title: Validate timestamp step by step with regex
body: <p>This regular expression validates timestamps e.g. 2018-02-12 00:55:22:</p>
  
  <pre><code>[0-9]{4}-(0[1-9]|1[0-2])-(0[1-9]|[1-2][0-9]|3[0-1]) (2[0-3]|[01][0-9]):[0-5][0-9]:[0-5][0-9]
  </code></pre>
  
  <p>However, the timestamp should be validated step by step:</p>
  
  <pre><code>201 =&gt; true
  201a =&gt; false
  2018- =&gt; true
  20189 =&gt; false
  </code></pre>
  
  <p>Is there a nice (short) regex extension?</p>
  
  <p>......</p>