitate such inspection.