Can you update lookups using workflows?

    This will have to be done with coding.  Workflow Rules can't process unrelated records.  And, Workflow Rule Field Updates can't dynamically set Lookup() field values.

State Validation Rule

The US State contains 2 Letters. 

1. Create a Text field and the set values as "2"

2. To write a below validation rule  

OR( ISNUMBER( State__c ), OR(CONTAINS(State__c , "!"),CONTAINS(State__c , "@"),CONTAINS(State__c , "#"),CONTAINS(State__c , "$"),CONTAINS(State__c , "$"),CONTAINS(State__c , "%"),CONTAINS(State__c , "^"),CONTAINS(State__c , "&"),CONTAINS(State__c , "*")))

Roll Up Summary using Trigger

trigger sumofLPT on loan__Loan_Payment_Transaction__c (after insert, after update, after delete) {
    List<loan__Loan_Account__c> llaList = new List<loan__Loan_Account__c>();
  
    Set<ID> setlpt =  new Set<ID>();
  
    for(loan__Loan_Payment_Transaction__c lpt: trigger.new)
    {
        setlpt.add(lpt.id);
    }
    Decimal sum1;
    Decimal sum2;
    Decimal sum3;
    Decimal sum4;
    for(loan__Loan_Account__c loan: [select name, (select name, loan__Cleared__c, loan__Rejected__c, loan__Rejection_Reason__c, loan__Reversed__c, loan__Reversal_Reason__c, loan__Transaction_Amount__c FROM loan__Loan_Payment_Transactions__r)
    FROM loan__Loan_Account__c ])
    {
      
        sum1 = 0;
        sum2 = 0;
        sum3 = 0;
        sum4 = 0;
        for(loan__Loan_Payment_Transaction__c lpt1:loan.loan__Loan_Payment_Transactions__r){
        if(lpt1.loan__Rejected__c == true){   
            sum1 += lpt1.loan__Transaction_Amount__c;
            loan.Rejected_Transaction_Amount__c = sum1;
            System.debug('!!!!!!!!!!!!!sum1'+sum1);
            System.debug('@@@@@@@@@@@@@loan.Rejected_Transaction_Amount__c'+loan.Rejected_Transaction_Amount__c);
          
           }
            /*else
            {
                for(loan__Loan_Payment_Transaction__c lpt2: Trigger.old){
                if((Trigger.isUpdate || Trigger.isDelete) && lpt1.loan__Rejected__c == true){
                loan__Loan_Payment_Transaction__c lpt1old = Trigger.oldMap.get(lpt1.Id);
                }
               }
            }*/
         if(lpt1.loan__Reversed__c == true){
            sum2 += lpt1.loan__Transaction_Amount__c;
            loan.Reversed_Transaction_Amount__c = sum2;
            System.debug('!!!!!!!!!!!!!sum2'+sum2);
            System.debug('@@@@@@@@@@@@@loan.Reversed_Transaction_Amount__c'+loan.Reversed_Transaction_Amount__c);
            }
         if(lpt1.loan__Cleared__c == true){
            sum3 += lpt1.loan__Transaction_Amount__c;
            loan.Cleared_Transaction_Amount__c = sum3;
            System.debug('!!!!!!!!!!!!!sum3'+sum3);
            System.debug('@@@@@@@@@@@@@loan.Cleared_Transaction_Amount__c'+loan.Cleared_Transaction_Amount__c);
            }
            if(lpt1.loan__Cleared__c == false && lpt1.loan__Reversed__c == false && lpt1.loan__Rejected__c == false){
            sum4 += lpt1.loan__Transaction_Amount__c;
            loan.Uncleared_Transaction_Amount__c = sum4;
             System.debug('!!!!!!!!!!!!!sum4'+sum4);
            System.debug('@@@@@@@@@@@@@loan.Uncleared_Transaction_Amount__c'+loan.Uncleared_Transaction_Amount__c);
            }
        
        }
        llaList.add(loan);
    }
    update llaList;
  
}