Custom Validation not working?

Hey there,

is there a trick to add an own validation?

i just have to validate, if it is a number and if it has 15 digits.

i tried lots of combination of pattern.
[0-9]{15,15} or just {15,} or 15 without {} … or .{15} …
i tried also a validation combination of my and the “number validation”…
and also a max-length of 15.

nothings working.

when i try {15,15}, there is the error message all the time and i cant send the order…