ast-grep

name	ast-grep
description	Use for ast-grep project setup, rule authoring, rule debugging, and structural search workflows.
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Ast-Grep

Use this skill for ast-grep project setup, rule authoring, rule debugging, and CLI workflows that go beyond a single structural query.

Routing

• Use exec_command for ast-grep queries, project scans, rule tests, debug output, and rewrite previews.
• Use ast-grep scan for project rules and ast-grep test for rule tests.
• Use ast-grep run --pattern=... --rewrite=... --json=compact --color=never for dry-run rewrite previews. Review every replacement before applying changes.
• Use ast-grep run --debug-query when a query matches an unexpected syntax node.
• Use --kind to match node kinds. Ast-grep supports ESQuery-style compound selectors and pseudo-selectors in supported releases.
• Keep this skill read-only unless the user explicitly authorises applying a rewrite.

Quick Start

• In VT Code, prefer vtcode dependencies install ast-grep before suggesting system package managers.
• External install routes such as Homebrew, Cargo, npm, pip, MacPorts, or Nix are fallback options when the user explicitly wants a system-managed install.
• After installation, validate availability with ast-grep --help.
• On Linux, prefer the full ast-grep binary name over sg because sg may already refer to setgroups.
• When running CLI patterns with shell metavariables like $PROP, use single quotes so the shell does not expand them before ast-grep sees the pattern.
• A good first rewrite example is optional chaining, for example rewriting $PROP && $PROP() to $PROP?.().

Command Overview

• ast-grep run: ad-hoc query execution and one-off rewrites.
• ast-grep scan: project rule scanning.
• ast-grep new: scaffold and rule generation.
• ast-grep test: rule-test execution.
• ast-grep lsp: editor integration via language server.

Built-In Languages

• ast-grep ships many built-in languages. Common aliases include bash, c, cc / cpp, cs, css, ex, go / golang, html, java, js / javascript / jsx, json, kt, lua, md / markdown, php, py / python, rb, rs / rust, swift, ts / typescript, tsx, and yml.
• --lang <alias> and YAML language: <alias> use those built-in aliases. File-system scans infer language from built-in extensions unless the project overrides them.
• Pass --lang <alias> to the CLI when extension inference is insufficient.
• Use languageGlobs when the repository needs a different extension mapping than ast-grep’s built-in defaults.

How Ast-Grep Works

• ast-grep accepts several query formats: pattern queries, YAML rules, and programmatic API usage.
• The core pipeline is parse first, match second. Tree-Sitter builds the syntax tree, then ast-grep’s Rust matcher finds the target nodes.
• The main usage scenarios are search, rewrite, lint, and analyze.
• ast-grep processes many files in parallel and is built to use multiple CPU cores on larger codebases.
• In VT Code, run ast-grep through exec_command and use this skill for YAML authoring, rule tests, rewrites, and API-level work.

Project Scaffolding

• A scan-ready ast-grep project needs workspace sgconfig.yml plus at least one rule directory, usually rules/.
• rule-tests/ and utils/ are optional scaffolding that ast-grep new can create for rule tests and reusable utility rules.
• If the repository already has sgconfig.yml and rules/, prefer working with the existing layout instead of recreating scaffolding.
• Use ast-grep new when the repository does not have ast-grep scaffolding yet.
• Use ast-grep new rule when the scaffold exists and the task is creating a new rule plus optional test case.

sgconfig.yml

• sgconfig.yml is the project-level ast-grep config file, not a rule file. Treat it like the repository root for rule discovery, tests, parser overrides, and embedded-language behavior.
• ruleDirs is required and is resolved relative to the directory containing sgconfig.yml.
• testConfigs is optional and configures ast-grep test discovery. Each entry needs testDir; snapshotDir is optional and otherwise defaults to __snapshots__ under that testDir.
• utilDirs declares directories for global utility rules shared across multiple rule files.
• languageGlobs remaps files to parsers and takes precedence over ast-grep’s default extension mapping, which is useful for similar-language reuse like TS -> TSX or C -> Cpp.
• customLanguages registers project-local parsers. libraryPath can be one relative library path or a target-triple map, extensions is required, expandoChar is optional, and languageSymbol defaults to tree_sitter_{name}.
• languageInjections is experimental. Each entry needs hostLanguage, rule, and injected.
• Use dynamic injected candidates when the rule captures $LANG and the embedded language must be chosen from a list such as css, scss, or less.
• Raw ast-grep project discovery walks upward from the current working directory until it finds sgconfig.yml, and --config <file> overrides that discovery with an explicit root config path.
• ast-grep scan requires project config and will error if no sgconfig.yml is found. ast-grep run can still search without project config, though it also benefits from discovered config for things like customLanguages and languageGlobs.
• ast-grep scan --inspect summary is the quickest way to confirm which project directory and config file ast-grep actually selected during discovery.
• ast-grep also recognizes a home-directory sgconfig.yml as a global fallback config. XDG config directories are not part of this behavior.

Rule Catalog

• Use the ast-grep catalog as inspiration when the user wants existing example rules, not as something to copy blindly.
• Start from examples in the same language family when possible.
• Read catalog markers as hints about rule complexity:
  • simple pattern examples are good starting points
  • Fix means the example includes a rewrite path
  • constraints, labels, utils, transform, and rewriters mean the example depends on more advanced rule features
• When adapting a catalog example, translate it to the current repository’s language, style, and safety constraints instead of preserving the example verbatim.
• Prefer the bundled skill workflow when the user asks to explain, adapt, or combine catalog examples.

VT Code Bundled Rules

VT Code ships a set of curated ast-grep rules under rules/ with matching tests under rule-tests/. Run them with vtcode check ast-grep. The bundled rules are organised by language:

Python (rules/python/)

• no-print: flags print() calls in production code
• no-walrus-source: flags walrus operators that harm readability
• no-unnecessary-list: flags list(...) wrapping an already-list expression
• no-identity-check-with-type: flags type(x) is T in favor of isinstance(x, T)
• optional-to-union: flags Optional[X] in favor of X | None
• prefer-dict-get: flags if k in d: d[k] in favor of d.get(k)
• prefer-generator-expression: flags list comprehensions passed to sum/any/all/min/max
• prefer-isinstance-tuple: flags isinstance(x, A) or isinstance(x, B) in favor of isinstance(x, (A, B))

Rust (rules/rust/)

• no-unsafe-fn-without-unsafe: flags unsafe fn bodies that contain no unsafe block
• avoid-duplicate-export: flags pub use when pub mod already exposes the module
• no-iterator-for-each: flags .iter().for_each() in favor of for loops
• no-redundant-closure: flags |x| foo(x) in favor of foo directly
• let-chain-candidate: flags nested if that could be collapsed with let-chains
• no-chars-enumerate: flags .chars().enumerate() when .char_indices() is more idiomatic
• no-alloc-digit-count: flags digit-count loops that allocate instead of using repeated division
• prefer-iterator-sum: flags manual accumulator loops in favor of .sum()
• prefer-retain-over-filter-collect: flags .filter().collect() on a Vec in favor of .retain()
• prefer-unwrap-or-default: flags .unwrap_or(Default::default()) in favor of .unwrap_or_default()

Kotlin (rules/kotlin/)

• no-var: flags mutable var declarations
• no-println: flags println/print calls
• no-lateinit: flags lateinit var usage
• no-unsafe-cast: flags as casts without null-safe as?
• no-unnecessary-let: flags let blocks that add no value
• prefer-is-empty: flags .count() == 0 in favor of .isEmpty()
• prefer-data-class: flags classes that should be data class
• clean-architecture-imports: flags imports that violate clean architecture layer boundaries

Ruby (rules/ruby/)

• no-path-traversal: flags string concatenation in File.join / Pathname that may cause traversal
• prefer-action-over-filter: flags before_filter / after_filter in favor of before_action / after_action
• prefer-symbol-over-proc: flags Proc.new with a symbol when (&:method) is cleaner

TypeScript (rules/typescript/)

• no-await-in-promise-all: flags await inside Promise.all() arrays (defeats parallelism)
• no-console-except-error: flags console.log/debug/warn/info/trace (allows console.error in catch blocks)
• no-debugger: flags debugger statements
• no-unnecessary-boolean-literal-compare: flags x === true or x === false
• no-useless-promise-resolve: flags return Promise.resolve(...) in async functions
• prefer-array-flat-map: flags .map(fn).flat() in favor of .flatMap(fn)
• prefer-nullish-coalescing: flags || in assignments/returns where ?? is more precise
• use-logical-assignment: flags $A = $A || $B in favor of $A ||= $B
• prefer-optional-chaining: flags a && a.b in favor of a?.b
• no-return-in-forEach: flags return inside .forEach() callbacks (does not return from caller)
• no-array-delete: flags delete arr[i] in favor of .splice()

TSX (rules/tsx/)

• avoid-jsx-short-circuit: flags {cond && <Elem />} in favor of {cond ? <Elem /> : null} (prevents rendering 0)
• no-nested-links: flags <a> elements nested inside other <a> elements (invalid HTML)
• no-unnecessary-usestate-type: flags useState<string>('hello') when TypeScript can infer the type
• rename-svg-attribute: flags hyphenated SVG attributes like stroke-linecap in favor of camelCase strokeLinecap

Examples (rules/examples/)

• no-console-log: starter rule scoped to __ast_grep_examples__/ for scaffold validation

Rust Catalog Highlights

• Avoid duplicated exports: a Rust lint-style rule can detect pub use foo::Bar; in the same source file that already exposes pub mod foo;. Treat this as API-surface cleanup, not a mechanical rewrite. The rule uses all to combine a pub use $A::$B; pattern with inside: { kind: source_file } and a has check for pub mod $A; with stopBy: end:

id: avoid-duplicate-export
language: Rust
severity: warning
message: Item re-exported via `pub use` when `pub mod` already exposes the module.
rule:
  all:
    - pattern: "pub use $A::$B;"
    - inside:
        kind: source_file
    - has:
        pattern: "pub mod $A;"
        stopBy: end

• Beware chars().enumerate(): the Rust catalog rewrite from $A.chars().enumerate() to $A.char_indices() is valid when the code needs byte offsets instead of character indexes. Do not apply blindly if the caller intentionally wants character positions:

id: no-chars-enumerate
language: Rust
severity: warning
message: Use `.char_indices()` instead of `.chars().enumerate()` when byte offsets are needed.
rule:
  pattern: "$A.chars().enumerate()"
fix: "$A.char_indices()"

• Count usize digits without allocation: the catalog rewrite from $NUM.to_string().chars().count() to $NUM.checked_ilog10().unwrap_or(0) + 1 is a good Rust-specific performance cleanup when the target is known to be an integer digit count. Do not over-apply if the expression is part of a more general formatting pipeline:

id: no-alloc-digit-count
language: Rust
severity: info
message: Count integer digits without heap allocation.
rule:
  pattern: "$NUM.to_string().chars().count()"
fix: "$NUM.checked_ilog10().unwrap_or(0) + 1"

• Unsafe function without unsafe block: the Rust catalog’s function_item rule that requires unsafe modifiers but rejects bodies containing unsafe_block is a good review rule for redundant unsafe markers. It is diagnostic-oriented and should usually stay a scan rule, not an automatic rewrite. The rule uses kind: function_item with has checking function_modifiers via regex: "^unsafe" and not rejecting bodies containing unsafe_block:

id: no-unsafe-fn-without-unsafe
language: Rust
severity: warning
message: Unsafe function contains no `unsafe` block.
rule:
  all:
    - kind: function_item
    - has:
        kind: function_modifiers
        regex: "^unsafe"
    - not:
        has:
          kind: unsafe_block
          stopBy: end

• Rust 2024 let-chain candidate: the catalog’s nested if/if let detection rule uses utils to define reusable matchers for sole-child statements, no-else if expressions, and no-else if let expressions. The root rule matches an if whose block contains only another if statement, suggesting the two can be collapsed into a single let-chain. Keep this as a hint-severity suggestion because let-chains require Rust 2024 edition:

id: let-chain-candidate
language: Rust
severity: hint
message: Nested `if`/`if let` can be collapsed into a Rust 2024 let-chain.
utils:
  sole-child:
    all:
      - nthChild: 1
      - nthChild: { position: 1, reverse: true }
  if-no-else:
    kind: if_expression
    not: { has: { field: alternative, kind: else_clause } }
  if-let-no-else:
    matches: if-no-else
    has: { field: condition, kind: let_condition }
  sole-inner-if-stmt:
    kind: expression_statement
    matches: sole-child
    has: { matches: if-no-else }
  sole-inner-if-let-stmt:
    kind: expression_statement
    matches: sole-child
    has: { matches: if-let-no-else }
rule:
  matches: if-no-else
  has:
    field: consequence
    kind: block
    has: { matches: sole-inner-if-stmt }
  any:
    - matches: if-let-no-else
    - has:
        field: consequence
        kind: block
        has: { matches: sole-inner-if-let-stmt }

• Rewrite indoc! macro: the catalog example that removes indoc! { r#"..."# } wrappers is a rewrite-oriented example. Keep it on the CLI skill path because the replacement is formatting-sensitive and should be reviewed interactively before broad apply. The CLI pattern is ast-grep --pattern ‘indoc! { r#"$$$A"# }’ --rewrite ‘$$$A’.
• Adapt these rules to the repository’s Rust style before using them directly. In VT Code, preserve existing lint policy, public API conventions, and the project’s bias against unnecessary rewrites.

TypeScript Catalog Highlights

• TypeScript vs TSX matters: keep .ts and .tsx rules separate unless the repository intentionally parses .ts as TSX through languageGlobs. Do not assume one pattern works unchanged across both parsers.
• Find import file without extension: good scan rule for ESM codebases that require explicit local file extensions on static or dynamic imports. It is policy-dependent, so only use it where the runtime or bundler actually requires explicit extensions.
• XState v4 to v5 migration: strong example of multi-rule YAML with utils, transform, and multi-document configs. Keep this sort of migration on the CLI skill path and review the generated diff instead of treating it as a one-line rewrite.
• No await inside Promise.all([...]): good rewrite rule when the awaited expression is directly inside the array literal. Keep the rewrite narrow so it does not change intentionally sequential logic hidden behind helper calls.
• No console except allowed cases: good scan rule for client-facing TypeScript, but it is repository-policy dependent. Adapt the allowed methods and environments before enabling it broadly.
• Find import usage or identifiers: these examples are useful for repository analysis and dependency cleanup, not just linting. They are often better treated as search/report rules than rewrite rules.
• Switch Chai should to expect: a useful migration example, but it is test-framework-specific and should be applied only where Chai is actually in use.
• Speed up barrel imports: strong rewriters / transform.rewrite example for splitting one import into many direct imports. Keep it on the CLI skill path because path conventions, default-vs-named exports, and formatting policy vary by repository.
• Missing Angular @Component() decorator: good example of labels plus pattern-object context and selector. Keep framework-specific rules tied to actual framework usage in the repository.
• Logical assignment operators: a compact rewrite example for $A = $A || $B to $A ||= $B, but only apply it where the project’s JS target and lint policy allow ES2021 operators.
• Adapt TypeScript catalog rules to the repository’s module system, framework stack, transpilation target, and lint policy before using them directly.

TSX Catalog Highlights

• TSX vs TypeScript matters for parsing: JSX-bearing patterns should stay on the TSX parser unless the repository intentionally routes .ts through TSX with languageGlobs.
• Unnecessary useState<T> primitives: good cleanup rewrite for useState<string|number|boolean>($A) when the initializer already gives TypeScript enough information to infer the state type. Bundled as rules/tsx/no-unnecessary-usestate-type.yml.
• Avoid && short-circuit in JSX: good React-facing rewrite from {cond && <View />} to {cond ? <View /> : null} when the left side can evaluate to renderable falsy values like 0. Bundled as rules/tsx/avoid-jsx-short-circuit.yml.
• Rewrite MobX component style: useful migration example when observer(() => ...) hides React hook linting from tooling. Keep it on the CLI skill path because naming, export shape, and component conventions vary by repository.
• Avoid unnecessary React hooks: good diagnostic rule for use* functions that do not actually call hooks. Treat it as a review rule first, because renaming or de-hooking can be API-affecting.
• Reverse React Compiler: clearly rewrite-oriented and intentionally opinionated. Keep it on the CLI skill path and only use it when the user explicitly wants that de-memoization behavior.
• Avoid nested links: good accessibility and correctness scan rule for JSX trees. Bundled as rules/tsx/no-nested-links.yml.
• Rename SVG attributes: strong TSX rewrite example for hyphenated SVG attribute names such as stroke-linecap to strokeLinecap. Keep it reviewable because generated markup can be formatting-sensitive. Bundled as rules/tsx/rename-svg-attribute.yml.
• Adapt TSX catalog rules to the repository’s React version, JSX runtime, lint rules, framework conventions, and browser-support target before using them directly.

YAML Catalog Highlights

• YAML scan rules are useful for configuration-policy checks where the repository needs to flag specific keys or values rather than rewrite source code.
• The catalog host/port example is a simple message-oriented rule that matches either host: $HOST or port: $PORT and attaches a diagnostic. Treat it as a starting point for config validation, not a complete policy by itself.
• For YAML rules, be explicit about whether the repository cares about the key name, the value, or both. If both matter together, move from separate any patterns to a more structured rule before relying on the result.
• Keep YAML config checks repository-specific. Hard-coded values like 8000 are only useful when they reflect an actual project policy.

Ruby Catalog Highlights

• Key Ruby tree-sitter node kinds for pattern authoring: call for method calls (e.g. $OBJ.method), method_call for keyword-style calls (e.g. puts "hello"), block for {{ }} blocks, do_block for do...end blocks, symbol for :name literals, assignment for variable assignments, method for method definitions, class for class definitions, if for conditionals, unless for negative conditionals, case for case/when, while and until for loops, return for return statements, yield for yield calls, super for super calls, self for self references.
• Ast-grep parses Ruby patterns with its bundled Ruby parser. Use ast-grep run --lang ruby --debug-query through exec_command to inspect surprising matches.
• Ruby’s $VAR meta-variable syntax works directly because $ is a valid Ruby global variable prefix. No expandoChar override is needed.
• Rails *_filter to *_action: useful migration rewrite for older Rails controllers. The catalog rule uses a transform with replace to swap _filter for _action on the captured $FILTER meta-variable. The pattern uses $$$ACTION to capture all arguments after the filter name. Keep it on the CLI skill path because framework version, controller style, and review expectations vary by repository:

id: migration-action-filter
language: Ruby
rule:
  any:
    - pattern: before_filter $$$ACTION
    - pattern: after_filter $$$ACTION
    - pattern: around_filter $$$ACTION
  has:
    pattern: $FILTER
    kind: identifier
fix:
  template: $FILTER_ACTION $$$ACTION
  transform:
    FILTER_ACTION:
      source: $FILTER
      replace:
        regex: _filter$
        by: _action

• Prefer symbol over proc: good Ruby cleanup rewrite for cases like .select { |v| v.even? } to .select(&:even?). The catalog rule constrains ITER to map|select|each via regex, and matches the block pattern $LIST.$ITER { |$V| $V.$METHOD }. The fix uses $LIST.$ITER(&:$METHOD) syntax. Only apply where the shorthand remains readable and matches local Ruby style. Extend the ITER regex to cover reject, find_all, detect, any?, all?, none?, count when appropriate.
• Path traversal detection in Rails: good security-oriented scan rule for Rails.root.join, File.join, or send_file fed by variables. Uses any with three patterns and severity: hint because this is a detection rule, not proof of exploitability. The surrounding validation path still matters. Advise File.basename() or allowlist validation as remediation.
• For bare block fragments like { |$V| $V.$METHOD } or do |$V| $V.$METHOD end, wrap in the enclosing method call and use selector: call to match the outer call. For symbol-to-proc, match the enclosing method call directly with $LIST.$ITER(&:$METHOD).
• Adapt Ruby catalog rules to the repository’s Rails version, Ruby style guide, and security posture before using them directly.

Python Catalog Highlights

• Key Python tree-sitter node kinds for pattern authoring: function_definition for functions, call for function calls, import_statement and import_from_statement for imports, assignment for assignments, decorated_definition for decorated functions/classes, with_statement for context managers, try_statement for try/except, if_statement for conditionals, for_statement for loops, return_statement for returns, async_function_definition for async functions, await for await expressions, type for type annotations, subscript for generic types like Optional[T], list_comprehension for list comprehensions, argument_list for function arguments, keyword_argument for keyword arguments, conditional_expression for ternary expressions, assert_statement for assertions.
• Ast-grep parses Python patterns before matching them. Use ast-grep run --lang python --debug-query to inspect the parsed query when metavariables or incomplete fragments behave unexpectedly.
• Python’s $VAR meta-variable syntax works directly because $ is not a valid Python identifier prefix in expression context. No expandoChar override is needed.
• OpenAI SDK migration: useful multi-rule migration example for legacy openai Python client code, but keep it on the CLI skill path because imports, client lifetime, response shapes, and surrounding application logic often need repository-specific review. The migration uses three rules separated by ---: import rewrite (import openai to from openai import Client), client initialization (openai.api_key = $KEY to client = Client($KEY)), and completion method (openai.Completion.create($$$ARGS) to client.completions.create($$$ARGS)).
• Prefer generator expressions: good example of narrowing a rewrite to contexts like any(...), all(...), or sum(...) where generator expressions are clearly valid. Do not generalize it to every list comprehension. The constraint-based variant uses to restrict to and to kind, then strips brackets with a transform:

id: prefer-generator-in-builtins
language: python
rule:
  pattern: $FUNC($LIST)
  constraints:
    FUNC:
      regex: ^(any|all|sum)$
    LIST:
      kind: list_comprehension
transform:
  INNER:
    substring:
      source: $LIST
      startChar: 1
      endChar: -1
fix: $FUNC($INNER)

• Walrus operator in if statements: useful paired-rule rewrite example, but only apply it where the repository targets Python 3.8+ and the style guide accepts assignment expressions. This is a multi-rule YAML using follows and precedes relational operators. The first rule rewrites the if to use :=, the second deletes the preceding assignment:

id: use-walrus-operator
language: python
rule:
  follows:
    pattern:
      context: $VAR = $$$EXPR
      selector: expression_statement
  pattern: "if $VAR: $$$B"
fix: |-
  if $VAR := $$$EXPR:
      $$$B
---
id: remove-walrus-source
language: python
rule:
  pattern: $VAR = $$$EXPR
  kind: expression_statement
  precedes:
    pattern: "if $VAR: $$$B"
fix: ‘’

• Remove async function: strong rewriters example for stripping async and inner await, but treat it as high-risk migration work because it changes call semantics and often requires broader control-flow review. Uses rewriters to strip await from inside the body before removing the async keyword:

id: remove-async
language: python
rule:
  pattern:
    context: ‘async def $FUNC($$$ARGS): $$$BODY’
    selector: function_definition
rewriters:
  remove-await-call:
    pattern: ‘await $$$CALL’
    fix: $$$CALL
transform:
  REMOVED_BODY:
    rewrite:
      rewriters: [remove-await-call]
      source: $$$BODY
fix: |-
  def $FUNC($$$ARGS):
      $REMOVED_BODY

• Pytest fixture refactors: good example of utils-driven context matching for fixture rename or type-hint updates. Uses utils to define reusable context matchers like is-fixture-function (function following a @pytest.fixture decorator) and is-test-function (function whose name starts with test_). Keep it tied to real pytest usage so similarly named non-test code is not swept in.
• Optional[T] to T | None and recursive union rewrites: useful typing-modernization examples, but only where the repository targets Python 3.10+ and static typing policy actually prefers PEP 604 unions. The simple variant uses context and selector to disambiguate Optional[$T] as a generic type:

id: optional-to-union
language: python
rule:
  pattern:
    context: ‘a: Optional[$T]’
    selector: generic_type
fix: $T | None

The recursive variant handles nested Union and Optional types using multiple rewriters that call each other, transforming deeply nested expressions like Optional[Union[List[Union[str, dict]], str]] into List[str | dict] | str | None.

• SQLAlchemy mapped_column to annotated Mapped[...]: useful ORM migration example, but keep it on the CLI skill path because ORM version, model style, and nullable semantics need review. Uses rewriters to filter out String positional args and nullable=True keyword args from the argument list, then wraps the result in Mapped[str | None].
• print detection: use kind: call with has: { field: function, pattern: print } to match print() calls. Scope with files to exclude test directories and scripts where console output is acceptable. For logging.debug() or similar, use regex: ^(debug|info|warning)$ on the function field inside a logging. attribute access.
• f-string preference: use kind: call with has: { field: function, pattern: $FN } and constraints restricting $FN to ^(str|int|float|repr)$ to find type-conversion calls that could be f-string expressions. This is a suggestion rule, not an enforcement rule, because some conversions are intentional type coercion.
• List comprehension vs map/filter: pattern $LIST = list(map($FUNC, $ITER)) can be rewritten to $LIST = [$FUNC($X) for $X in $ITER] when $FUNC is a simple lambda or single-argument call. Keep it on the CLI skill path because readability depends on the complexity of $FUNC.
• dict.get with default: pattern $D[$KEY] inside a try_statement with except KeyError can often be rewritten to $D.get($KEY) or $D.get($KEY, $DEFAULT). Use kind: subscript with inside to scope within the try body. Treat as review material because some dict access patterns intentionally propagate KeyError.
• Assert vs unittest assertions: pattern assert $EXPR == $VAL can be rewritten to self.assertEqual($EXPR, $VAL) in unittest contexts, or left as-is in pytest contexts. Use files to scope by test framework convention.
• isinstance tuple consolidation: pattern can be rewritten to . This is a safe autofix when both calls check the same variable.

Kotlin Catalog Highlights

• Clean-architecture import checks: good scan-rule example for enforcing architectural boundaries with files plus import-path constraints. Treat it as repository-policy enforcement rather than a universal Kotlin rule.
• The Kotlin catalog example is diagnostic-oriented, not rewrite-oriented. Keep it on the scan path because import-boundary violations usually need design review instead of blind mutation.
• File-scoped package constraints are the point of the example: adapt the files glob and package regexes to the repository’s actual module layout before relying on the result.
• Ast-grep parses Kotlin patterns with its bundled Kotlin parser. Use ast-grep run --lang kotlin --debug-query through exec_command to inspect surprising matches.
• Unsafe cast detection ($EXPR as $TYPE): good warning-level scan rule for catching runtime ClassCastException risks. The safe cast as? is a different AST node, so this pattern does not false-positive on safe casts. Treat as review material; some casts are intentionally unsafe after exhaustive when or is checks.
• var vs val preference: use kind: property_declaration with has: { field: property_delegate, pattern: var } to match mutable property declarations. A naive var $NAME: $TYPE pattern may over-match in contexts where the parser attaches different node structure. The kind plus has plus field approach is more robust.
• println detection: use an any composite to cover Kotlin’s top-level println($$$ARGS), Java’s System.out.println($$$ARGS), and System.err.println($$$ARGS). Scope with files to exclude test directories where console output is acceptable.
• isEmpty() preference: straightforward rewrite rule from $X.size == 0 or $X.length == 0 to $X.isEmpty(). Also cover $X.count() == 0 and $X.size <= 0. This is a safe autofix because Kotlin’s isEmpty() is semantically equivalent for standard collections and strings.
• lateinit detection: pattern lateinit var $NAME: $TYPE is a direct structural match. Use severity: info because is sometimes justified in dependency injection and test setup contexts. Teams should adjust severity to match their policy.

Java Catalog Highlights

• Unused local variable detection: useful educational example for has plus ordered all plus precedes, but prefer the project’s established linter or IDE for real unused-variable enforcement because Java variable scopes are broader than the sample rule covers. The rule uses all to guarantee that the meta-variable $IDENT is captured by the first has clause before the not/precedes check runs. Without that ordering, the meta-variable would not be available for the later comparison:

id: no-unused-vars
language: java
rule:
  kind: local_variable_declaration
  all:
    - has:
        has:
          kind: identifier
          pattern: $IDENT
    - not:
        precedes:
          stopBy: end
          has:
            stopBy: end
            any:
              - { kind: identifier, pattern: $IDENT }
              - { has: { kind: identifier, pattern: $IDENT, stopBy: end } }
fix: ‘’

Treat matches as review candidates, not conclusive unused-variable proofs. Java variable scopes are broader than this sample covers, and the project’s established linter or IDE is usually a better fit for real unused-variable enforcement.

• Field declarations of type String: good structural scan example showing why field_declaration plus has: { field: type } is more robust than a naive pattern when modifiers and annotations are present. A naive String $F; pattern fails because it ignores modifiers and annotations. A $MOD String $F; pattern also fails because tree-sitter does not consider $MOD a valid modifier and produces an ERROR node. The structural rule approach works regardless of how many modifiers or annotations precede the type:

id: find-field-with-type
language: java
rule:
  kind: field_declaration
  has:
    field: type
    regex: ^String$

Use this kind plus has plus field plus regex pattern whenever a naive code pattern fails because Java modifiers, annotations, or access qualifiers change the surface syntax. The field: type constraint targets the semantic type child of the declaration, not the raw text, so it is robust against private static final String, @Nullable String, or other decorated forms.

• The Java catalog examples are primarily search/diagnostic material, not high-confidence autofix rules. Keep them review-oriented unless the repository explicitly wants ast-grep-based cleanup instead of compiler or linter diagnostics.
• Adapt Java catalog rules to the repository’s package conventions, annotation usage, style tooling, and existing static-analysis stack before using them directly.

HTML Catalog Highlights

• HTML parser reuse for framework templates: useful when Vue, Svelte, Astro, or similar files are mostly HTML, but keep parser caveats in mind because framework-specific control flow or frontmatter may require a custom language instead. Use languageGlobs in sgconfig.yml to parse .vue, .svelte, or .astro files as HTML when the framework syntax is minimal enough for the HTML parser.
• Key HTML node kinds for pattern authoring: element for full HTML elements, tag_name for tag names, attribute_name for attribute names, attribute_value for attribute values, text for text content, and comment for HTML comments. Use these with kind to match specific HTML structures without writing full pattern syntax.
• Matching elements by tag name: use kind: element with has: { field: tag_name, pattern: $TAG } to match elements by their tag name. For regex-based tag matching (e.g. all heading tags), use kind: tag_name with regex: "^h[1-6]$" and inside: { kind: element }.
• Matching elements by attribute: use kind: element with has: { kind: attribute_name, regex: "^class$" } to find elements with a specific attribute. To also match the attribute value, add a nested has on the attribute node to capture attribute_value.
• Scoping with inside and stopBy: HTML inside with stopBy: { kind: element } scopes matches to the nearest enclosing element. This is essential for avoiding cross-element matches in deeply nested HTML. The inside-tag utility pattern from the catalog demonstrates wrapping inside with kind: element and has to capture the enclosing tag name, then using constraints to restrict which tags match.
• Ant Design Vue visible to open: good framework-specific attribute rewrite using enclosing-tag checks plus constraints. The pattern uses kind: attribute_name with regex: :visible to match the attribute, inside to find the enclosing element, has to capture the , and to restrict to specific components (). Keep it on the CLI skill path because framework version and component set must be confirmed first.

Go Catalog Highlights

• Problematic defer with nested function calls: strong Go-specific scan example for catching cases where deferred arguments are evaluated immediately instead of at function exit. In Go, defer evaluates arguments when the defer statement is encountered, not when the deferred function runs. This is particularly problematic with assertion libraries in tests:

id: problematic-defer-call
language: go
rule:
  pattern:
    context: ‘{ defer $A.$B(t, failpoint.$M($$$)) }’
    selector: defer_statement

Treat matches as correctness and test-reliability review items. The fix is wrapping in a closure: defer func() { require.NoError(t, failpoint.Disable("...")) }(). Adapt to the repository’s test conventions before enabling broadly.

• Function declarations by name pattern: good example of using kind plus has plus regex when a meta-variable pattern cannot express the naming constraint directly. A plain Test$_ pattern fails because it is not valid syntax; use a YAML rule instead:

id: test-functions
language: go
rule:
  kind: function_declaration
  has:
    field: name
    regex: Test.*

Useful for test discovery, migration targeting, or repository audits where meta-variable patterns are too limited.

• Contextual matching for function calls: Go’s tree-sitter grammar parses fmt.Println($A) as a type conversion, not a call expression, because Go syntax allows both. Use a contextual pattern with selector: call_expression through the CLI skill path via exec_command:

id: match-function-call
language: go
rule:
  pattern:
    context: ‘func t() { fmt.Println($A) }’
    selector: call_expression

Use this pattern whenever a plain call-expression pattern under-matches or parses as a conversion in Go.

• Package-import detection: useful search/scan rule for dependency auditing, compliance checks, or migration prep. Adapt the import regex to the repository’s actual dependency boundaries instead of hard-coding example packages:

id: match-package-import
language: go
rule:
  kind: import_spec
  has:
    regex: github.com/golang-jwt/jwt

• Problematic JSON tags with -,: high-signal security-oriented scan rule for Go struct tags. When a struct field has a JSON tag starting with -,, it can be unexpectedly unmarshaled with the - key, bypassing the developer’s intent to omit the field. This is a real unmarshaling footgun, not just a style preference:

id: unmarshal-tag-is-dash
severity: error
message: Struct field can be decoded with the `-` key because the JSON tag
  starts with a `-` but is followed by a comma.
rule:
  pattern: ‘`$TAG`’
  inside:
    kind: field_declaration
constraints:
  TAG:
    regex: json:"-,.+

Treat matches as actionable security review items. The fix is using just - without a comma: json:"-".

• Adapt Go catalog rules to the repository’s Go version, test conventions, package layout, security posture, and existing static-analysis tooling before using them directly.

Cpp Catalog Highlights

• Reuse Cpp rules for C only when the repository intentionally parses C sources as Cpp via languageGlobs; do not assume mixed C/C++ projects want that parser tradeoff by default.
• Ast-grep bundles C++ parser support. Use --lang cpp for ad-hoc CLI patterns and language: Cpp in YAML rules. Run ast-grep run --lang cpp --debug-query through exec_command to inspect surprising matches.
• Format-string vulnerability rewrite: strong security-oriented example for fprintf/sprintf-style calls missing an explicit format string. Uses constraints with regex on $PRINTF and kind on $VAR to distinguish vulnerable calls from safe ones. The fix inserts "%s" as the format argument:

id: fix-format-string
language: cpp
rule:
  pattern: $PRINTF($S, $VAR)
  constraints:
    PRINTF:
      regex: ^sprintf|fprintf$
    VAR:
      not: { kind: string_literal }
      not: { kind: concatenated_string }
fix: $PRINTF($S, "%s", $VAR)

Keep it reviewable because real C/C++ codebases may prefer safer API migrations (e.g. snprintf) over mechanical "%s" insertion in some contexts.

• Struct inheritance matching: useful example of AST-shaped pattern authoring in C++. A shortened surface pattern like struct $SOMETHING: $INHERITS produces an ERROR node because tree-sitter-cpp requires the full syntactic form. Use the complete pattern with the body block:

id: find-struct-inheritance
language: cpp
rule:
  pattern: struct $NAME : $BASE { $$$BODY; }

This matches structs that use inheritance via base class clauses. The full struct ... : ... { ... } shape is required for the parser to produce a valid struct_specifier node instead of an ERROR node.

• Adapt Cpp catalog rules to the repository’s C-vs-C++ parser choice, security posture, libc usage, and coding-standard expectations before using them directly.

C Catalog Highlights

• Parsing C as Cpp can reduce duplicated rule authoring, but only use that route when the repository intentionally opts into the parser tradeoff with languageGlobs; do not blur C and C++ semantics by default.
• Match function calls in C with contextual patterns: tree-sitter-c parses code fragments differently depending on surrounding syntax. A bare test($A) becomes macro_type_specifier, while test($A); becomes expression_statement -> call_expression. Use context plus selector: call_expression through the CLI skill path via exec_command:

id: match-function-call
language: c
rule:
  pattern:
    context: $M($$$);
    selector: call_expression

• Rewrite method-style calls to function calls: useful migration example for C codebases that emulate methods with structs or function pointers. Uses transform with replace to derive a conditional comma from $$$ARGS:

id: method_receiver
language: c
rule:
  pattern: $R.$METHOD($$$ARGS)
transform:
  MAYBE_COMMA:
    replace:
      source: $$$ARGS
      replace: ‘^.+’
      by: ‘, ‘
fix:
  $METHOD(&$R$MAYBE_COMMA$$$ARGS)

Keep it on the CLI skill path because it changes calling conventions and may affect ownership, pointer semantics, or naming policy.

• Yoda-condition rewrite: clearly style-driven and repository-policy-sensitive. Uses constraints to restrict $B to number_literal and inside to scope within if_statement:

id: may-the-force-be-with-you
language: c
rule:
  pattern: $A == $B
  inside:
    kind: parenthesized_expression
    inside: {kind: if_statement}
constraints:
  B: { kind: number_literal }
fix: $B == $A

Treat it as optional rewrite material only where the project explicitly prefers constant-on-the-left comparisons.

• Adapt C catalog rules to the repository’s parser choice, macro usage, pointer conventions, coding style, and safety policy before using them directly.

Markdown Catalog Highlights

• Markdown became a first-class language in ast-grep 0.43. Query with --lang md or lang: markdown in YAML rules.
• Queryable node kinds include atx_heading for ATX-style headings, fenced_code_block for fenced code blocks, and list_item for list items.
• Combine node kinds with compound selectors for broader sweeps, for example atx_heading, fenced_code_block matches both headings and code blocks.
• Markdown parsing is powered by tree-sitter-md and still has known parsing bugs and edge cases. Use it for inspection, indexing, outline extraction, and lightweight automation rather than critical rewrites.
• Ast-grep infers a language from recognised file extensions during path scans. Pass --lang md for stdin or whenever extension inference is unavailable.
• Adapt Markdown catalog rules to the repository’s documentation conventions, heading hierarchy, and content structure before using them directly.

JavaScript API Highlights

• Use @ast-grep/napi only when rule YAML and CLI commands are insufficient. The programmatic API suits computed replacements, ordered-match logic, cross-node inspection, or edit orchestration that would be awkward in pure rule syntax.
• Core objects are SgRoot and SgNode: parse(Lang.<X>, source) creates the tree, root() returns the root node, and find / findAll / traversal / refinement / edit APIs live on SgNode.
• Matcher inputs can be pattern strings, numeric kind ids, or NapiConfig objects. Prefer patterns or NapiConfig unless there is a concrete reason to drop to raw kind ids.
• getMatch and getMultipleMatches expose captured metavariables, but replace does not interpolate metavariables for you. Build replacement strings explicitly in JavaScript from matched nodes before calling commitEdits.
• Prefer the CLI path for ordinary query, scan, test, and rewrite flows. Use NAPI only when the task is genuinely programmatic.
• registerDynamicLanguage and extra language packages exist, but that path is still experimental. Prefer established parsers and repo-native tooling unless dynamic-language support is actually needed.

Python API Highlights

• Use ast-grep-py when the task needs programmatic AST traversal or computed edits but a Python host environment is a better fit than JavaScript or Rust. Prefer it only after rule YAML and direct CLI commands stop being a good fit.
• Core objects are again SgRoot and SgNode: SgRoot(source, language) parses the source, root() returns the root node, and search, refinement, traversal, and edit APIs live on SgNode.
• find and find_all support either direct rule keyword arguments or a config object. Prefer keyword-rule searches for simple cases and config objects when constraints or utility rules make the query more expressive.
• get_match, get_multiple_matches, and __getitem__ expose captured metavariables. __getitem__ is useful when you want a stricter access pattern and are willing to let missing captures raise instead of returning None.
• replace and commit_edits generate source edits, but they do not interpolate metavariables for you. Build replacement text explicitly from matched nodes before applying edits.
• Use the Python API only for genuinely programmatic transformations, rather than ordinary CLI queries or rewrites.

NAPI Performance Highlights

• NAPI is not automatically faster than host-language traversal. Performance usually comes from reducing Rust↔JavaScript FFI crossings and letting ast-grep do more work per boundary crossing.
• Prefer parseAsync over parse when many parse jobs can benefit from Node’s libuv thread pool and the main JS thread is already busy.
• Prefer findAll over manual recursive traversal in JavaScript. One bulk Rust-side search is usually cheaper than repeated kind(), children(), and recursion calls across the FFI boundary.
• Prefer findInFiles when scanning many files and you can use its file-path-oriented search model. It avoids unnecessary round-tripping source strings through JavaScript and can parallelize work in Rust threads.
• findInFiles has a callback-completion caveat: its returned promise can resolve before all callbacks run. If completion ordering matters, track callback counts explicitly before treating the scan as finished.
• Apply these performance tips only when scale justifies them. For small inputs or one-off transformations, simpler synchronous code is often the better tradeoff.

Rule Essentials

• Start rule files with id, language, and root rule.
• Treat the root rule as a rule object that matches one target AST node per result.
• A rule object still needs a positive anchor. In practice, start with pattern or kind; regex is a filter, not a sufficient root rule by itself.
• Atomic rules are pattern, kind, regex, nthChild, and range.
• Use atomic fields such as pattern, kind, regex, nthChild, and range for direct node checks.
• On the CLI, use --kind alone to match by node type or combine it with a pattern to filter matches by node kind.
• Use relational fields such as inside, has, follows, and precedes when the match depends on surrounding nodes.
• Use composite fields such as all, any, not, and matches to combine sub-rules or reuse utility rules.
• kind values support ESQuery-style pseudo-selectors (:has(), :not(), :is(), :nth-child()) for matching nodes by descendant structure, exclusion, alternatives, or sibling position without writing separate relational rules.
• Rule object fields are effectively unordered and conjunctive; if matching becomes order-sensitive, rewrite the logic with an explicit all sequence instead of assuming YAML key order matters.
• language controls how patterns parse. Syntax that is valid in one language can fail in another.

Rule Cheat Sheet

• Atomic rules check properties of one node. Start here when a single syntax shape is enough.
• pattern, kind, and regex are the common atomic fields. pattern can also be an object with context, selector, and optional strictness.
• Pattern objects are for invalid, incomplete, or ambiguous snippets. context is required; selector picks the real target node inside that context; strictness tunes how literally the pattern matches.
• Use pattern objects when the bare snippet would parse as the wrong node kind, such as JavaScript class fields or Go/C call expressions inside ambiguous fragments.
• kind is usually a plain node kind name, but ast-grep 0.42+ supports ESQuery-style pseudo-selectors in kind strings. Use :has(selector) or :has(> selector) to match nodes containing descendants or direct children, :not(selector) to exclude nodes, :is(selector, ...) for or-logic in compound selectors, and :nth-child(An+B) or :nth-child(An+B of selector) for positional matching. These pseudo-selectors also work in CLI selector values.
• ast-grep 0.43+ further expands kind with compound selector operators: A > B (direct child), A B (descendant), A + B (immediate sibling), A ~ B (general sibling), and A, B (either). This syntax works in YAML rule kind fields and the CLI --kind / -k flag. It is ESQuery-style, not full ESQuery: class selectors, attribute selectors, and wildcard selectors are not supported.
• Separate kind and pattern checks do not change how the pattern is parsed. If parse shape is the problem, switch to one pattern object with context and selector.
• regex matches the whole node text. Reach for nthChild when position among named siblings matters and range when the match must be limited to a known source span.
• Regex syntax follows Rust regex, not PCRE. Do not assume look-around or backreferences are available, and usually pair with or so the expensive text check only runs on the right node shapes.

ESQuery-Style Kind Selectors

Ast-grep supports ESQuery-style selectors in YAML rule kind fields and the CLI --kind or -k flag. Ast-grep parses the selector internally.

Relationship Selectors

• Child selector (>): matches a direct child node. kind: call_expression > identifier is equivalent to kind: identifier with inside: { kind: call_expression }.
• Descendant selector (space): matches a descendant node. kind: call_expression identifier is equivalent to kind: identifier with inside: { kind: call_expression, stopBy: end }.
• Adjacent sibling selector (+): matches the next sibling node. kind: decorator + method_definition is equivalent to kind: method_definition with follows: { kind: decorator }.
• Following sibling selector (~): matches any following sibling node. kind: decorator ~ method_definition is equivalent to kind: method_definition with follows: { kind: decorator, stopBy: end }.

Comma Selector

• Comma-separated selectors are converted to any. kind: identifier, number is equivalent to any: [{ kind: identifier }, { kind: number }].

Pseudo-classes

• :has(selector): matches a node if it has a descendant matching the inner selector. kind: function_declaration:has(return_statement) matches function declarations that contain a return statement. Use > inside :has to match a direct child: kind: expression_statement:has(> call_expression).
• :not(selector): negates the inner selector. kind: identifier:not(number) matches identifiers that are not numbers.
• :is(selector, ...): accepts comma-separated selectors and is converted to any. kind: :is(identifier, number) matches either identifiers or numbers. Can be combined with relationship selectors: kind: call_expression > :is(identifier, number).
• :nth-child(An+B): maps to ast-grep's nthChild rule. kind: array > number:nth-child(2n+1) matches odd-numbered number elements in arrays.
• :nth-child(An+B of selector): supports of syntax for filtering. kind: array > :nth-child(1 of number) matches the first number element in an array.
• :nth-last-child(position): equivalent to nthChild with reverse: true. kind: array > number:nth-last-child(1) matches the last number element in an array.

Compound Selectors

Compound selectors are combined with all. kind: function_declaration:has(return_statement):not(generator_function) is equivalent to all: [{ kind: function_declaration }, { has: { kind: return_statement, stopBy: end } }, { not: { kind: generator_function } }].

Examples

# Match identifiers that are direct children of call expressions
kind: call_expression > identifier

# Match any identifier or number node
kind: identifier, number

# Match function declarations containing return statements
kind: function_declaration:has(return_statement)

# Match identifiers that are not numbers
kind: identifier:not(number)

# Match either identifiers or numbers
kind: :is(identifier, number)

# Match the first number element in an array
kind: array > :nth-child(1 of number)

# Match odd-indexed elements
kind: array > number:nth-child(2n+1)

# Combine with pattern: match fn declarations that have return statements
pattern: "fn $NAME() {}"
kind: function_item:has(return_statement)

# C++: match class definitions that have virtual methods
kind: class_specifier:has(virtual_function_specifier)

# C++: match template function declarations
kind: template_declaration > function_definition

# C++: match delete expressions (potential memory management issues)
kind: delete_expression

# Python: match function definitions with decorators
kind: decorated_definition > function_definition

Current Limitations

• Class selectors like .body are tokenized but rejected as unsupported.
• Supported pseudo-classes are only :has, :not, :is, :nth-child, and :nth-last-child.
• :has(...), :not(...), and of ... parse a single complex selector, not a comma selector list.
• :is(...) is the one pseudo-class that accepts comma-separated selector lists.
• Identifiers can include letters, digits, _ and -, but cannot start with a digit.

Config Cheat Sheet

• Basic info keys define the rule itself. Use id for the unique rule name, language for the parser target, url for rule documentation, and metadata for custom project data. Native JSON includes metadata only when the scan uses --include-metadata.
• One YAML file can hold multiple rules when you separate documents with ---.
• Finding keys define what gets matched. rule is the core matcher, constraints narrows meta-variable captures, and utils holds reusable helper rules that you call through matches.
• utils can be purely local to the current file or can supplement global utility-rule files loaded through utilDirs. Keep shared building blocks global only when multiple rule files genuinely need them.
• constraints runs after rule matched, only targets single meta variables like $ARG, and is a poor fit inside not.
• Patching keys define reusable fixes. Use transform to derive new meta-variables before replacement, fix for either a string replacement or a template object with expandStart / expandEnd, and rewriters when the transformation is too complex for one inline fix.
• Linting keys define what scan results report. Use severity, message, note, and labels for diagnostics, then files and ignores to scope where the rule applies.
• Severity levels are error, warning, info, hint, and off. hint is the default severity in ast-grep project scans.
• An error-severity match makes ast-grep scan exit with status 1. Inspect the native human-readable or JSON findings before treating that status as an execution failure.
• severity: off disables the rule during scanning. note supports Markdown but cannot interpolate meta variables.
• Use scan flags such as , , , , and to override named rule severities for one CLI run. Use to select configured rule identifiers.

Transformation Objects

• transform builds new strings from captured meta variables before fix runs.
• Each transform entry introduces a new variable name without a leading $. Inside the transform object, source still points at an existing capture or prior transform result using the normal $VAR form.
• Later transforms can consume variables created by earlier transforms, so transform order matters when you are stacking multiple string operations.
• Transforms are evaluated in declaration order. A transform that references a variable created by an earlier transform in the same transform block will see the already-transformed value.
• Transforms only run after the rule matches. If the rule does not match, no transforms execute and no fix is applied.

replace

• replace uses a Rust regex over one meta variable. source must be $VAR style, replace is the regex, by is the replacement text, and regex capture groups can be reused in by.
• Regex capture groups are only available inside the replace field of the replace transform and can only be referenced from the by field of that same transform. Regular regex rules do not expose those capture groups.
• Rust regex syntax applies: no look-around, no backreferences in the traditional PCRE sense, but capture groups () work and are referenced as $1, $2, etc. in by.

# Strip leading underscore from a variable name
transform:
  CLEAN_NAME:
    replace:
      replace: "^_"
      by: ""
      source: $VAR

# Extract domain from email using capture group
transform:
  DOMAIN:
    replace:
      replace: "^[^@]+@(.+)$"
      by: "$1"
      source: $EMAIL

# String-form (ast-grep 0.38.3+)
transform:
  CLEAN_NAME: replace($VAR, replace="^_", by="")

substring

• substring slices a meta variable by character index with inclusive startChar and exclusive endChar. Negative indexes count from the end, and slicing is based on Unicode characters rather than raw bytes.
• substring behaves like Python string slicing, so omit either bound when the slice should stay open-ended.

# Remove first and last character (e.g., strip quotes)
transform:
  UNQUOTED:
    substring:
      startChar: 1
      endChar: -1
      source: $STR

# Get first 3 characters
transform:
  PREFIX:
    substring:
      endChar: 3
      source: $ID

# Remove first character only
transform:
  NO_PREFIX:
    substring:
      startChar: 1
      source: $NAME

# String-form (ast-grep 0.38.3+)
transform:
  UNQUOTED: substring($STR, startChar=1, endChar=-1)

convert

• convert changes identifier-style casing through toCase. Common outputs are lowerCase, upperCase, capitalize, camelCase, snakeCase, kebabCase, and pascalCase.
• Use separatedBy to control how convert splits words before rebuilding the target case. Supported separators include dash, dot, space, slash, underscore, and CaseChange.
• CaseChange splits at transitions such as astGrep, ASTGrep, or XMLHttpRequest, which matters when converting mixed acronym identifiers.
• When separatedBy is omitted, all known separators are used. This is usually the right default.

# Convert camelCase to snake_case
transform:
  SNAKE_NAME:
    convert:
      toCase: snakeCase
      source: $CAMEL

# Convert only by underscore, preserving camelCase within segments
transform:
  KEBAB_FROM_UNDERSCORE:
    convert:
      toCase: kebabCase
      separatedBy: [underscore]
      source: $UNDERSCORE_NAME

# Convert PascalCase to camelCase (using CaseChange separator)
transform:
  CAMEL:
    convert:
      toCase: camelCase
      separatedBy: [CaseChange]
      source: $PASCAL

# String-form (ast-grep 0.38.3+)
transform:
  SNAKE_NAME: convert($CAMEL, toCase=snakeCase)

Chaining Transforms

• Later transforms can consume variables created by earlier transforms. This is the standard way to build multi-step string pipelines.
• Transform order is the declaration order in the YAML transform map.

# Pipeline: strip prefix, then convert case
transform:
  RAW_NAME:
    replace:
      replace: "^get"
      by: ""
      source: $METHOD_NAME
  SNAKE:
    convert:
      toCase: snakeCase
      source: $RAW_NAME
# Input: "getUserName" -> RAW_NAME="UserName" -> SNAKE="user_name"

Conditional Separators from Multi-Capture

• Use replace transforms for conditional punctuation or whitespace when a multi-capture may be empty. The common pattern is deriving MAYBE_COMMA or similar from $$$ARGS so the extra separator only appears when matches exist.

# Add comma only when there are arguments
rule:
  pattern: "foo($$$ARGS)"
transform:
  MAYBE_COMMA:
    replace:
      replace: ".+"
      by: ", "
      source: $ARGS
fix: "bar($MAYBE_COMMA$newArg)"
# If $$$ARGS matched "a, b" -> MAYBE_COMMA=", " -> "bar(, newArg)"
# If $$$ARGS matched nothing -> MAYBE_COMMA="" -> "bar(newArg)"

String-Form Transforms

• String-form transforms such as replace(...), substring(...), convert(...), and rewrite(...) are valid shorthand in ast-grep 0.38.3+.
• Prefer object form when compatibility or debugging clarity matters.
• String-form syntax: operator($SOURCE, key1=value1, key2=value2).
• Array values use [item1, item2] syntax inside the string form.

Rewriters

• rewriters is an experimental feature for advanced multi-node rewrites. Prefer ast-grep’s API instead when the YAML starts carrying too much control flow or state.
• Rewriters allow replacing multiple sub-nodes with different fixes in one rule. The normal fix replaces one matched node at a time; rewriters plus transform.rewrite handle the one-to-many case.
• The three-step workflow is:
  1. Define rewriters at the YAML rule root. Each rewriter needs id, rule, and fix. Optional fields are constraints, transform, and utils.
  2. Apply the rewriter to a metavariable via transform using the rewrite operator. rewriters lists which rewriter ids to try; source points at the metavariable whose sub-nodes are rewritten.
  3. Use the resulting transformed metavariable in the outer fix.
• Concrete example converting Python dict(a=1, b=2) to {‘a’: 1, ‘b’: 2}:
  • Define a rewriter that matches keyword_argument nodes and rewrites $KEY=$VAL to ’$KEY’: $VAL.
  • Apply it to $$$ARGS captured from dict($$$ARGS) via transform: { LITERAL: { rewrite: { rewriters: [dict-rewrite], source: $$$ARGS } } }.
  • Use fix: ‘{ $LITERAL }’ on the outer rule to wrap the rewritten arguments in braces.
• Multiple rewriters can be listed in one transform.rewrite call. Each sub-node is transformed by the first matching rewriter in declaration order. If two rewriters could match the same node, only the one that appears earlier in the rewriters list is applied. Order matters.
• joinBy controls how transformed sub-nodes are stitched together. By default, sub-nodes are replaced in-place preserving original separators. Set joinBy to a string like ’ + ‘ or ’\n’ to override the joiner.
• A rewriter can call other rewriters from the same rewriters list inside its own transform section, enabling multi-pass rewrite pipelines.

Pattern Syntax

• Pattern code must be valid code that tree-sitter can parse.
• Patterns match syntax trees, so a query can match nested expressions instead of only top-level text.
• $VAR matches one named AST node.
• $$$ARGS matches zero or more AST nodes in places like arguments, parameters, or statements.
• Reusing the same captured name means both occurrences must match the same syntax.
• Prefixing a meta variable with _ disables capture, so repeated $_X occurrences do not need to match the same content.
• $$VAR captures unnamed nodes when named-node matching is too narrow.
• If a short snippet is ambiguous, move to an object-style pattern with more context plus selector instead of guessing.

Pattern Parsing Deep Dive

• Pattern creation has four stages: preprocess meta-variable text when the language needs a custom expandoChar, parse the snippet, choose the effective node, then detect meta variables inside that effective node.
• Invalid pattern code usually fails because a meta variable is standing in for syntax that the parser treats as an operator or keyword. Patterns like $LEFT $OP $RIGHT or { $KIND foo() {} } should become rule objects using parseable code plus kind, regex, has, or other rule fields.
• Incomplete or ambiguous snippets can appear to work only because tree-sitter recovered from an error. Treat that as best-effort behavior, not a stable contract across ast-grep upgrades, and prefer valid context plus selector.
• The default effective node is the leaf node or the innermost node with more than one child. Override it with selector when the real match should be a statement instead of the inner expression, especially for follows and precedes.
• Meta variables are detected only when the whole AST node text matches meta-variable syntax. Mixed text like obj.on$EVENT, lowercase names like $jq, or string-content fragments do not become meta variables.
• $$VAR captures unnamed nodes such as operators when the grammar exposes them only as anonymous tokens. $$$ARGS is lazy: it stops before the next node that satisfies the rest of the pattern.
• When pattern behaviour is surprising, inspect the parsed tree and effective node with ast-grep run --debug-query; in the Playground, use the pattern view for the same questions.

Pattern Core Concepts

• ast-grep is structural search, not plain text search. pattern matches syntax-tree shape, while regex is the escape hatch when node text itself matters.
• Tree-Sitter gives ast-grep a concrete syntax tree, not a stripped-down abstract syntax tree. That CST detail is why punctuation and modifiers can still matter even when matching stays syntax-aware.
• Named nodes carry a kind; unnamed nodes are punctuation or literal tokens. Meta variables match named nodes by default, and $$VAR is the opt-in when unnamed nodes matter.
• kind belongs to the node itself. field belongs to the parent-child relationship, so use relational has or inside with field when role matters more than raw node kind.
• Pseudo-selectors extend kind matching with CSS/ESQuery-style combinators. They are string-level refinements on node kind names, not separate rule fields, so they compose naturally with pattern and relational rules.