title: RegEx match exactly 4 digits
body: <p>Ok, i have a regex pattern like this <code>/^([SW])\w+([0-9]{4})$/</code></p>
  
  <p>This pattern should match a string like <code>SW0001</code> with <code>SW</code>-Prefix and 4 digits.</p>
  
  <p>I thougth <code>[0-9]{4}</code> would do the job, but it also matches strings with 5 digits and so on.</p>
  
  <p>Any suggestions on how to get this to work to only match strings with <code>SW</code>  and 4 digits?</p>