$lens->validation = 'USERID^=[a-zA-Z][a-zA-Z0-9]{3,16}$^UserID must be 4-16 chars long and cannot start with a number';
Alternatively (not used in this example), you can use an input mask such as:
$lens->validation = 'USERID^=XXXXxxxxxxxxxxxx^UserID must be at least 4 chars long';