The following regular expression will allow to validate values with a specific number of values.

The particular example below allows to validate 6-digit values:

^([0-9]{6}$

This way, only digits from 0 to 9 will be accepted with a total of 6 digits, neither more nor less.

You can change the value {6} for the number of digits that will be required for the field. For example, if you need to validate 9-digit values, the regular expression will be as follows:

^[0-9]{9}$