title: Trouble determining the pattern for NSRegularExpression...?
body: <p>i am relatively new to NSRegularExpression and just can't come up with a pattern to find a string within a string.... 
  here is the string...</p>
  
  <pre><code>#@$294@001@[12345-678[123-456-7@15665@2
  </code></pre>
  
  <p>I want to extract the string.. </p>
  
  <p><code>@001@[12345-678[123-456-7@</code></p>
  
  <p>for more info I know that there will be 3 digits(like <code>001</code>) between two <code>@</code> 's and 20 characters between the last two <code>@</code> 's..</p>
  
  <p>I have tried n number of combinations but nothing seem to work. any help is appreciated.</p>