title: use sed to replace string which has non alpha numeric char before and after it
body: <p>I want to use sed command, in order to replace specific string with other string.
  However, I want to skip the string occurrences which the last char before them or the first char after them is alphanumeric.
  This means I want to replace these string only if it has non alpha numeric char before and after it.</p>
  
  <p>I tried many ways and search the web, without result.</p>
  
  <p>For example:
  The string is: </p>
  
  <pre><code>aaaBCD] BCD1 [BCD] BCD.
  </code></pre>
  
  <p>If I'll try to swap <code>BCD</code> with <code>BBB</code>, I want only the last two <code>BCD</code> will be replaced.</p>