Give slices of a pattern starting with a capture group no namespace

ArchUnitNET/Fluent/Slices/SliceRuleInitializer.cs

@@ -161,8 +161,14 @@ private static SliceIdentifier AssignFunc(
                 );
             }
 
+            // An empty prefix means the pattern starts with its capture group; there is no
+            // namespace to nest the slice under, so report none rather than an empty one.
             return fullName
-                ? SliceIdentifier.Of(slicePrefix + sliceString, countOfSingleAsterisk, slicePrefix)
+                ? SliceIdentifier.Of(
+                    slicePrefix + sliceString,
+                    countOfSingleAsterisk,
+                    slicePrefix == "" ? null : slicePrefix
+                )
                 : SliceIdentifier.Of(sliceString, countOfSingleAsterisk);
         }
     }

ArchUnitNETTests/Domain/PlantUml/PlantUmlSliceDiagramTests.cs

@@ -103,6 +103,14 @@ public Task BuildUmlBySlicesMatchingWithPackages_NonContiguousCaptureGroups()
             );
         }
 
+        [Fact]
+        public Task BuildUmlBySlicesMatchingWithPackages_CaptureGroupAtStart()
+        {
+            // Nothing precedes the capture group, so there is no namespace to nest the slices
+            // under and they are drawn flat, like those of Matching.
+            return VerifySlices(SliceRuleDefinition.Slices().MatchingWithPackages("(**).Service"));
+        }
+
         [Fact]
         public Task BuildUmlBySlicesMatchingWithPackages_LimitDependencies()
         {

ArchUnitNETTests/Domain/PlantUml/Snapshots/PlantUmlSliceDiagramTests.BuildUmlBySlicesMatchingWithPackages_CaptureGroupAtStart.verified.txt

@@ -0,0 +1,11 @@
+@startuml
+
+!include https://raw.githubusercontent.com/plantuml-stdlib/C4-PlantUML/master/C4_Container.puml
+
+HIDE_STEREOTYPE()
+
+[SlicesTestAssembly.DotDotSemantics.Alpha]
+[SlicesTestAssembly.MultipleSubnamespaces.Slice1]
+[SlicesTestAssembly.MultipleSubnamespaces.Slice2]
+[SlicesTestAssembly.MultipleSubnamespaces.Slice1] --|> [SlicesTestAssembly.MultipleSubnamespaces.Slice2]
+@enduml