Domain check constraint not honored?

I have created a custom type as a domain based on text, which adds a
check constraint using a regexp to limit it to containing digits and
'.'. However I am finding I can add values with other characters to a
column of this type. Is this to be expected for some reason?

Or alternately, did I define the constraint wrong somehow? It is defined
thus:

CREATE DOMAIN hierpath AS text
CHECK(
VALUE ~ '[0-9.]+'
);

Checking the docs I'm pretty sure the '.' doesn't need escaping but I
also tried a test leaving it out ( '[0-9]+') and the result is the same.
It lets me store letters in a column defined to be of this type.

The version() function tells me
"PostgreSQL 9.4.5 on x86_64-unknown-linux-gnu, compiled by gcc (Ubuntu
4.8.2-19ubuntu1) 4.8.2, 64-bit"

Thanks,
Eric