The fact that the first digit has to be in the range 5-9 only applies in case of two digits. So, check for that in the case of 2 digits, and allow any more digits directly:

^([5-9]d|d{3,})$

This regexp has beginning/ending anchors to make sure you're checking all digits, and the string actually represents a number. The | means or, so either [5-9]d or any number with 3 or more digits. d is simply a shortcut for [0-9].

Edit: To disallow numbers like 001:

^([5-9]d|[1-9]d{2,})$

This forces the first digit to be not a zero in the case of 3 or more digits.