8 answers
We have ISCHANGED method in Saleforce which would return a TRUE if a particular field used on the ISCHANGED function has changed. Hence you can modify your Validation Rule as below -
NOT(REGEX(Email_Domain__c, "[a-zA-Z0-9.']+")) && ISCHANGED(Email_Domain__c)
This validation rule would only fire if the
Email_Domain__c was changed and changed to an unacceptable value. Now the Validation Rule will not fire as the trigger is trying to update the Support_Email__c field and not the Email_Domain__c field on Account.Feel free to get back to me if you have any doubts.