Support underscore-prefixed API version identifiers

NamespaceParser now recognizes API version identifiers starting with '_', in addition to 'v' and 'V'. Added comprehensive tests to ensure correct parsing of underscore-prefixed version formats in various namespace scenarios.

Author: xavierjohn

src/Abstractions/src/Asp.Versioning.Abstractions/NamespaceParser.cs

@@ -157,16 +157,17 @@ protected virtual bool TryParse( Text identifier, out ApiVersion? apiVersion )
             return false;
         }
 
-        // 'v' | 'V' : [<year> : ['_'] : <month> : ['_'] : <day>] : ['_'] : [<major> ['_' : <minor>]] : ['_'] : [<status>]
+        // 'v' | 'V' | '_' : [<year> : ['_'] : <month> : ['_'] : <day>] : ['_'] : [<major> ['_' : <minor>]] : ['_'] : [<status>]
         //
         // - v1
         // - v1_1
         // - v2_0_Beta
         // - v20180401
         // - v2018_04_01_1_1_Beta
+        // - _2018_04_01
         var ch = identifier[0];
 
-        if ( ch != 'v' && ch != 'V' )
+        if ( ch != 'v' && ch != 'V' && ch != '_' )
         {
             apiVersion = default;
             return false;

src/Abstractions/test/Asp.Versioning.Abstractions.Tests/NamespaceParserTest.cs

@@ -70,6 +70,17 @@ public void parse_should_return_no_versions()
         { "Contoso.Api.v2018_04_01_1_0_Beta.Controllers", "2018-04-01.1.0-Beta" },
         { "MyRestaurant.Vegetarian.Food.v1_1.Controllers", "1.1" },
         { "VersioningSample.V5.Controllers", "5.0" },
+        { "_1", "1" },
+        { "_1_1", "1.1" },
+        { "_0_9_Beta", "0.9-Beta" },
+        { "_20180401", "2018-04-01" },
+        { "_2018_04_01", "2018-04-01" },
+        { "_20180401_Beta", "2018-04-01-Beta" },
+        { "_2018_04_01_Beta", "2018-04-01-Beta" },
+        { "_2018_04_01_1_0_Beta", "2018-04-01.1.0-Beta" },
+        { "Contoso.Api._1.Controllers", "1" },
+        { "Contoso.Api._2018_04_01.Controllers", "2018-04-01" },
+        { "Contoso.Api._2018_04_01_Beta.Controllers", "2018-04-01-Beta" },
     };
 
     public static TheoryData<string, string[]> NamespaceWithMultipleVersions => new()