title: Javascript regex to validate if number begins with leading zero
body: <p>I need to validate some inputs using a regex.</p>
  
  <p>Here are some sample use cases and expected results.</p>
  
  <pre><code>0001     - Matched
  001      - Matched
  01       - Matched
  00.12312 - Matched
  000.1232 - Matched
  1        - Not matched
  20       - Not matched
  0.1      - Not matched
  0.123123 - Not matched
  </code></pre>
  
  <p>What would a regex like this look like? If first char is <code>0</code> and second char is <code>numerical[0-9]</code> then it is invalid.</p>
  
  <p>I've tried this but it doesn't work.</p>
  
  <pre><code>[0][0-9]
  </code></pre>