How to delete characters at the end of form field?

I have a digital signature field and when it is signed, it copies the name to another field like below:

var f = this.getField("Signature");
var sigInfo = f.signatureInfo();
var cert = sigInfo.certificates[0];
this.getField("Signature Name").value = cert.subjectDN.cn;

However, because the DN has the serial number baked into it, it appears there is no way for me to omit it.  So the only work around I can think of is to remove all the numbers from it or just delete the last six characters (which are numbers).

Question is how can I do that?