Why doesn't [01-12] range work as expected?

Question

I\'m trying to use the range pattern [01-12] in regex to match two digit mm, but this doesn\'t work as expected.

Answer 1

This also works:

^([1-9]|[0-1][0-2])$

[1-9] matches single digits between 1 and 9

[0-1][0-2] matches double digits between 10 and 12

There are some good examples here