title: Insert text in a List<String> fallowing a pattern
body: <p>I have a <code>List&lt;String&gt;</code> of some extracted text, and I want to verify if the list meets this criteria (could contains many times this pattern and every one is an item in the list):</p>
  
  <pre><code>0      // A zero should always be here when two numbers are together
  \r\n   // New line
  number // any positive number
  \r\n   // New line
  number // Positive number, .length &lt; = 4
  \r\n   // New line
  </code></pre>
  
  <p>What I want is to verify that the first zero is always present, If not, insert it to match the previous list format. </p>
  
  <pre><code>text  --&gt; Insert a zero after this text
  \r\n
  4
  \r\n
  1234
  \r\n
  </code></pre>
  
  <p>To...</p>
  
  <pre><code>text
  \r\n
  0     --&gt; the inserted zero
  \r\n
  4
  \r\n
  1234
  \r\n
  </code></pre>
  
  <p>So, I know I could use the <code>.Insert(index, string)</code> inside a loop, in fact I'm using a for to loop the list with a lot of ugly validations</p>
  
  <pre><code>public Regex isNumber = new Regex(@"^\d+$");
  
  // When the list is been build and a possible match is found call this method:
  private void CheckIfZeroMustBeAdded(List&lt;string&gt; stringList)
  {
      int counter = 0;
  
      for (int i = stringList.Count - 1; i &gt; 1; i--)
      {
          if (stringList[i].Equals(Environment.NewLine))
          {
              // Do nothing
          }
          else if (counter == 2) 
          {
              if (!stringList[i].Equals("0"))
              {
                  stringList.Insert(i, string.Format("{0}{1}", Environment.NewLine,"0"));
                  break;
              }
          }
          else if (ExtractionConst.isNumber.Match(stringList[i]).Success &amp;&amp; !stringList[i].Equals("0")
          {
              // There are two numbers together
              counter++;
          }
          else
          {
              break;
          }
      }
  }
  </code></pre>
  
  <p><br/>
  but.. Is there any efficient way of doing this?</p>