In order to be able to create a custom validator you need to have some programming knowledge. Complexity depends of what you want to achieve. It could be a very simple script or very hard. Below are few examples:
All information who to deplo such a custom script you can find in the documentation:
Hey Community! My name is James and I am a product manager on the Server Mobile team. I am excited to announce the new Jira Server Mobile apps for both iPhone and Android are now available. This is...
Connect with like-minded Atlassian users at free events near you!Find an event
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no Community Events near you at the moment.Host an event
You're one step closer to meeting fellow Atlassian users at your local event. Learn more about Community Events