title: Regex: word boundary but for white space, beginning of line or end of line only
body: <p>I am looking for some word boundary to cover those 3 cases:</p>
  
  <ol>
  <li>beginning of string</li>
  <li>end of string</li>
  <li>white space</li>
  </ol>
  
  <p>Is there something like that since \b covers also -,/ etc.?</p>
  
  <p>Would like to replace \b in this pattern by something described above:</p>
  
  <pre><code>(\b\d*\sx\s|\b\d*x|\b)
  </code></pre>