You aren't using the attribute correctly. The attribute take a regular expression and compares the input to that regular expression. If it is invalid, then the ErrorMessage is displayed.
[RegularExpression(@"^[a-zA-Z]+$", ErrorMessage = "Invalid search")]
public string SearchText { get; set; }
So something like this would only be valid if the SearchText input on your html page has letters only. Obviously, you should adjust the regex to meet your requirements.
If you want a "default value", then you may want to use the placeholder HTML attribute. http://www.w3schools.com/tags/att_input_placeholder.asp