regex for digits only code example

Example 1: just number regex

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

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

Example 3: regex pater for only 4 or 6 digits

^([0-9]{4}|[0-9]{6})$