How to write custom RegularExpressionValidator which takes the values from the config file?

I have to use a Regular expression validator for username property in my model. I am getting this regular expression from the config file.

[RegularExpression(UsernameValidationExpression)] //UsernameValidationExpression = value from the config file
public string UserName { get; set; }

Here i am getting an error "An attribute argument must be a constant expression, typeof expression or array creation expression of an attribute parameter type"

How can i solve this??