I am trying to fetch data from description fied and update that in other field called "Email Sub". I am getting following errors

1. system.finalexception record is read-only

I have also tried before insert trigger, but field is not getting updated

trigger ParseBodySubject on BMCServiceDesk__Incident__c (after Insert, before update) {

    for(BMCServiceDesk__Incident__c incident : Trigger.new){

              String  description = String.valueof([Select id, BMCServiceDesk__incidentDescription__c from BMCServiceDesk__Incident__c where id=:trigger.new limit 1]);                                System.debug('Desc:'+description );

              String Sub = description.substringAfterLast('Subject:');              

              System.debug('Sub:' +Sub);

              incident.Email_Subject__c= Sub;    // Error Line       

       }

}

Appreciate your help

I am trying to fetch data from description fied and update that in other field called "Email Sub". I am getting following errors

1. system.finalexception record is read-only

I have also tried before insert trigger, but field is not getting updated

trigger ParseBodySubject on BMCServiceDesk__Incident__c (after Insert, before update) {

    for(BMCServiceDesk__Incident__c incident : Trigger.new){

              String  description = String.valueof([Select id, BMCServiceDesk__incidentDescription__c from BMCServiceDesk__Incident__c where id=:trigger.new limit 1]);                                System.debug('Desc:'+description );

              String Sub = description.substringAfterLast('Subject:');              

              System.debug('Sub:' +Sub);

              incident.Email_Subject__c= Sub;    // Error Line       

       }

}

Appreciate your help