Introduction to Parsers

Parsing is also known as syntactic analysis, is the process of analyzing a sequence of tokens to determine their grammatical structure according to the rules of a language. It organizes tokens into a parse tree or syntax tree, which represents the structural relationship between different parts of the input.

• Converts tokens into a structured parse tree
• Helps understand the grammatical structure of text or code
• Ensures the input follows syntax rules
• Used in compilers, interpreters, and NLP applications
• Supports further tasks like semantic analysis and code execution

Role of Parser

A parser performs syntactic and semantic analysis of source code by converting tokens into a structured intermediate representation while identifying errors in the code.

• Checks whether code follows grammar and syntax rules
• Supports context-sensitive analysis such as type checking
• Generates parse trees or Abstract Syntax Trees (ASTs)
• Detects and reports syntax errors clearly
• Attempts basic error recovery during parsing

Types of Parsing

The parsing is divided into two types

• Top-down Parsing
• Bottom-up Parsing

1. Top-Down Parsing

Top-down parsing builds the parse tree from the start symbol toward the input symbols.

• Starts from the start symbol
• Expands non-terminals using production rules
• Uses leftmost derivation
• Also known as recursive descent or predictive parsing
• Easier to implement for simple grammars

Types of Top-Down Parsing

With Backtracking

• Tries multiple production rules
• Backtracks if one rule fails
• Flexible but slower

Without Backtracking

• Does not retry other rules once a choice is made.
• Faster and more efficient.
• Works only for suitable grammars (e.g., LL grammars).

2. Bottom-Up Parsing

Bottom-up parsing builds the parse tree from input symbols toward the start symbol.

• Starts from the input string
• Reduces substrings into non-terminals
• Uses rightmost derivation in reverse
• Also known as Shift-Reduce Parsing
• Efficient for complex grammars and compiler design

Types of Bottom-Up Parsing

• LR Parsing
• SLR(1) Parsing
• LALR Parsing
• CLR Parsing
• Operator Precedence Parsing

Bottom-Up vs Top-Down Parser