Regular Expression schema C# .Net 5

Swagger schema is preventing the execution of the Swagger UI testing utility.  The pattern is applied, without applying the ValidationAttribute.ErrorMessage.  

[RegularExpression(@"^[A-Za-z0-9\-_\.:]+$", ErrorMessage = "Invalid foo name.")]

 "x-generator": "NSwag v13.10.6.0 (NJsonSchema v10.3.8.0 (Newtonsoft.Json v12.0.0.0))",
  "swagger": "2.0",

"foo": { "type": "string", "maxLength": 50, "pattern": "^[A-Za-z0-9\\-_\\.:]+$" }