Syntax Analysis


In compiler design, syntax analysis (or parsing) is the second phase of the compilation process, following lexical analysis. It involves verifying whether the linear sequence of tokens generated by the lexical analyzer conforms to the grammar of the programming language. The goal of syntax analysis is to construct a representation, usually a parse tree or an abstract syntax tree (AST), depicting the hierarchical syntactic structure of the source code.

Key Concepts

  • Grammar: A set of rules defining the correct structure of sentences in a programming language. Context-free grammars (CFGs) are commonly used to describe programming languages.
  • Tokens: The basic units of code recognized by the lexical analyzer (e.g., keywords, identifiers, operators, punctuation).
  • Parse Tree: A tree-like structure representing the derivation of the input program from the grammar. The root of the tree is the start symbol of the grammar, and the leaves are the tokens of the program.
  • Abstract Syntax Tree (AST): A simplified and more structured representation of the source code, focusing on the program's essential elements and their relationships.

Types of Parsers

  • Top-down Parsers: These parsers start from the root of the parse tree and attempt to derive the input string of tokens. They use predictive techniques to choose grammar productions. Examples include LL parsers and recursive descent parsers.
  • Bottom-up Parsers: Start at the leaves of the parse tree (the tokens) and work upwards towards the root. They use techniques like shifting and reducing actions to construct the parse tree. Examples include LR parsers (e.g., SLR, LALR) and operator-precedence parsers.

Error Recovery

Syntax analyzers are expected to detect and report syntax errors in the source code. Error recovery strategies are implemented in parsers to allow them to continue parsing the rest of the input and avoid cascading errors. Common techniques include:

  • Panic Mode: Skipping tokens until a synchronization point is found.
  • Phrase-level Recovery: Attempting to repair local errors.
  • Error Productions: Including special productions in the grammar to handle common errors.


  • Compilers: Parsing is a fundamental component of compilers, ensuring that code is syntactically correct before further stages of translation and optimization.
  • Interpreters: Parsers are used to analyze and execute code on the fly.
  • Natural Language Processing: Parsing techniques are applied in the analysis of natural language structures.
  • IDEs (Integrated Development Environments): Parsers power features like syntax highlighting and code completion.


Consider a simple grammar for arithmetic expressions:

expr   -> term + expr | term
term   -> factor * term | factor
factor -> ( expr ) | number

A syntax analyzer would process an input string like "2 + (3 * 4)" and construct a parse tree or AST representing the expression's hierarchical structure.


Syntax analysis plays a critical role in ensuring the well-formedness of programs. By validating code structure against the language's grammar, the compiler can detect and report errors early on, aiding in the development of correct and reliable software.