Query to select 'Customer_Num' with digit characters using regular expression
Neither is correct because both only check for a single digit that can appear anywhere in the string.
A regex that only selects digits in the complete string would be:
where REGEXP_LIKE(customer_num, '^[0-9]+$')


^ anchors the expression at the beginning of the string (to avoid leading non-digits)
[0-9]+ matches multiple digits but at least one
$ anchors the expression at the end of the string (to avoid trailing non-digits)

There is no difference between using [0-9] and using [[:digit:]]