diff --git a/cli/npm/dsl.d.ts b/cli/npm/dsl.d.ts
index fdf43b55..78b654f3 100644
--- a/cli/npm/dsl.d.ts
+++ b/cli/npm/dsl.d.ts
@@ -1,19 +1,19 @@
-type AliasRule = {type: 'ALIAS'; named: boolean; content: Rule; value: string};
-type BlankRule = {type: 'BLANK'};
-type ChoiceRule = {type: 'CHOICE'; members: Rule[]};
-type FieldRule = {type: 'FIELD'; name: string; content: Rule};
-type ImmediateTokenRule = {type: 'IMMEDIATE_TOKEN'; content: Rule};
-type PatternRule = {type: 'PATTERN'; value: string};
-type PrecDynamicRule = {type: 'PREC_DYNAMIC'; content: Rule; value: number};
-type PrecLeftRule = {type: 'PREC_LEFT'; content: Rule; value: number};
-type PrecRightRule = {type: 'PREC_RIGHT'; content: Rule; value: number};
-type PrecRule = {type: 'PREC'; content: Rule; value: number};
-type Repeat1Rule = {type: 'REPEAT1'; content: Rule};
-type RepeatRule = {type: 'REPEAT'; content: Rule};
-type SeqRule = {type: 'SEQ'; members: Rule[]};
-type StringRule = {type: 'STRING'; value: string};
-type SymbolRule<Name extends string> = {type: 'SYMBOL'; name: Name};
-type TokenRule = {type: 'TOKEN'; content: Rule};
+type AliasRule = { type: 'ALIAS'; named: boolean; content: Rule; value: string };
+type BlankRule = { type: 'BLANK' };
+type ChoiceRule = { type: 'CHOICE'; members: Rule[] };
+type FieldRule = { type: 'FIELD'; name: string; content: Rule };
+type ImmediateTokenRule = { type: 'IMMEDIATE_TOKEN'; content: Rule };
+type PatternRule = { type: 'PATTERN'; value: string };
+type PrecDynamicRule = { type: 'PREC_DYNAMIC'; content: Rule; value: number };
+type PrecLeftRule = { type: 'PREC_LEFT'; content: Rule; value: number };
+type PrecRightRule = { type: 'PREC_RIGHT'; content: Rule; value: number };
+type PrecRule = { type: 'PREC'; content: Rule; value: number };
+type Repeat1Rule = { type: 'REPEAT1'; content: Rule };
+type RepeatRule = { type: 'REPEAT'; content: Rule };
+type SeqRule = { type: 'SEQ'; members: Rule[] };
+type StringRule = { type: 'STRING'; value: string };
+type SymbolRule<Name extends string> = { type: 'SYMBOL'; name: Name };
+type TokenRule = { type: 'TOKEN'; content: Rule };
 
 type Rule =
   | AliasRule
@@ -48,8 +48,8 @@ type RuleBuilders<
   RuleName extends string,
   BaseGrammarRuleName extends string
 > = {
-  [name in RuleName]: RuleBuilder<RuleName | BaseGrammarRuleName>;
-};
+    [name in RuleName]: RuleBuilder<RuleName | BaseGrammarRuleName>;
+  };
 
 interface Grammar<
   RuleName extends string,
@@ -102,7 +102,7 @@ interface Grammar<
   externals?: (
     $: Record<string, SymbolRule<string>>,
     previous: Rule[],
-  ) => SymbolRule<string>[];
+  ) => (SymbolRule<string> | RegExp | string)[];
 
   /**
    * An array of tokens that may appear anywhere in the language. This
@@ -153,8 +153,8 @@ interface Grammar<
 
 type GrammarSchema<RuleName extends string> = {
   [K in keyof Grammar<RuleName>]: K extends 'rules'
-    ? Record<RuleName, Rule>
-    : Grammar<RuleName>[K];
+  ? Record<RuleName, Rule>
+  : Grammar<RuleName>[K];
 };
 
 /**