title: How to use conditionals when replacing in Notepad++ via regex
body: <p>Consider the following regex:</p>
  
  <p><code>([a-zA-Z])([a-zA-Z]?)/([a-zA-Z])([a-zA-Z]?)</code></p>
  
  <p>If the text is: <code>a/b</code>
  the capturing groups will be:</p>
  
  <pre><code>/1  'a'
  /2  ''
  /3  'b'
  /4  ''
  </code></pre>
  
  <p>And if the text is: <code>aa/b</code>
  the capturing groups will be:</p>
  
  <pre><code>/1  'a'
  /2  'a'
  /3  'b'
  /4  ''
  </code></pre>
  
  <p>Suppose, I want to find and replace this string in Notepad++ such that if <code>/2</code> or <code>/4</code> are empty (as in the first case above), I prepend <code>c</code>.</p>
  
  <p>So, the text <code>a/b</code> becomes <code>ca/cb</code>.
  And the text <code>aa/b</code> becomes <code>aa/cb</code></p>
  
  <p>I use the following regex for replacing:</p>
  
  <pre><code>(?(2)\1\2|0\1)/(?(4)\3\4|0\3)
  </code></pre>
  
  <p>But Notepad++ is treating <code>?</code> literally in this case, and not as a conditional identifier. Any idea what am I doing wrong?</p>