When the search provider for a CMS site is Microsoft's FAST Search for SharePoint 2010, some string refinements containing special characters can result in a malformed query error when applied to search criteria.
An error (malformed query) occurs when submitting a query with a refinement known to include a special character, such as an exclamation mark (!). Zero results are returned.
Refinements that include a special character, such as an exclamation mark (!), can result in a malformed query error being returned from Microsoft FAST Search for SharePoint 2010. Typically, this would occur when the refinement represents a single word phrase with the special character interspersed. For example, a refinement with a value of "ref!nement" would generate such an error.
Reconsider using metadata and other properties as refinements when they may contain values with special characters as described above.