diff --git a/src/rules/no-duplicate-definitions.js b/src/rules/no-duplicate-definitions.js
index 70ef7cd3..0d1d5a40 100644
--- a/src/rules/no-duplicate-definitions.js
+++ b/src/rules/no-duplicate-definitions.js
@@ -7,7 +7,7 @@
 // Imports
 //-----------------------------------------------------------------------------
 
-import { normalizeIdentifier } from "micromark-util-normalize-identifier";
+import { normalizeIdentifier } from "../util.js";
 
 //-----------------------------------------------------------------------------
 // Type Definitions
@@ -81,13 +81,11 @@ export default /** @satisfies {NoDuplicateDefinitionsRuleDefinition} */ ({
 
 	create(context) {
 		const allowDefinitions = new Set(
-			context.options[0].allowDefinitions.map(identifier =>
-				normalizeIdentifier(identifier).toLowerCase(),
-			),
+			context.options[0].allowDefinitions.map(normalizeIdentifier),
 		);
 		const allowFootnoteDefinitions = new Set(
-			context.options[0].allowFootnoteDefinitions.map(identifier =>
-				normalizeIdentifier(identifier).toLowerCase(),
+			context.options[0].allowFootnoteDefinitions.map(
+				normalizeIdentifier,
 			),
 		);
 		const [{ checkFootnoteDefinitions }] = context.options;
diff --git a/src/rules/no-empty-definitions.js b/src/rules/no-empty-definitions.js
index 824734bd..9adbb068 100644
--- a/src/rules/no-empty-definitions.js
+++ b/src/rules/no-empty-definitions.js
@@ -7,8 +7,7 @@
 // Imports
 //-----------------------------------------------------------------------------
 
-import { normalizeIdentifier } from "micromark-util-normalize-identifier";
-import { htmlCommentPattern } from "../util.js";
+import { htmlCommentPattern, normalizeIdentifier } from "../util.js";
 
 //-----------------------------------------------------------------------------
 // Type Definitions
@@ -96,13 +95,11 @@ export default /** @satisfies {NoEmptyDefinitionsRuleDefinition} */ ({
 
 	create(context) {
 		const allowDefinitions = new Set(
-			context.options[0].allowDefinitions.map(identifier =>
-				normalizeIdentifier(identifier).toLowerCase(),
-			),
+			context.options[0].allowDefinitions.map(normalizeIdentifier),
 		);
 		const allowFootnoteDefinitions = new Set(
-			context.options[0].allowFootnoteDefinitions.map(identifier =>
-				normalizeIdentifier(identifier).toLowerCase(),
+			context.options[0].allowFootnoteDefinitions.map(
+				normalizeIdentifier,
 			),
 		);
 		const [{ checkFootnoteDefinitions }] = context.options;
diff --git a/src/rules/no-reference-like-urls.js b/src/rules/no-reference-like-urls.js
index 68b95203..794e8241 100644
--- a/src/rules/no-reference-like-urls.js
+++ b/src/rules/no-reference-like-urls.js
@@ -7,7 +7,7 @@
 // Imports
 //-----------------------------------------------------------------------------
 
-import { normalizeIdentifier } from "micromark-util-normalize-identifier";
+import { normalizeIdentifier } from "../util.js";
 
 //-----------------------------------------------------------------------------
 // Type Definitions
@@ -79,8 +79,7 @@ export default /** @satisfies {NoReferenceLikeUrlsRuleDefinition} */ ({
 						const { label, destination } = match.groups;
 						const { type, title } = node;
 						const prefix = type === "image" ? "!" : "";
-						const url =
-							normalizeIdentifier(destination).toLowerCase();
+						const url = normalizeIdentifier(destination);
 
 						if (definitionIdentifiers.has(url)) {
 							context.report({
diff --git a/src/rules/no-unused-definitions.js b/src/rules/no-unused-definitions.js
index 71315032..7a231314 100644
--- a/src/rules/no-unused-definitions.js
+++ b/src/rules/no-unused-definitions.js
@@ -7,7 +7,7 @@
 // Imports
 //-----------------------------------------------------------------------------
 
-import { normalizeIdentifier } from "micromark-util-normalize-identifier";
+import { normalizeIdentifier } from "../util.js";
 
 //-----------------------------------------------------------------------------
 // Type Definitions
@@ -81,13 +81,11 @@ export default /** @satisfies {NoUnusedDefinitionsRuleDefinition} */ ({
 
 	create(context) {
 		const allowDefinitions = new Set(
-			context.options[0].allowDefinitions.map(identifier =>
-				normalizeIdentifier(identifier).toLowerCase(),
-			),
+			context.options[0].allowDefinitions.map(normalizeIdentifier),
 		);
 		const allowFootnoteDefinitions = new Set(
-			context.options[0].allowFootnoteDefinitions.map(identifier =>
-				normalizeIdentifier(identifier).toLowerCase(),
+			context.options[0].allowFootnoteDefinitions.map(
+				normalizeIdentifier,
 			),
 		);
 		const [{ checkFootnoteDefinitions }] = context.options;
diff --git a/src/util.js b/src/util.js
index b69e80eb..314ce671 100644
--- a/src/util.js
+++ b/src/util.js
@@ -3,6 +3,12 @@
  * @author Nicholas C. Zakas
  */
 
+//------------------------------------------------------------------------------
+// Imports
+//------------------------------------------------------------------------------
+
+import { normalizeIdentifier as micromarkUtilNormalizeIdentifier } from "micromark-util-normalize-identifier";
+
 //-----------------------------------------------------------------------------
 // Regex Patterns
 //-----------------------------------------------------------------------------
@@ -48,6 +54,15 @@ export function frontmatterHasTitle(value, pattern) {
 	return false;
 }
 
+/**
+ * Normalizes a Markdown reference identifier.
+ * @param {string} identifier The identifier to normalize.
+ * @returns {string} The normalized lowercase identifier.
+ */
+export function normalizeIdentifier(identifier) {
+	return micromarkUtilNormalizeIdentifier(identifier).toLowerCase();
+}
+
 /**
  * Replaces all HTML comments with whitespace.
  * This preserves offsets and locations of characters
diff --git a/tests/util.test.js b/tests/util.test.js
index ccaaf5f8..64e3bb87 100644
--- a/tests/util.test.js
+++ b/tests/util.test.js
@@ -8,7 +8,11 @@
 //------------------------------------------------------------------------------
 
 import assert from "node:assert";
-import { frontmatterHasTitle, stripHtmlComments } from "../src/util.js";
+import {
+	frontmatterHasTitle,
+	normalizeIdentifier,
+	stripHtmlComments,
+} from "../src/util.js";
 
 //------------------------------------------------------------------------------
 // Tests
@@ -72,6 +76,50 @@ describe("util", () => {
 		});
 	});
 
+	describe("normalizeIdentifier()", () => {
+		it("should preserve an already normalized identifier", () => {
+			const input = "example";
+			const result = normalizeIdentifier(input);
+			assert.strictEqual(result, "example");
+		});
+
+		it("should convert a mixed-case identifier to lowercase", () => {
+			const input = "ExAmPlE";
+			const result = normalizeIdentifier(input);
+			assert.strictEqual(result, "example");
+		});
+
+		it("should trim leading and trailing spaces", () => {
+			const input = "  example  ";
+			const result = normalizeIdentifier(input);
+			assert.strictEqual(result, "example");
+		});
+
+		it("should collapse consecutive internal spaces into a single space", () => {
+			const input = "foo   bar";
+			const result = normalizeIdentifier(input);
+			assert.strictEqual(result, "foo bar");
+		});
+
+		it("should normalize tabs and line breaks to a single space", () => {
+			const input = "foo\t\r\nbar";
+			const result = normalizeIdentifier(input);
+			assert.strictEqual(result, "foo bar");
+		});
+
+		it("should preserve an empty string", () => {
+			const input = "";
+			const result = normalizeIdentifier(input);
+			assert.strictEqual(result, "");
+		});
+
+		it("should return an empty string for Markdown whitespace only", () => {
+			const input = " \t\r\n ";
+			const result = normalizeIdentifier(input);
+			assert.strictEqual(result, "");
+		});
+	});
+
 	describe("stripHtmlComments()", () => {
 		it("should replace single-line HTML comments with spaces", () => {
 			const input = "Hello<!--1234567-->World";