title: regex to allow 1 to 4 alphabet at starting of string and then followed by any number of digits
body: <p>I am trying to validate a field which can take input as following ways:</p>
  
  <ol>
  <li>should take 1 to 4 alpha charcters.(but should start with alpha)</li>
  <li>from 5th position to so on should take numbers.(no where from 5th should accept  alphabets)</li>
  <li>in between 1-4 characters of alpha it should not allow numbers.
  4.even if first 4 characters are entered it should accept.(that 4 characters should be alpha.ex:"asdf")
  ^[a-zA-Z][0-9]$</li>
  </ol>
  
  <p>i have many things and searched many sites.i could not find it.please help me.
  Thank you in advance.</p>