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

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.

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.

  yamlCopy

  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).

  yamlCopy

  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.

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.

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.

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.

all:
  - kind: call_expression
  - pattern: console.log($ARG)

any: Disjunction (OR) of Rules

Matches a node if any sub-rules in the list match.

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.

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.

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:

rule:
  kind: function_declaration
  has:
    pattern: await $EXPR
    stopBy: end

Finding Code Inside Specific Contexts

Find console.log calls inside class methods:

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:

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:

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