title: Match a specific pattern after first match is found and correlate columns between them
body: <p>I need to find the difference in timestamps between two related lines in a file that do not share a common identifier. For example:</p>
  
  <ol>
  <li>text</li>
  <li>2018-01-29 15:05:11,592 Validating control... </li>
  <li>text</li>
  <li>text</li>
  <li>text</li>
  <li>2018-01-29 15:05:12,725 Done validating control.</li>
  </ol>
  
  <p>Since there is no unique identifier for each validation process, I need to:</p>
  
  <ol>
  <li>First match a line with "Validating control..."</li>
  <li>Save the timestamp to an array</li>
  <li>Match the next line in file that contains "Done validating control."</li>
  <li>Save the timestamp</li>
  </ol>
  
  <p>I have a function to compute the difference in times- I am having a hard time 
  parsing out and correlating each duo of validating/done validating lines. Although there will be hundreds of each of those lines, they always occur in sequence and are processed serially. So I always know that when I find "validating", the next occurrence of "done validating" (no matter how far down) will correspond to it.</p>
  
  <p>I was thinking I could parse out all the lines independent of one another (all the "validating" into file A and all the "done validating" into file B) and then correlate line-by-line. Would that be the best approach or is there a way I can do this without generating additional files?</p>