Parser

A parser is a software component that analyses a sequence of input tokens to determine its grammatical structure with respect to a given formal grammar. In programming language implementation, a parser processes source code produced by a lexer to produce a parse tree or an abstract syntax tree (AST) used by later stages such as semantic analysis and code generation. Parsers are used beyond compilers, in data processing to parse data formats like XML, JSON, or custom configuration languages, and in natural language processing to obtain syntactic structures from sentences. Output structures encode relationships such as precedence, nesting, and scope.

Parsers are broadly categorized by parsing strategy. Top-down parsers, including recursive-descent and LL(k) parsers, build structures from the start symbol using lookahead. Bottom-up parsers, such as shift-reduce parsers and LR, LALR, or canonical parsers, assemble parse trees by reducing input to the start symbol. Many grammars require elimination of left recursion or left factoring for practical LL parsing. Ambiguity in a grammar yields multiple parses; unambiguous grammars or disambiguation strategies are used. Parsers may produce concrete parse trees or abstract syntax trees, depending on the desired representation.

Useful tools include parser generators like Yacc/Bison, ANTLR, JavaCC, or Pegen, which emit code from grammar specifications. Libraries exist for many languages to implement custom or domain-specific languages and data formats. Parsing can be streaming or incremental; error handling and recovery are important for resilience. In NLP, statistical or neural parsers complement rule-based approaches. Performance considerations include time complexity, memory usage, and the ability to handle large inputs.