title: replace all combinations of non-alphanumeric characters
body: <p>I need to to create url from various strings.<br>
  So trying to replace all non-alphanumeric characters <code>and all of their combinations</code> - with a hyphen character (<code>-</code>)</p>
  
  <pre><code>$string = "blue - sky";
  $string = preg_replace("/[^A-Za-z0-9 ]/", '-', $string);
  echo $string;  
  </code></pre>
  
  <p>result - <code>blue---sky</code><br>
  expected - <code>blue-sky</code>.</p>