diff --git a/README.md b/README.md
index 113d10f..418919a 100644
--- a/README.md
+++ b/README.md
@@ -60,8 +60,10 @@ Throws a `TypeError` if the object contains an invalid type or parameter names.
 
 This package exposes the validation functions used by `format`:
 
-- `isTypeValid` Validates the MIME type against RFC 9110.
-- `isTokenValid` Validates a token against RFC 9110 (used for the parameter name).
+- `isTypeValid(str, start?, end?)` Validates the MIME type against RFC 9110.
+- `isTokenValid(str, start?, end?)` Validates a token against RFC 9110 (used for the parameter name).
+
+Passing `start` and `end` allows for validating a subset of a string, instead of using `str#slice`.
 
 ## License
 
diff --git a/src/index.ts b/src/index.ts
index a8f2cc0..a89b04e 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -68,15 +68,18 @@ export interface ContentType {
 /**
  * Validate a type string against RFC 9110.
  */
-export function isTypeValid(type: string): boolean {
-  const len = type.length;
+export function isTypeValid(
+  type: string,
+  start = 0,
+  end = type.length,
+): boolean {
   let hasSlash = false;
 
-  for (let index = 0; index < len; index++) {
+  for (let index = start; index < end; index++) {
     const code = type.charCodeAt(index);
 
     if (code === 47 /* / */) {
-      if (hasSlash || index === 0 || index === len - 1) return false;
+      if (hasSlash || index === start || index >= end - 1) return false;
       hasSlash = true;
     } else if (!isTokenCode(code)) {
       return false;
@@ -89,12 +92,15 @@ export function isTypeValid(type: string): boolean {
 /**
  * Validate a token against RFC 9110.
  */
-export function isTokenValid(name: string): boolean {
-  const len = name.length;
-  if (len === 0) return false;
+export function isTokenValid(
+  token: string,
+  start = 0,
+  end = token.length,
+): boolean {
+  if (start >= end) return false;
 
-  for (let index = 0; index < len; index++) {
-    if (!isTokenCode(name.charCodeAt(index))) return false;
+  for (let index = start; index < end; index++) {
+    if (!isTokenCode(token.charCodeAt(index))) return false;
   }
 
   return true;
diff --git a/src/validation.spec.ts b/src/validation.spec.ts
index 3f36e29..888cd9c 100644
--- a/src/validation.spec.ts
+++ b/src/validation.spec.ts
@@ -28,6 +28,30 @@ describe("isTypeValid(type)", function () {
   ])("should reject invalid type: %s", function (type) {
     assert.strictEqual(isTypeValid(type), false);
   });
+
+  it("should default the end index to the string length", function () {
+    assert.strictEqual(isTypeValid(";text/html", 1), true);
+  });
+
+  it.each([
+    ["text/html;", 0, 9, true],
+    [";text/html;", 1, 10, true],
+    [";a/b;", 1, 4, true],
+    [";text/html;", 0, 10, false],
+    [";text/html;", 1, 11, false],
+    [";text/html;", 5, 10, false],
+    [";text/html;", 1, 6, false],
+    [";text/html;", 1, 5, false],
+    [";text/html/extra;", 1, 16, false],
+    [";text/ plain;", 1, 12, false],
+    [";text/html;", 1, 1, false],
+    [";text/html;", 10, 1, false],
+  ])(
+    "should validate type %j from %i to %i as %s",
+    function (type, start, end, expected) {
+      assert.strictEqual(isTypeValid(type, start, end), expected);
+    },
+  );
 });
 
 describe("isTokenValid(name)", function () {
@@ -62,4 +86,26 @@ describe("isTokenValid(name)", function () {
   ])("should reject invalid parameter name: %s", function (name) {
     assert.strictEqual(isTokenValid(name), false);
   });
+
+  it("should default the end index to the string length", function () {
+    assert.strictEqual(isTokenValid(";charset", 1), true);
+  });
+
+  it.each([
+    ["charset=", 0, 7, true],
+    [";charset=", 1, 8, true],
+    [";a=", 1, 2, true],
+    [";charset=", 0, 8, false],
+    [";charset=", 1, 9, false],
+    [";char set=", 1, 9, false],
+    [";char/set=", 1, 9, false],
+    [";chärset=", 1, 8, false],
+    [";charset=", 1, 1, false],
+    [";charset=", 8, 1, false],
+  ])(
+    "should validate parameter name %j from %i to %i as %s",
+    function (name, start, end, expected) {
+      assert.strictEqual(isTokenValid(name, start, end), expected);
+    },
+  );
 });