Question

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]');  if (regex.IsMatch(compare)) {      //true }  regex = new Regex('[0-9]');  if (regex.IsMatch(compare)) {      //true } 

In case it matters, I’m using C# and .NET2.0.

Answer 1

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.