diff --git a/README.md b/README.md
index 37cf569..7b5ed63 100644
--- a/README.md
+++ b/README.md
@@ -96,7 +96,15 @@ ip.cidrSubnet('192.168.1.134/26').contains('192.168.1.190') // true
 Additional features:
 ```ts
 ip.isV4Format('255.255.255.256')   // true
+ip.isV6Format('127.0.0.1')         // true
+
+ip.setMode('strict')               // or 'legacy'
+ip.isV4Format('255.255.255.256')   // false
+ip.isV6Format('127.0.0.1')         // false
+
+// new methods are always strict
 ip.isV4('255.255.255.256')         // false
+ip.isV6('127.0.0.1')               // false
 ```
 
 ## License
diff --git a/src/main/ts/core.ts b/src/main/ts/core.ts
index 8e57b7b..98ab609 100644
--- a/src/main/ts/core.ts
+++ b/src/main/ts/core.ts
@@ -8,11 +8,23 @@ export const V6_RE = /^(?=.+)(::)?(((\d{1,3}\.){3}\d{1,3})?|([0-9a-f]{0,4}:{0,2}
 export const V4_S_RE = /^((25[0-5]|(2[0-4]|1\d|[1-9]|)\d)\.?\b){4}$/
 export const V6_S_RE = /(([\dA-Fa-f]{1,4}:){7}[\dA-Fa-f]{1,4}|([\dA-Fa-f]{1,4}:){1,7}:|([\dA-Fa-f]{1,4}:){1,6}:[\dA-Fa-f]{1,4}|([\dA-Fa-f]{1,4}:){1,5}(:[\dA-Fa-f]{1,4}){1,2}|([\dA-Fa-f]{1,4}:){1,4}(:[\dA-Fa-f]{1,4}){1,3}|([\dA-Fa-f]{1,4}:){1,3}(:[\dA-Fa-f]{1,4}){1,4}|([\dA-Fa-f]{1,4}:){1,2}(:[\dA-Fa-f]{1,4}){1,5}|[\dA-Fa-f]{1,4}:((:[\dA-Fa-f]{1,4}){1,6})|:((:[\dA-Fa-f]{1,4}){1,7}|:)|fe80:(:[\dA-Fa-f]{0,4}){0,4}%[\dA-Za-z]+|::(ffff(:0{1,4}){0,1}:){0,1}((25[0-5]|(2[0-4]|1{0,1}\d){0,1}\d)\.){3}(25[0-5]|(2[0-4]|1{0,1}\d){0,1}\d)|([\dA-Fa-f]{1,4}:){1,4}:((25[0-5]|(2[0-4]|1{0,1}\d){0,1}\d)\.){3}(25[0-5]|(2[0-4]|1{0,1}\d){0,1}\d))$/
 
-export const isV4Format = (ip: string): boolean => V4_RE.test(ip) // Legacy
-export const isV6Format = (ip: string): boolean => V6_RE.test(ip) // Legacy
+export const isV4Format = (ip: string): boolean => defs.V4_RE.test(ip) // Legacy
+export const isV6Format = (ip: string): boolean => defs.V6_RE.test(ip) // Legacy
 export const isV4 = (ip: string): boolean => V4_S_RE.test(ip)
 export const isV6 = (ip: string): boolean => V6_S_RE.test(ip)
 
+const defs = {
+  V4_RE,
+  V6_RE,
+}
+
+export const setMode = (mode: 'legacy' | 'strict'): void => {
+  if (mode === 'legacy') { Object.assign(defs, { V4_RE, V6_RE }); return }
+  if (mode === 'strict') { Object.assign(defs, { V4_RE: V4_S_RE, V6_RE: V6_S_RE }); return }
+
+  throw new Error('mode must be either "legacy" or "strict"')
+}
+
 export function readUInt16BE(buf: Buffer | Uint8Array | DataView, offset: number = 0): number {
   if (typeof (buf as Buffer).readUInt16BE === 'function') {
     // Node.js Buffer or feross/buffer polyfill
diff --git a/src/test/ts/core.test.ts b/src/test/ts/core.test.ts
index 0a0b20c..4aaace1 100644
--- a/src/test/ts/core.test.ts
+++ b/src/test/ts/core.test.ts
@@ -28,7 +28,8 @@ import {
   cidr,
   cidrSubnet,
   or,
-  not
+  not,
+  setMode
 } from '../../main/ts/core.ts'
 
 describe('core', () => {
@@ -372,4 +373,22 @@ describe('core', () => {
       assert.equal(isPublic(input), !expected, `isPublic(${input}) === ${!expected}`)
     }
   })
+
+  describe('setMode()', () => {
+    test('sets ipv4/ipv6 mode', () => {
+      setMode('legacy')
+      assert.equal(isV4Format('999.999.999.999'), true)
+      assert.equal(isV4('999.999.999.999'), false)
+      assert.equal(isV6Format('127.0.0.1'), true)
+      assert.equal(isV6('127.0.0.1'), false)
+
+      setMode('strict')
+      assert.equal(isV4Format('999.999.999.999'), false)
+      assert.equal(isV4('999.999.999.999'), false)
+      assert.equal(isV6Format('127.0.0.1'), false)
+      assert.equal(isV6('127.0.0.1'), false)
+
+      setMode('legacy')
+    })
+  })
 })
diff --git a/target/cjs/core.cjs b/target/cjs/core.cjs
index 64fbc9d..cb50cdb 100644
--- a/target/cjs/core.cjs
+++ b/target/cjs/core.cjs
@@ -35,6 +35,7 @@ __export(core_exports, {
   not: () => not,
   or: () => or,
   readUInt16BE: () => readUInt16BE,
+  setMode: () => setMode,
   subnet: () => subnet,
   toBuffer: () => toBuffer,
   toLong: () => toLong,
@@ -47,10 +48,25 @@ var V4_RE = /^(\d{1,3}(\.|$)){4}$/;
 var V6_RE = /^(?=.+)(::)?(((\d{1,3}\.){3}\d{1,3})?|([0-9a-f]{0,4}:{0,2})){1,8}(::)?$/i;
 var V4_S_RE = /^((25[0-5]|(2[0-4]|1\d|[1-9]|)\d)\.?\b){4}$/;
 var V6_S_RE = /(([\dA-Fa-f]{1,4}:){7}[\dA-Fa-f]{1,4}|([\dA-Fa-f]{1,4}:){1,7}:|([\dA-Fa-f]{1,4}:){1,6}:[\dA-Fa-f]{1,4}|([\dA-Fa-f]{1,4}:){1,5}(:[\dA-Fa-f]{1,4}){1,2}|([\dA-Fa-f]{1,4}:){1,4}(:[\dA-Fa-f]{1,4}){1,3}|([\dA-Fa-f]{1,4}:){1,3}(:[\dA-Fa-f]{1,4}){1,4}|([\dA-Fa-f]{1,4}:){1,2}(:[\dA-Fa-f]{1,4}){1,5}|[\dA-Fa-f]{1,4}:((:[\dA-Fa-f]{1,4}){1,6})|:((:[\dA-Fa-f]{1,4}){1,7}|:)|fe80:(:[\dA-Fa-f]{0,4}){0,4}%[\dA-Za-z]+|::(ffff(:0{1,4}){0,1}:){0,1}((25[0-5]|(2[0-4]|1{0,1}\d){0,1}\d)\.){3}(25[0-5]|(2[0-4]|1{0,1}\d){0,1}\d)|([\dA-Fa-f]{1,4}:){1,4}:((25[0-5]|(2[0-4]|1{0,1}\d){0,1}\d)\.){3}(25[0-5]|(2[0-4]|1{0,1}\d){0,1}\d))$/;
-var isV4Format = (ip) => V4_RE.test(ip);
-var isV6Format = (ip) => V6_RE.test(ip);
+var isV4Format = (ip) => defs.V4_RE.test(ip);
+var isV6Format = (ip) => defs.V6_RE.test(ip);
 var isV4 = (ip) => V4_S_RE.test(ip);
 var isV6 = (ip) => V6_S_RE.test(ip);
+var defs = {
+  V4_RE,
+  V6_RE
+};
+var setMode = (mode) => {
+  if (mode === "legacy") {
+    Object.assign(defs, { V4_RE, V6_RE });
+    return;
+  }
+  if (mode === "strict") {
+    Object.assign(defs, { V4_RE: V4_S_RE, V6_RE: V6_S_RE });
+    return;
+  }
+  throw new Error('mode must be either "legacy" or "strict"');
+};
 function readUInt16BE(buf, offset = 0) {
   if (typeof buf.readUInt16BE === "function") {
     return buf.readUInt16BE(offset);
@@ -306,6 +322,7 @@ var isPublic = (addr) => !isPrivate(addr);
   not,
   or,
   readUInt16BE,
+  setMode,
   subnet,
   toBuffer,
   toLong,
diff --git a/target/dts/core.d.ts b/target/dts/core.d.ts
index 4d3c19a..0d33804 100644
--- a/target/dts/core.d.ts
+++ b/target/dts/core.d.ts
@@ -8,6 +8,7 @@ export declare const isV4Format: (ip: string) => boolean;
 export declare const isV6Format: (ip: string) => boolean;
 export declare const isV4: (ip: string) => boolean;
 export declare const isV6: (ip: string) => boolean;
+export declare const setMode: (mode: "legacy" | "strict") => void;
 export declare function readUInt16BE(buf: Buffer | Uint8Array | DataView, offset?: number): number;
 export type Family = typeof IPV4 | typeof IPV6;
 export declare function normalizeFamily(family?: string | number): Family;
diff --git a/target/dts/index.d.ts b/target/dts/index.d.ts
index e303292..1a127d6 100644
--- a/target/dts/index.d.ts
+++ b/target/dts/index.d.ts
@@ -14,6 +14,7 @@ export declare const ip: {
     isV6Format: (ip: string) => boolean;
     isV4: (ip: string) => boolean;
     isV6: (ip: string) => boolean;
+    setMode: (mode: "legacy" | "strict") => void;
     normalizeAddress: (addr: string | number) => string;
     normalizeToLong: (addr: string) => number;
     isLoopback: (addr: string | number) => boolean;
diff --git a/target/esm/core.mjs b/target/esm/core.mjs
index b0435ad..a02ff21 100644
--- a/target/esm/core.mjs
+++ b/target/esm/core.mjs
@@ -5,10 +5,25 @@ var V4_RE = /^(\d{1,3}(\.|$)){4}$/;
 var V6_RE = /^(?=.+)(::)?(((\d{1,3}\.){3}\d{1,3})?|([0-9a-f]{0,4}:{0,2})){1,8}(::)?$/i;
 var V4_S_RE = /^((25[0-5]|(2[0-4]|1\d|[1-9]|)\d)\.?\b){4}$/;
 var V6_S_RE = /(([\dA-Fa-f]{1,4}:){7}[\dA-Fa-f]{1,4}|([\dA-Fa-f]{1,4}:){1,7}:|([\dA-Fa-f]{1,4}:){1,6}:[\dA-Fa-f]{1,4}|([\dA-Fa-f]{1,4}:){1,5}(:[\dA-Fa-f]{1,4}){1,2}|([\dA-Fa-f]{1,4}:){1,4}(:[\dA-Fa-f]{1,4}){1,3}|([\dA-Fa-f]{1,4}:){1,3}(:[\dA-Fa-f]{1,4}){1,4}|([\dA-Fa-f]{1,4}:){1,2}(:[\dA-Fa-f]{1,4}){1,5}|[\dA-Fa-f]{1,4}:((:[\dA-Fa-f]{1,4}){1,6})|:((:[\dA-Fa-f]{1,4}){1,7}|:)|fe80:(:[\dA-Fa-f]{0,4}){0,4}%[\dA-Za-z]+|::(ffff(:0{1,4}){0,1}:){0,1}((25[0-5]|(2[0-4]|1{0,1}\d){0,1}\d)\.){3}(25[0-5]|(2[0-4]|1{0,1}\d){0,1}\d)|([\dA-Fa-f]{1,4}:){1,4}:((25[0-5]|(2[0-4]|1{0,1}\d){0,1}\d)\.){3}(25[0-5]|(2[0-4]|1{0,1}\d){0,1}\d))$/;
-var isV4Format = (ip) => V4_RE.test(ip);
-var isV6Format = (ip) => V6_RE.test(ip);
+var isV4Format = (ip) => defs.V4_RE.test(ip);
+var isV6Format = (ip) => defs.V6_RE.test(ip);
 var isV4 = (ip) => V4_S_RE.test(ip);
 var isV6 = (ip) => V6_S_RE.test(ip);
+var defs = {
+  V4_RE,
+  V6_RE
+};
+var setMode = (mode) => {
+  if (mode === "legacy") {
+    Object.assign(defs, { V4_RE, V6_RE });
+    return;
+  }
+  if (mode === "strict") {
+    Object.assign(defs, { V4_RE: V4_S_RE, V6_RE: V6_S_RE });
+    return;
+  }
+  throw new Error('mode must be either "legacy" or "strict"');
+};
 function readUInt16BE(buf, offset = 0) {
   if (typeof buf.readUInt16BE === "function") {
     return buf.readUInt16BE(offset);
@@ -263,6 +278,7 @@ export {
   not,
   or,
   readUInt16BE,
+  setMode,
   subnet,
   toBuffer,
   toLong,