Required Field Validator based on condition

Question

Hi All,

I have a scenario , where user has to select any one role from radiobuttonlist to register himself as below

• Administrator
• Shop floor User
• Key User
• Others

if he selects Role "Others" from radio-buttonlist, a text box will be appeared to enter the role manually.

Can i have a requiredfieldvalidator for text-box if only user selects role "Others" else if he selects other roles validation is not required. How can i achieve this validation based on the condition 

Thanks & Regards

Vishwas