[SalesForce] Check if string contains specific characters- Validation Rule

Nope. Only Hierarchy settings can be accessed using $Setup global in validation formula context.

Take the advantage of order of execution of triggers ,validation rules and workflows.

1)Your custom validations run after before triggers

2)Your workflow rules run after your triggers .

3)Triggers and Merge Statement document https://www.salesforce.com/us/developer/docs/apexcode/Content/apex_triggers_merge_statements.htm

Using the above conclusion ,create a boolean field on the record named "ByPassValidation" and avoid this from pagelayout .

As per document when merge happens ,after delete trigger on looser record gives a MasterRecordId ,Query records with Ids equivalent to MasterRecordId and update the boolean ByPassValidation=true

Now In validation rules use this flag and AND your current validation with this field .Make sure by default your ByPassValidation is unchecked .

Draft a workflow rule now to again uncheck the ByPassValidation flag .In the entire process there can be chance for trigger to fire once more due to wrokflow making an update ,Use Static boolean to avoid this condition .