I haven't used regular expressions at all, so I'm having difficulty troubleshooting. I want the regex to match only when the contained string is all numbers; but with the two examples below it is matching a string that contains all numbers plus an equals sign like "1234=4321". I'm sure there's a way to change this behavior, but as I said, I've never really done much with regular expressions.
string compare = "1234=4321";
Regex regex = new Regex(@"[\d]");
regex = new Regex("[0-9]");
Use the beginning and end anchors.
Regex regex = new Regex(@"^\d$");
"^\d+$" if you need to match more than one digit.
"\d" will match
[0-9] and other digit characters like the Eastern Arabic numerals
"^[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.