title: Regular Expression - Only match Alphanumerics and a SINGLE whitespace between words
body: <p>I'm new to Regular Expressions...
  I've been asked a regular expression that accepts Alphanumerics, a few characters more, and only ONE whitespace between words.</p>
  
  <p>For example : 
  This should match : </p>
  
  <pre><code>"Hello world"
  </code></pre>
  
  <p>This shouldn't : </p>
  
  <pre><code>"Hello  world"
  </code></pre>
  
  <p>Any ideas?</p>
  
  <p>This was my expression:</p>
  
  <pre><code>[\w':''.'')''(''\[''\]''{''}''-''_']+$
  </code></pre>
  
  <p>I already tried the \s? (the space character once or never - right? ) but I didn't get it to work.</p>