text: Parsing Expression Grammars (PEGs) are a formalism used to describe top-down parsers with backtracking. As PEGs do not provide a good error recovery mechanism, PEG-based parsers usually do not recover from syntax errors in the input, or recover from syntax errors using ad-hoc, implementation-specific features. The lack of proper error recovery makes PEG parsers unsuitable for using with Integrated Development Environments (IDEs), which need to build syntactic trees even for incomplete, syntactically invalid programs. We propose a conservative extension, based on PEGs with labeled failures, that adds a syntax error recovery mechanism for PEGs. This extension associates recovery expressions to labels, where a label now not only reports a syntax error but also uses this recovery expression to reach a synchronization point in the input and resume parsing. We give an operational semantics of PEGs with this recovery mechanism, and use an implementation based on such semantics to build a robust parser for the Lua language. We evaluate the effectiveness of this parser, alone and in comparison with a Lua parser with automatic error recovery generated by ANTLR, a popular parser generator.
  @cite_14: This second edition of Grune and Jacobs brilliant work presents new developments and discoveries that have been made in the field. Parsing, also referred to as syntax analysis, has been and continues to be an essential part of computer science and linguistics. Parsing techniques have grown considerably in importance, both in computer science, ie. advanced compilers often use general CF parsers, and computational linguistics where such parsers are the only option. They are used in a variety of software products including Web browsers, interpreters in computer devices, and data compression programs; and they are used extensively in linguistics.
summary: In this section, we discuss other error recovery approaches used by top-down parsers with backtracking, focusing on PEGs. Error handling in top-down parsing based on CFGs is a well-studied subject. Grune and Jacobs @cite_14 presents an overview of several error handling techniques used in this context.