howto-ast-grep

name	howto-ast-grep
description	Use when searching for or transforming code patterns structurally - provides ast-grep CLI syntax, metavariable patterns, and language-specific examples for precise AST-based code search and rewriting

Using ast-grep for Structural Code Search

Overview

ast-grep (sg) matches code by AST structure, not text. Use it instead of grep/ripgrep when you need to match code patterns regardless of whitespace, variable names, or formatting. Patterns must be valid, parsable code in the target language.

When to Use

Prefer ast-grep over grep when:

• Finding function calls with specific argument shapes
• Matching patterns where variable names differ but structure is the same
• Rewriting code patterns across a codebase (migrations, deprecations)
• Finding anti-patterns that regex can't express (e.g. "await inside a loop")

Use grep instead when:

• Searching for string literals, comments, or non-code text
• Simple keyword search (import names, error messages)
• ast-grep is not installed

CLI Quick Reference

# Search for a pattern
sg run -p 'console.log($$$ARGS)' -l javascript src/

# Search and replace (interactive)
sg run -p '$A && $A()' -r '$A?.()' -l typescript --interactive src/

# Apply all replacements without prompting
sg run -p 'var $X = $Y' -r 'const $X = $Y' -l javascript -U src/

# JSON output for programmatic use
sg run -p '$FUNC($$$)' -l python --json src/

# Read from stdin
echo 'let x = 1' | sg run -p 'let $X = $Y' -l javascript --stdin

Metavariable Syntax

Syntax	Matches	Example
$NAME	Exactly one AST node	console.log($MSG) matches console.log("hi") but not console.log("hi", 2)
$$$NAME	Zero or more nodes	console.log($$$ARGS) matches any number of arguments
$_NAME	One node, non-capturing	$_OBJ.$METHOD() matches without binding $_OBJ

Reuse enforces equality: $A == $A matches x == x but not x == y.

Language-Specific Patterns

Always pass -l <language>. The language determines how patterns are parsed.

JavaScript/TypeScript:

# Find React hooks
sg run -p 'use$HOOK($$$)' -l typescript src/

# Optional chaining candidates
sg run -p '$A && $A.$B' -r '$A?.$B' -l typescript src/

# Async/await patterns
sg run -p 'await $PROMISE' -l javascript src/

Python:

# Find print statements
sg run -p 'print($$$)' -l python src/

# Dictionary access patterns
sg run -p '$DICT[$KEY]' -l python src/

# With statement patterns
sg run -p 'with open($$$ARGS) as $F: $$$BODY' -l python src/

Go:

# Error handling
sg run -p 'if $ERR != nil { return $$$VALS }' -l go ./

# Function calls
sg run -p '$PKG.$FUNC($$$)' -l go ./

Rust:

# Unwrap calls (potential panics)
sg run -p '$EXPR.unwrap()' -l rust src/

# Match expressions
sg run -p 'match $VAL { $$$ARMS }' -l rust src/

Pattern Pitfalls

Mistake	Why It Fails	Fix
Using regex in -p	Patterns must be valid code	Write the code structure you want to find
$VAR + Name	Parsed as $VARN + ame	Use constraints or transform in YAML rules
Single quotes in C/Rust patterns	'x' is a char literal, not a string	Use "x" for strings
Forgetting -l flag	ast-grep cannot infer language from patterns	Always specify language explicitly
$A($B, $C) for variadic calls	Only matches exactly 2 args	Use $A($$$ARGS) for any arity

YAML Rules for Complex Matching

When CLI patterns aren't expressive enough, use YAML rules:

id: no-unwrap-in-production
language: rust
rule:
  pattern: $EXPR.unwrap()
  not:
    inside:
      kind: function_item
      has:
        field: name
        regex: '^test_'
message: "Use expect() or proper error handling instead of unwrap()"
severity: warning

Key rule combinators:

• all: All sub-rules must match (AND)
• any: Any sub-rule can match (OR)
• not: Sub-rule must NOT match
• inside: Target is inside another node
• has: Target contains a descendant
• follows/precedes: Sibling ordering

Run rules: sg scan --rule path/to/rule.yml src/

Red Flags

• Using grep with complex regex when ast-grep would match the structure directly
• Forgetting $$$ and missing variadic matches
• Writing patterns that aren't valid code in the target language
• Not specifying -l and getting parse failures