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---
name: ast-grep
description: Guide for writing ast-grep rules to perform structural code search and analysis. Use when users need to search codebases using Abstract Syntax Tree (AST) patterns, find specific code structures, or perform complex code queries that go beyond simple text search. This skill should be used when users ask to search for code patterns, find specific language constructs, or locate code with particular structural characteristics.
---
# ast-grep Code Search
## Overview
This skill helps translate natural language queries into ast-grep rules for structural code search. ast-grep uses Abstract Syntax Tree (AST) patterns to match code based on its structure rather than just text, enabling powerful and precise code search across large codebases.
## When to Use This Skill
Use this skill when users:
- Need to search for code patterns using structural matching (e.g., "find all async functions that don't have error handling")
- Want to locate specific language constructs (e.g., "find all function calls with specific parameters")
- Request searches that require understanding code structure rather than just text
- Ask to search for code with particular AST characteristics
- Need to perform complex code queries that traditional text search cannot handle
## General Workflow
Follow this process to help users write effective ast-grep rules:
### Step 1: Understand the Query
Clearly understand what the user wants to find. Ask clarifying questions if needed:
- What specific code pattern or structure are they looking for?
- Which programming language?
- Are there specific edge cases or variations to consider?
- What should be included or excluded from matches?
### Step 2: Create Example Code
Write a simple code snippet that represents what the user wants to match. Save this to a temporary file for testing.
**Example:**
If searching for "async functions that use await", create a test file:
```javascript
// test_example.js
async function example() {
const result = await fetchData();
return result;
}
```
### Step 3: Write the ast-grep Rule
Translate the pattern into an ast-grep rule. Start simple and add complexity as needed.
**Key principles:**
- Always use `stopBy: end` for relational rules (`inside`, `has`) to ensure search goes to the end of the direction
- Use `pattern` for simple structures
- Use `kind` with `has`/`inside` for complex structures
- Break complex queries into smaller sub-rules using `all`, `any`, or `not`
**Example rule file (test_rule.yml):**
```yaml
id: async-with-await
language: javascript
rule:
kind: function_declaration
has:
pattern: await $EXPR
stopBy: end
```
See `references/rule_reference.md` for comprehensive rule documentation.
### Step 4: Test the Rule
Use ast-grep CLI to verify the rule matches the example code. There are two main approaches:
**Option A: Test with inline rules (for quick iterations)**
```bash
echo "async function test() { await fetch(); }" | ast-grep scan --inline-rules "id: test
language: javascript
rule:
kind: function_declaration
has:
pattern: await \$EXPR
stopBy: end" --stdin
```
**Option B: Test with rule files (recommended for complex rules)**
```bash
ast-grep scan --rule test_rule.yml test_example.js
```
**Debugging if no matches:**
1. Simplify the rule (remove sub-rules)
2. Add `stopBy: end` to relational rules if not present
3. Use `--debug-query` to understand the AST structure (see below)
4. Check if `kind` values are correct for the language
### Step 5: Search the Codebase
Once the rule matches the example code correctly, search the actual codebase:
**For simple pattern searches:**
```bash
ast-grep run --pattern 'console.log($ARG)' --lang javascript /path/to/project
```
**For complex rule-based searches:**
```bash
ast-grep scan --rule my_rule.yml /path/to/project
```
**For inline rules (without creating files):**
```bash
ast-grep scan --inline-rules "id: my-rule
language: javascript
rule:
pattern: \$PATTERN" /path/to/project
```
## ast-grep CLI Commands
### Inspect Code Structure (--debug-query)
Dump the AST structure to understand how code is parsed:
```bash
ast-grep run --pattern 'async function example() { await fetch(); }' \
--lang javascript \
--debug-query=cst
```
**Available formats:**
- `cst`: Concrete Syntax Tree (shows all nodes including punctuation)
- `ast`: Abstract Syntax Tree (shows only named nodes)
- `pattern`: Shows how ast-grep interprets your pattern
**Use this to:**
- Find the correct `kind` values for nodes
- Understand the structure of code you want to match
- Debug why patterns aren't matching
**Example:**
```bash
# See the structure of your target code
ast-grep run --pattern 'class User { constructor() {} }' \
--lang javascript \
--debug-query=cst
# See how ast-grep interprets your pattern
ast-grep run --pattern 'class $NAME { $$$BODY }' \
--lang javascript \
--debug-query=pattern
```
### Test Rules (scan with --stdin)
Test a rule against code snippet without creating files:
```bash
echo "const x = await fetch();" | ast-grep scan --inline-rules "id: test
language: javascript
rule:
pattern: await \$EXPR" --stdin
```
**Add --json for structured output:**
```bash
echo "const x = await fetch();" | ast-grep scan --inline-rules "..." --stdin --json
```
### Search with Patterns (run)
Simple pattern-based search for single AST node matches:
```bash
# Basic pattern search
ast-grep run --pattern 'console.log($ARG)' --lang javascript .
# Search specific files
ast-grep run --pattern 'class $NAME' --lang python /path/to/project
# JSON output for programmatic use
ast-grep run --pattern 'function $NAME($$$)' --lang javascript --json .
```
**When to use:**
- Simple, single-node matches
- Quick searches without complex logic
- When you don't need relational rules (inside/has)
### Search with Rules (scan)
YAML rule-based search for complex structural queries:
```bash
# With rule file
ast-grep scan --rule my_rule.yml /path/to/project
# With inline rules
ast-grep scan --inline-rules "id: find-async
language: javascript
rule:
kind: function_declaration
has:
pattern: await \$EXPR
stopBy: end" /path/to/project
# JSON output
ast-grep scan --rule my_rule.yml --json /path/to/project
```
**When to use:**
- Complex structural searches
- Relational rules (inside, has, precedes, follows)
- Composite logic (all, any, not)
- When you need the power of full YAML rules
**Tip:** For relational rules (inside/has), always add `stopBy: end` to ensure complete traversal.
## Tips for Writing Effective Rules
### Always Use stopBy: end
For relational rules, always use `stopBy: end` unless there's a specific reason not to:
```yaml
has:
pattern: await $EXPR
stopBy: end
```
This ensures the search traverses the entire subtree rather than stopping at the first non-matching node.
### Start Simple, Then Add Complexity
Begin with the simplest rule that could work:
1. Try a `pattern` first
2. If that doesn't work, try `kind` to match the node type
3. Add relational rules (`has`, `inside`) as needed
4. Combine with composite rules (`all`, `any`, `not`) for complex logic
### Use the Right Rule Type
- **Pattern**: For simple, direct code matching (e.g., `console.log($ARG)`)
- **Kind + Relational**: For complex structures (e.g., "function containing await")
- **Composite**: For logical combinations (e.g., "function with await but not in try-catch")
### Debug with AST Inspection
When rules don't match:
1. Use `--debug-query=cst` to see the actual AST structure
2. Check if metavariables are being detected correctly
3. Verify the node `kind` matches what you expect
4. Ensure relational rules are searching in the right direction
### Escaping in Inline Rules
When using `--inline-rules`, escape metavariables in shell commands:
- Use `\$VAR` instead of `$VAR` (shell interprets `$` as variable)
- Or use single quotes: `'$VAR'` works in most shells
**Example:**
```bash
# Correct: escaped $
ast-grep scan --inline-rules "rule: {pattern: 'console.log(\$ARG)'}" .
# Or use single quotes
ast-grep scan --inline-rules 'rule: {pattern: "console.log($ARG)"}' .
```
## Common Use Cases
### Find Functions with Specific Content
Find async functions that use await:
```bash
ast-grep scan --inline-rules "id: async-await
language: javascript
rule:
all:
- kind: function_declaration
- has:
pattern: await \$EXPR
stopBy: end" /path/to/project
```
### Find Code Inside Specific Contexts
Find console.log inside class methods:
```bash
ast-grep scan --inline-rules "id: console-in-class
language: javascript
rule:
pattern: console.log(\$\$\$)
inside:
kind: method_definition
stopBy: end" /path/to/project
```
### Find Code Missing Expected Patterns
Find async functions without try-catch:
```bash
ast-grep scan --inline-rules "id: async-no-trycatch
language: javascript
rule:
all:
- kind: function_declaration
- has:
pattern: await \$EXPR
stopBy: end
- not:
has:
pattern: try { \$\$\$ } catch (\$E) { \$\$\$ }
stopBy: end" /path/to/project
```
## Resources
### references/
Contains detailed documentation for ast-grep rule syntax:
- `rule_reference.md`: Comprehensive ast-grep rule documentation covering atomic rules, relational rules, composite rules, and metavariables
Load these references when detailed rule syntax information is needed.

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# ast-grep Rule Reference
This document provides comprehensive documentation for ast-grep rule syntax, covering all rule types and metavariables.
## Introduction to ast-grep Rules
ast-grep rules are declarative specifications for matching and filtering Abstract Syntax Tree (AST) nodes. They enable structural code search and analysis by defining conditions an AST node must meet to be matched.
### Rule Categories
ast-grep rules are categorized into three types:
* **Atomic Rules**: Match individual AST nodes based on intrinsic properties like code patterns (`pattern`), node type (`kind`), or text content (`regex`).
* **Relational Rules**: Define conditions based on a target node's position or relationship to other nodes (e.g., `inside`, `has`, `precedes`, `follows`).
* **Composite Rules**: Combine other rules using logical operations (AND, OR, NOT) to form complex matching criteria (e.g., `all`, `any`, `not`, `matches`).
## Anatomy of an ast-grep Rule Object
The ast-grep rule object is the core configuration unit defining how ast-grep identifies and filters AST nodes. It's typically written in YAML format.
### General Structure
Every field within an ast-grep Rule Object is optional, but at least one "positive" key (e.g., `kind`, `pattern`) must be present.
A node matches a rule if it satisfies all fields defined within that rule object, implying an implicit logical AND operation.
For rules using metavariables that depend on prior matching, explicit `all` composite rules are recommended to guarantee execution order.
### Rule Object Properties
| Property | Type | Category | Purpose | Example |
| :--- | :--- | :--- | :--- | :--- |
| `pattern` | String or Object | Atomic | Matches AST node by code pattern. | `pattern: console.log($ARG)` |
| `kind` | String | Atomic | Matches AST node by its kind name. | `kind: call_expression` |
| `regex` | String | Atomic | Matches node's text by Rust regex. | `regex: ^[a-z]+$` |
| `nthChild` | number, string, Object | Atomic | Matches nodes by their index within parent's children. | `nthChild: 1` |
| `range` | RangeObject | Atomic | Matches node by character-based start/end positions. | `range: { start: { line: 0, column: 0 }, end: { line: 0, column: 10 } }` |
| `inside` | Object | Relational | Target node must be inside node matching sub-rule. | `inside: { pattern: class $C { $$$ }, stopBy: end }` |
| `has` | Object | Relational | Target node must have descendant matching sub-rule. | `has: { pattern: await $EXPR, stopBy: end }` |
| `precedes` | Object | Relational | Target node must appear before node matching sub-rule. | `precedes: { pattern: return $VAL }` |
| `follows` | Object | Relational | Target node must appear after node matching sub-rule. | `follows: { pattern: import $M from '$P' }` |
| `all` | Array<Rule> | Composite | Matches if all sub-rules match. | `all: [ { kind: call_expression }, { pattern: foo($A) } ]` |
| `any` | Array<Rule> | Composite | Matches if any sub-rules match. | `any: [ { pattern: foo() }, { pattern: bar() } ]` |
| `not` | Object | Composite | Matches if sub-rule does not match. | `not: { pattern: console.log($ARG) }` |
| `matches` | String | Composite | Matches if predefined utility rule matches. | `matches: my-utility-rule-id` |
## Atomic Rules
Atomic rules match individual AST nodes based on their intrinsic properties.
### pattern: String and Object Forms
The `pattern` rule matches a single AST node based on a code pattern.
**String Pattern**: Directly matches using ast-grep's pattern syntax with metavariables.
```yaml
pattern: console.log($ARG)
```
**Object Pattern**: Offers granular control for ambiguous patterns or specific contexts.
* `selector`: Pinpoints a specific part of the parsed pattern to match.
```yaml
pattern:
selector: field_definition
context: class { $F }
```
* `context`: Provides surrounding code context for correct parsing.
* `strictness`: Modifies the pattern's matching algorithm (`cst`, `smart`, `ast`, `relaxed`, `signature`).
```yaml
pattern:
context: foo($BAR)
strictness: relaxed
```
### kind: Matching by Node Type
The `kind` rule matches an AST node by its `tree_sitter_node_kind` name, derived from the language's Tree-sitter grammar. Useful for targeting constructs like `call_expression` or `function_declaration`.
```yaml
kind: call_expression
```
### regex: Text-Based Node Matching
The `regex` rule matches the entire text content of an AST node using a Rust regular expression. It's not a "positive" rule, meaning it matches any node whose text satisfies the regex, regardless of its structural kind.
### nthChild: Positional Node Matching
The `nthChild` rule finds nodes by their 1-based index within their parent's children list, counting only named nodes by default.
* `number`: Matches the exact nth child. Example: `nthChild: 1`
* `string`: Matches positions using An+B formula. Example: `2n+1`
* `Object`: Provides granular control:
* `position`: `number` or An+B string.
* `reverse`: `true` to count from the end.
* `ofRule`: An ast-grep rule to filter the sibling list before counting.
### range: Position-Based Node Matching
The `range` rule matches an AST node based on its character-based start and end positions. A `RangeObject` defines `start` and `end` fields, each with 0-based `line` and `column`. `start` is inclusive, `end` is exclusive.
## Relational Rules
Relational rules filter targets based on their position relative to other AST nodes. They can include `stopBy` and `field` options.
### inside: Matching Within a Parent Node
Requires the target node to be inside another node matching the `inside` sub-rule.
```yaml
inside:
pattern: class $C { $$$ }
stopBy: end
```
### has: Matching with a Descendant Node
Requires the target node to have a descendant node matching the `has` sub-rule.
```yaml
has:
pattern: await $EXPR
stopBy: end
```
### precedes and follows: Sequential Node Matching
* `precedes`: Target node must appear before a node matching the `precedes` sub-rule.
* `follows`: Target node must appear after a node matching the `follows` sub-rule.
Both include `stopBy` but not `field`.
### stopBy and field: Refining Relational Searches
**stopBy**: Controls search termination for relational rules.
* `"neighbor"` (default): Stops when immediate surrounding node doesn't match.
* `"end"`: Searches to the end of the direction (root for `inside`, leaf for `has`).
* `Rule object`: Stops when a surrounding node matches the provided rule (inclusive).
**field**: Specifies a sub-node within the target node that should match the relational rule. Only for `inside` and `has`.
**Best Practice**: When unsure, always use `stopBy: end` to ensure the search goes to the end of the direction.
## Composite Rules
Composite rules combine atomic and relational rules using logical operations.
### all: Conjunction (AND) of Rules
Matches a node only if all sub-rules in the list match. Guarantees order of rule matching, important for metavariables.
```yaml
all:
- kind: call_expression
- pattern: console.log($ARG)
```
### any: Disjunction (OR) of Rules
Matches a node if any sub-rules in the list match.
```yaml
any:
- pattern: console.log($ARG)
- pattern: console.warn($ARG)
- pattern: console.error($ARG)
```
### not: Negation (NOT) of a Rule
Matches a node if the single sub-rule does not match.
```yaml
not:
pattern: console.log($ARG)
```
### matches: Rule Reuse and Utility Rules
Takes a rule-id string, matching if the referenced utility rule matches. Enables rule reuse and recursive rules.
## Metavariables
Metavariables are placeholders in patterns to match dynamic content in the AST.
### $VAR: Single Named Node Capture
Captures a single named node in the AST.
* **Valid**: `$META`, `$META_VAR`, `$_`
* **Invalid**: `$invalid`, `$123`, `$KEBAB-CASE`
* **Example**: `console.log($GREETING)` matches `console.log('Hello World')`.
* **Reuse**: `$A == $A` matches `a == a` but not `a == b`.
### $$VAR: Single Unnamed Node Capture
Captures a single unnamed node (e.g., operators, punctuation).
**Example**: To match the operator in `a + b`, use `$$OP`.
```yaml
rule:
kind: binary_expression
has:
field: operator
pattern: $$OP
```
### $$$MULTI_META_VARIABLE: Multi-Node Capture
Matches zero or more AST nodes (non-greedy). Useful for variable numbers of arguments or statements.
* **Example**: `console.log($$$)` matches `console.log()`, `console.log('hello')`, and `console.log('debug:', key, value)`.
* **Example**: `function $FUNC($$$ARGS) { $$$ }` matches functions with varying parameters/statements.
### Non-Capturing Metavariables (_VAR)
Metavariables starting with an underscore (`_`) are not captured. They can match different content even if named identically, optimizing performance.
* **Example**: `$_FUNC($_FUNC)` matches `test(a)` and `testFunc(1 + 1)`.
### Important Considerations for Metavariable Detection
* **Syntax Matching**: Only exact metavariable syntax (e.g., `$A`, `$$B`, `$$$C`) is recognized.
* **Exclusive Content**: Metavariable text must be the only text within an AST node.
* **Non-working**: `obj.on$EVENT`, `"Hello $WORLD"`, `a $OP b`, `$jq`.
The ast-grep playground is useful for debugging patterns and visualizing metavariables.
## Common Patterns and Examples
### Finding Functions with Specific Content
Find functions that contain await expressions:
```yaml
rule:
kind: function_declaration
has:
pattern: await $EXPR
stopBy: end
```
### Finding Code Inside Specific Contexts
Find console.log calls inside class methods:
```yaml
rule:
pattern: console.log($$$)
inside:
kind: method_definition
stopBy: end
```
### Combining Multiple Conditions
Find async functions that use await but don't have try-catch:
```yaml
rule:
all:
- kind: function_declaration
- has:
pattern: await $EXPR
stopBy: end
- not:
has:
pattern: try { $$$ } catch ($E) { $$$ }
stopBy: end
```
### Matching Multiple Alternatives
Find any type of console method call:
```yaml
rule:
any:
- pattern: console.log($$$)
- pattern: console.warn($$$)
- pattern: console.error($$$)
- pattern: console.debug($$$)
```
## Troubleshooting Tips
1. **Rule doesn't match**: Use `dump_syntax_tree` to see the actual AST structure
2. **Relational rule issues**: Ensure `stopBy: end` is set for deep searches
3. **Wrong node kind**: Check the language's Tree-sitter grammar for correct kind names
4. **Metavariable not working**: Ensure it's the only content in its AST node
5. **Pattern too complex**: Break it down into simpler sub-rules using `all`