<!DOCTYPE html> <html> <head> <script language="javascript"> var reg = /^\(?([0-9]{3})\)?[-. ]?([0-9]{3})[-. ]?([0-9]{4})$/; function testPhone(){ var PhoneNumber = document.getElementById('tbPhone'); var tagP = document.getElementById('01'); var OK = reg.exec(PhoneNumber.value); if (!OK) tagP.innerHTML = "Định dạng phone không hợp lệ !"; else tagP.innerHTML = "Phone OK !"; } </script> </head> <body> <input id="tbPhone" type="text" value="" placeholder="Nhập Phone" onkeyup="testPhone()"/> <p id="01"></p> </body> </html>