title: Check if string has at least one alphanumeric character
body: <p>I'm an absolute beginner trying to learn string validation. I have a variable about to store user input:</p>
  
  <pre><code>Text_input = raw_input('Type anything: ')
  </code></pre>
  
  <p>I want to check if <code>Text_input</code> contains at least one alphanumeric character. (If not, the program should print a message such as "Try again!" and ask the user to type again.) So, typing "A#" should pass but "#" should not. Any suggestions?</p>