ast-grep

Use ast-grep (sg) when the task depends on code structure: calls, imports, functions, components, conditionals, object literals, nested constructs, or safe repeated rewrites.

Prefer rg for plain text/content search.

Default commands

Check availability:

ast-grep --version || sg --version

Search by AST pattern:

ast-grep run --lang <language> -p '<pattern>' [path]
# short form when sg is installed:
sg run --lang <language> -p '<pattern>' [path]

Use JSON when output may be parsed or large:

ast-grep run --lang ts -p 'console.log($$$ARGS)' --json=stream

Preview rewrites before applying:

ast-grep run --lang js -p 'var $NAME = $VALUE' -r 'let $NAME = $VALUE'
ast-grep run --lang js -p 'var $NAME = $VALUE' -r 'let $NAME = $VALUE' --update-all

Only use --update-all after inspecting the preview.

Pattern reminders

• Patterns should look like real source code in the target language.
• $A captures one AST node.
• $$$ARGS captures zero or more nodes.
• Always set --lang explicitly (js, ts, jsx, tsx, json, yaml, python, go, rust, etc.).
• Quote patterns with single quotes to avoid shell expansion.
• If a pattern fails unexpectedly, inspect it with --debug-query=ast.

Common patterns

# console calls
ast-grep run --lang ts -p 'console.log($$$ARGS)'

# function declarations
ast-grep run --lang ts -p 'function $NAME($$$ARGS) { $$$BODY }'

# React hook usage
ast-grep run --lang tsx -p 'useEffect($$$ARGS)'

# imports from a package
ast-grep run --lang ts -p 'import { $$$IMPORTS } from "$PKG"'

# YAML key/value shape
ast-grep run --lang yaml -p '$KEY: $VALUE'

Rule workflow

For complex searches or reusable rules:

1. Start from a small code example that should match.
2. Write the simplest working -p pattern first.
3. Add YAML rule constraints only after the basic pattern works.
4. Use --json=stream and/or --debug-query=ast to inspect matches.
5. If it fails, remove constraints and debug one condition at a time.
6. Preview rewrites; apply only after reviewing matches.