title: PHP regex replace white space by &nbsp; if it is following single letter
body: <p>I have user defined string (html formated string to be saved and used in web) and need to find a way to replace each white space which is right after a single letter by <code>&amp;nbsp;</code>. </p>
  
  <p>For example <code>"this is a string"</code> should become <code>"this is a&amp;nbsp;string"</code>, </p>
  
  <p><code>"bla bla b l abla b la blabla"</code> should become <code>"bla bla b&amp;nbsp;l&amp;nbsp;abla b&amp;nbsp;la blabla"</code> ...etc... </p>