title: HTML5 phone number validation with pattern
body: <p>I'm using HTML5 form validation to validate phone numbers from India.</p>
  
  <p>Phone numbers from India are 10 digits long, and start with 7, 8, or 9.</p>
  
  <p>For example:</p>
  
  <ol>
  <li>7878787878</li>
  <li>9898989898</li>
  <li>8678678878</li>
  </ol>
  
  <p>These phone numbers are valid, but </p>
  
  <ol>
  <li>1212121212</li>
  <li>3434545464</li>
  <li>6545432322</li>
  </ol>
  
  <p>are invalid.</p>
  
  <p>Suggest a pattern that can detect valid India phone numbers.</p>
  
  <p>So far, my pattern is <code>[0-9]{10}</code>, but it doesn't check if the first digit is 7, 8, or 9.</p>