title: Regex with constraints, look-ahead function
body: <p>I'm trying to write a regex expression between 10 and 12 digits. There will be optional leading 0 (zeros) between {0,5} then a numeric string between 10-12 digits. Regardless of the number of zeros (0 to 5), I want 10-12 digits after leading zeros</p>
  
  <p>Example:</p>
  
  <pre><code>0000012345 should not be passing 
  0012345678 should not be passing as there are only 8 digits after leading zeros
  </code></pre>
  
  <p>I've tried:</p>
  
  <pre><code>^(0{0,5}(?=\d{10,12}$)^\d{1,2}?\s?(\d{10})$
  </code></pre>