ast-grep

Use ast-grep with patterns written in the target language syntax, where $VAR matches one AST node and $$$ matches zero or more nodes. Run the helper's search subcommand with --lang set, for example searching 'console.log($MSG)' across TypeScript files.

Run the helper's replace subcommand with a pattern and rewrite template; it defaults to a dry-run preview showing matches and affected files. Add --apply to mutate files, which internally runs a second pass with --update-all since sg silently ignores it when --json is set.

Use ast-grep when the answer depends on code structure, such as function shapes, call patterns, imports, or control flow. Use ripgrep when the target is text-shaped, like string literal contents, comments, license headers, or cross-language regex with alternation.

Most failures come from regex syntax like \w, .*, or | which ast-grep does not support, or from incomplete patterns like a Python def with a trailing colon. Run the helper's validate subcommand offline, then inspect the parsed pattern with --debug-query=ast.

Yes, the skill ships install.sh for POSIX systems and install.ps1 for Windows, trying brew, npm, cargo, pip, scoop, and winget before falling back to a pinned GitHub release binary. On Linux, prefer the ast-grep binary name since sg collides with the setgroups command.

ast-grep is a structural matcher with no scope, type, or data-flow analysis, so it cannot tell whether two identifiers refer to the same variable or whether a function throws. For semantic questions, use LSP tools, Pyright, or Semgrep with type inference.