regex for numbers

// https://regex101.com/r/qyq3PG/1
// or 
/^[0-9]*$/

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

^[0-9]

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