hi, can we assign multiple textbox to validate by a single regular expression validator?? the property validation group? what's its use??

I think you can only validate one textbox with one RegularExpressionValidator. To validate multiple textboxes with one control you'd need to use a CustomValidator.

The ValidationGroup property groups various controls together so you can validate only the controls in a specific group, e.g. if you had a section of your page for logging in and one for searching, each with required field validators, you wouldn't want to be told to enter your username & password when you hit the "Search" button.

Try to check this link:

http://www.codeproject.com/KB/validation/MultiDependValidator.aspx

May be it can help you  

Regards

Ahmad