get numbers only question

[Jassm Rahma]

I tried this:

SELECT employee_number FROM employees WHERE employee_number REGEXP ('[0-9]');

it's working excellent and I getting these numbers

7, 8, 9, 11, 1.222, 12, 13, 9, 15, 9, 9, 9

BUT....

when I run:

SELECT MAX(employee_number) FROM employees WHERE employee_number REGEXP ('[0-9]');

I get 9 where i should get 15.

why is that?