How to Search Workflow Rules or Validation Rules etc in Apex i.e. Metadata Search via Apex?

UPDATE: 11th Nov, I discovered the WorkflowRule (as apposed to the Workflow) metadata type! Combined with the ValidationRule metadata type. This means that the Metadata listMetadata API call now returns exactly the two lists you need to implement your search tool.

This will allow you to list the Validation Rules and Workflow Rules from Apex.

MetadataService.MetadataPort service = MetadataServiceExamples.createService();     
List<MetadataService.ListMetadataQuery> queries = new List<MetadataService.ListMetadataQuery>();        
MetadataService.ListMetadataQuery queryWorkflow = new MetadataService.ListMetadataQuery();
queryWorkflow.type_x = 'WorkflowRule';
queries.add(queryWorkflow);     
MetadataService.ListMetadataQuery queryValidationRule = new MetadataService.ListMetadataQuery();
queryValidationRule.type_x = 'ValidationRule';
queries.add(queryValidationRule);           
MetadataService.FileProperties[] fileProperties = service.listMetadata(queries, 25);
for(MetadataService.FileProperties fileProperty : fileProperties)
    System.debug(fileProperty.fullName);

For more information and the MetadataService and MetadataServiceExamples class used in the above code. Please take a look at this Github repo and Readme file.