I have two regular expressions that validate the values entered.
One that allows any length of Alpha-Numeric value:
@\"^\\s*(?[A-Z0-9]+)
I think what you're trying to say is that you don't want to allow any more than 10 digits. So, just add a $
at the end to specify the end of the regex.
Example: @"^\s*(?[0-9]{10})$"
Here's my original answer, but I think I read you too exact.
string myRegexString = `@"(?!(^\d{11}$)` ... your regex here ... )";
That reads "while ahead is not, start, 11 digits, end"