title: Finding syntax-error in a file that have a custom format using RexEx
body: <p>I have a big text file (144000 line) that have a custom format like following:</p>
  
  <pre><code>xxx
  XXXfield1XXX
  value1
  xxx
  xxx
  XXXfield2XXX
  value2
  xxx
  xxx
  XXXfield3XXX
  value3
  xxx
  </code></pre>
  
  <p>But there is a syntax-error (perhaps more) in the file (Because total line number of file is not dividable to four)</p>
  
  <p>How can I find the line number of error using just  <strong>RegExp</strong>? </p>