title: RegEx for string starts with numbers and followed by letters
body: <p>I want Regular expression for such inputs.  </p>
  
  <pre><code>1
  1a
  1b
  1c
  1d
  2
  2a
  2b
  2c
  </code></pre>
  
  <p>But if i write following inputs then it should not allow.  </p>
  
  <pre><code>a
  b
  c
  </code></pre>
  
  <p>The string must start with 1 or 2 (only once and mandatory) and then followed by any character between a to z(only once)  </p>
  
  <p>So total string length is 2 only</p>
  
  <p>the first letter will be always 1 or 2 (first letter is mandatory)<br>
  second letter will be a to z (not mandatory)  </p>
  
  <p>I tried this <code>[1-2]?[a-zA-Z]?</code> but it allow me to enter string starts with any character..  </p>
  
  <p>I want this RegEx for C#.Net  </p>