Use the following regex with Regex.IsMatch method
^[0-9]{5}$
^ and $ will anchors the match to the beginning and the end of the string (respectively) to prevent a match to be found in the middle of a long string, such as 1234567890 or abcd12345efgh.
[0-9] indicates a character class that specifies a range of characters from 0 to 9. The range is defined by the Unicode code range that starts and ends with the specified characters. The {5} followed behind is a quantifier indicating to repeat the [0-9] 5 times.
Note that the solution of ^\d{5}$ is only equivalent to the above solution, when RegexOptions.ECMAScript is specified, otherwise, it will be equivalent to \p{Nd}, which matches any Unicode digits - here is the list of all characters in Nd category. You should always check the documentation of the language you are using as to what the shorthand character classes actually matches.
I strongly suggest that you read through the documentation. You can use other resources, such as http://www.regular-expressions.info/, but always check back on the documentation of the language that you are using.
