title: Checking for valid mobile number in android
body: <p>I am currently making a registration for an application in android. I want to trap invalid mobile numbers entered in the EditText box. Once I click the submit button, the app will check if the mobile number entered is valid. </p>
  
  <p>All valid number should start with <code>"639"</code>. My question is how am I going to read the first three digits that the user entered? For example, the user enters <code>639158716271</code>, this a valid number. While <code>197837281919</code> is invalid.</p>
  
  <p>Can anyone suggest how to accomplish this?</p>
  
  <p>Thanks in advance!</p>