Symptoms
Needing more dynamic validation on a List View.
Diagnoses
Possible with parameters, an expression, and some rules.
Resolution
Essentially it involves grabbing the desired sub-string in one For loop, and then validating each row against it in another loop, using text parameter to store the sub-string and a boolean parameter to hold the final result of validation. Nested looping can be achieved by creating the inner loop in an unbound rule, and then using the 'Execute another rule" action to call the unbound rule from within the other loop. At the end you can show a message warning of invalidation.
