title: Regular expression exclude double character
body: <p>I need to have regular expression which find a character which not following by same character after it. Mean exclude double even multiple same character.</p>
  
  <p>For example, when i need to find <code>'e'</code> character from string: <code>"need only single characteeer"</code>, it is mean will find <code>'e'</code> on each words breakdown as below:</p>
  
  <p><code>"need"</code> > not match because it has double <code>'e'</code></p>
  
  <p><code>"only"</code> > not match because no <code>'e'</code></p>
  
  <p><code>"single"</code> > match because has only single <code>'e'</code></p>
  
  <p><code>"characteeer"</code> > not match because has multiple of <code>'e'</code></p>
  
  <p>Not sure whether it is possible or not. Any answer or comment will be highly appreciated. Thanks in advance.</p>
  
  <p><strong>UPDATE</strong></p>
  
  <p>Maybe my question above still ambiguous. Actually i need to find the <code>'e'</code> character only instead the words. I am going to replace it with double character. So the one which already has double character will not replaced.</p>
  
  <p>The main purpose is to replace <code>'e'</code> with <code>'ee'</code> for example. But the one which has <code>'ee'</code> or <code>'eee'</code> already, or even more <code>'e'</code>, will be untouched.</p>