regex to see check for numbers code example

Example 1: regex for numbers

Regex regex = new Regex(@"^\d$");

Example 2: regex match any number of digits

For matching an integer number of one single char/digit, specify:
[0-9]
But for matching any number of more than one char/digit; add "+":
[0-9]+
Example for matching hour with minutes of HH:MM format in a file:
grep "[0-9]\+\:[0-9]\+" file.txt