title: regular expression for getting single occourance of a character from a string
body: <p>I have some alphanumeric strings. From that I have to find out those strings which satisfy the following condition,</p>
  
  <ol>
  <li>There should be only one character in the whole string and that should be 'e'</li>
  <li>'e' should not present at the beginning or end of the string it should be present at the middle.</li>
  </ol>
  
  <p>I want to pick strings like <code>43e4234,435345e5</code></p>
  
  <p>I can do the same thing in ruby, but as i have huge number of strings i want to go with regular expression only</p>