AST-Grep (sg) - Structural Code Search and Refactoring

ast-grep is a syntax-aware code search and refactoring tool. It understands code structure and semantics, making it superior to regex-based searches for code analysis.

Quick Start

# Find function calls
sg -p 'functionName($$)' -l javascript .

# Find class definitions
sg -p 'class $NAME { $$ }' -l typescript .

# Find imports
sg -p 'import { $$ } from "$MODULE"' -l javascript .

# Interactive rewrite
sg -p '$OLD_PATTERN' --rewrite '$NEW_PATTERN' -l python --interactive .

When to Use ast-grep

Use sg (ast-grep) instead of plain regex or text search when:

• Structural code patterns are involved (e.g., finding all function calls, class definitions, or method implementations)
• Language-aware refactoring is required (e.g., renaming variables, updating function signatures, or changing imports)
• Complex code analysis is needed (e.g., finding all usages of a pattern across different syntactic contexts)
• Cross-language searches are necessary (e.g., working with both JavaScript and TypeScript in a monorepo)
• Semantic code understanding is important (e.g., finding patterns based on code structure, not just text)

Command Templates

Basic Search Pattern

sg -p '$PATTERN' -l $LANGUAGE $PATH

Search with Context

sg -p '$PATTERN' -l $LANGUAGE -C 3 $PATH  # Show 3 lines of context
sg -p '$PATTERN' -l $LANGUAGE -A 5 $PATH  # Show 5 lines after
sg -p '$PATTERN' -l $LANGUAGE -B 5 $PATH  # Show 5 lines before

Interactive Rewrite

sg -p '$OLD_PATTERN' --rewrite '$NEW_PATTERN' -l $LANGUAGE --interactive $PATH

JSON Output for Processing

sg -p '$PATTERN' -l $LANGUAGE --json $PATH

Pattern Syntax Reference

• $VAR — matches any single node and captures it
• $$ — matches zero or more nodes (wildcard)
• $$$ — matches one or more nodes
• Literal code — matches exactly as written
• Indentation insensitive — matches regardless of whitespace/formatting

Common Use Cases and Patterns

JavaScript/TypeScript Patterns

# Find function calls
sg -p 'functionName($$)' -l javascript .

# Find class definitions
sg -p 'class $NAME { $$ }' -l typescript .

# Find method calls on objects
sg -p '$OBJ.$METHOD($$)' -l typescript .

# Find import statements
sg -p 'import { $$ } from "$MODULE"' -l javascript .

# Find React hooks
sg -p 'const [$STATE, $SETTER] = useState($$)' -l typescript .

# Find async functions
sg -p 'async function $NAME($$) { $$ }' -l javascript .

Python Patterns

# Find function definitions
sg -p 'def $NAME($$): $$' -l python .

# Find class definitions
sg -p 'class $NAME: $$' -l python .

# Find method calls
sg -p '$OBJ.$METHOD($$)' -l python .

# Find imports
sg -p 'from $MODULE import $$' -l python .

# Find decorators
sg -p '@$DECORATOR' -l python .

Ruby Patterns

# Find class definitions
sg -p 'class $NAME < $$; $$; end' -l ruby .

# Find method definitions
sg -p 'def $NAME($$); $$; end' -l ruby .

# Find method calls
sg -p '$OBJ.$METHOD($$)' -l ruby .

General Patterns

# Find variable assignments
sg -p '$VAR = $$' -l $LANG .

# Find conditional statements
sg -p 'if $CONDITION { $$ }' -l $LANG .

Supported Languages

• Web: javascript, typescript, html, css
• Backend: python, ruby, go, rust, java, c, cpp
• Config: yaml, json, toml
• And many more - see full list

Integration Workflow

Using ast-grep workflow:

1. Identify if the task involves structural code patterns or language-aware refactoring
2. Determine the appropriate language(s) to search
3. Construct the pattern using ast-grep syntax
4. Run ast-grep to gather precise structural information
5. Use results to inform code edits, refactoring, or further analysis

Example Workflow

When asked to "find all service objects that call perform method":

1. Run ast-grep search:

   sg -p 'perform($$)' -l ruby app/services/
   

2. Analyze results structurally
3. Use Agent or Read tools for additional context if needed
4. Make informed edits based on structural understanding

Advanced Usage

Rewrite Patterns

# Simple rewrite
sg -p '$PROP && $PROP()' --rewrite '$PROP?.()' -l typescript

# Interactive rewrite session
sg -p '$OLD' --rewrite '$NEW' -l python --interactive

# Auto-apply all changes
sg -p '$OLD' --rewrite '$NEW' -l javascript --update-all

Complex Patterns

# Find nested patterns
sg -p 'if ($COND) { if ($INNER) { $$ } }' -l javascript

# Find patterns with specific context
sg -p 'class $NAME { constructor($$) { $$ } }' -l typescript

Key Benefits Over Regex

1. Language-aware — understands syntax and semantics
2. Structural matching — finds patterns regardless of formatting
3. Cross-language — works consistently across different languages
4. Precise refactoring — makes structural changes safely
5. Context-aware — understands code hierarchy and scope

When NOT to Use ast-grep

Use plain text search (rg) instead when:

• Searching for plain text comments or documentation
• Looking for string literals or hardcoded values
• Searching across non-code files (markdown, text, etc.)
• Simple substring matching without structural context

Use fd instead when:

• Searching for files by name or path
• Finding files by extension
• Locating files in specific directories

Decision Matrix

Notes

• ast-grep is pre-installed and available, just like ripgrep (rg) and fd
• No availability checks needed
• All ast-grep commands are executed through the Bash tool
• Always prefer ast-grep for code structure analysis over regex-based approaches