diff --git a/src/NzbDrone.Core/Validation/RuleBuilderExtensions.cs b/src/NzbDrone.Core/Validation/RuleBuilderExtensions.cs
index 231a7fbeb..df1d8056b 100644
--- a/src/NzbDrone.Core/Validation/RuleBuilderExtensions.cs
+++ b/src/NzbDrone.Core/Validation/RuleBuilderExtensions.cs
@@ -25,18 +25,18 @@ public static IRuleBuilderOptions<T, string> HaveHttpProtocol<T>(this IRuleBuild
         public static IRuleBuilderOptions<T, string> ValidHost<T>(this IRuleBuilder<T, string> ruleBuilder)
         {
             ruleBuilder.SetValidator(new NotEmptyValidator(null));
-            return ruleBuilder.SetValidator(new RegularExpressionValidator("^[-a-z0-9.]+$", RegexOptions.IgnoreCase)).WithMessage("must be valid Host without http://");
+            return ruleBuilder.SetValidator(new RegularExpressionValidator("^[-_a-z0-9.]+$", RegexOptions.IgnoreCase)).WithMessage("must be valid Host without http://");
         }
 
         public static IRuleBuilderOptions<T, string> ValidRootUrl<T>(this IRuleBuilder<T, string> ruleBuilder)
         {
             ruleBuilder.SetValidator(new NotEmptyValidator(null));
-            return ruleBuilder.SetValidator(new RegularExpressionValidator("^https?://[-a-z0-9.]+", RegexOptions.IgnoreCase)).WithMessage("must be valid URL that starts with http(s)://");
+            return ruleBuilder.SetValidator(new RegularExpressionValidator("^https?://[-_a-z0-9.]+", RegexOptions.IgnoreCase)).WithMessage("must be valid URL that starts with http(s)://");
         }
 
         public static IRuleBuilderOptions<T, string> ValidUrlBase<T>(this IRuleBuilder<T, string> ruleBuilder)
         {
-            return ruleBuilder.SetValidator(new RegularExpressionValidator(@"^(?!\/?https?://[-a-z0-9.]+)", RegexOptions.IgnoreCase)).WithMessage("Must be a valid URL path (ie: '/sonarr')");
+            return ruleBuilder.SetValidator(new RegularExpressionValidator(@"^(?!\/?https?://[-_a-z0-9.]+)", RegexOptions.IgnoreCase)).WithMessage("Must be a valid URL path (ie: '/sonarr')");
         }
 
         public static IRuleBuilderOptions<T, int> ValidPort<T>(this IRuleBuilder<T, int> ruleBuilder)