diff --git a/api_guide.md b/api_guide.md
index dc6f650..0607ff7 100644
--- a/api_guide.md
+++ b/api_guide.md
@@ -587,9 +587,9 @@ By default, `nix` files are formatted with `nixpkgs-fmt` and `md`, `json`, and
 To disable default formatters, set the `flakelight.builtinFormatters` option to
 false.
 
-You can set `formatters` to an attribute set, for which the keys are a bash case
-pattern and the value is the formatting command. `formatters` can optionally be
-a function that takes packages and returns the above.
+You can set `formatters` to an attribute set, for which the keys are a file name
+pattern and the value is the corresponding formatting command. `formatters` can
+optionally be a function that takes packages and returns the above.
 
 Formatting tools should be added to `devShell.packages` and all packages in
 `devShell.packages` will be available for formatting commands.
diff --git a/builtinModules/builtinFormatters.nix b/builtinModules/builtinFormatters.nix
index 4908e5f..1ab35ad 100644
--- a/builtinModules/builtinFormatters.nix
+++ b/builtinModules/builtinFormatters.nix
@@ -18,7 +18,9 @@ in
 
     formatters = {
       "*.nix" = "nixpkgs-fmt";
-      "*.md | *.json | *.yml" = "prettier --write";
+      "*.md" = "prettier --write";
+      "*.json" = "prettier --write";
+      "*.yml" = "prettier --write";
     };
   };
 }