title: Regular expression to allow only one space between words
body: <p>I've a textbox in an ASP.NET application, for which I need to use a regular expression to validate the user input string. Requirements for regex are - </p>
  
  <ol>
  <li>It should allow only one space between words. That is, total number of spaces between words or characters should only be one.</li>
  <li>It should ignore leading and trailing spaces.</li>
  </ol>
  
  <p>Matches: </p>
  
  <ul>
  <li>Test</li>
  <li>Test abc</li>
  </ul>
  
  <p>Non Matches:</p>
  
  <ul>
  <li>Test abc def</li>
  <li>Test        abc  --> I wanted to include multiple spaces between the 2 words. However the editor ignores these extra spaces while posting a question.</li>
  </ul>