DNS Regular Expression - Number of letters

[#1]

Hello,

I use Regular Expressions to regulate the possible entries for DNS Host Naming. I have a problem to use the rule to set maximum and minimal number of letters.

The Admin Guide shows the following rule \{m,n\}

An example is also added a\{3,5\}

I have tried own rules and because nothing worked I have also tested the easy example out of the Admin Guide. But also this does not work. 

Has anyone an idea how I have to set the rule to define the possible number of letters on a DNS Host Name entry?

Thank you very much.

Sebastian