title: Detecting custom url schemes from user typed in web address
body: <p>I was trying out the tutorial of building a simple web browser using UIWebView.
  In which there is feature to prefix the scheme "http://" to the url if user just types in the address without any scheme. For Ex: user types in google.com instead of <a href="http://google.com" rel="nofollow">http://google.com</a>
  This is achieved using the NSURL's scheme property.
  All is well until the address is simple.</p>
  
  <p>If the address is something like this: "test1.samplewebsite.com:81"
  NSURL scheme detect the string "test1.samplewebsite.com" as scheme instead of no scheme.</p>
  
  <p>This works fine on safari, it detects no scheme and promptly prefixes http.</p>
  
  <p>Before jumping into writing code to detect scheme, I wanted to check, Is there any system API's for detecting valid scheme?</p>