Use the beginning and end anchors.
Regex regex = new Regex(@"^\d$");
Use "^\d+$" if you need to match more than one digit.
Note that "\d" will match [0-9] and other digit characters like the Eastern Arabic numerals ٠١٢٣٤٥٦٧٨٩. Use "^[0-9]+$" to restrict matches to just the Arabic numerals 0 – 9.
If you need to include any numeric representations other than just digits (like decimal values for starters), then see @tchrist’s comprehensive guide to parsing numbers with regular expressions.