aⁿbⁿcⁿ

This is the classic, textbook example of a simple language that cannot be described with a context-free-grammar. This example illustrates how syntactic predicates can extend a parser beyond the bounds of context-free parsing. It is an adaptation of the grammar found in the Wikipedia PEG article.

aⁿbⁿcⁿ Input

To see a demonstration of how this grammar works:

• Click Generate above
• Click Configure
• Check "Save Parser Trace"
• Under "Traced Operators" check PRD then Save
• Click Parse
• Click "Display Trace" and "Full Screen"