[csharp] Improved regex support (#16269)

* improved regex support

* better parsing of options

* better handling of options

* spacing change

* typo

* rename the x-modifier- extension

* trivial change

* added to the sample

samples/client/petstore/csharp/OpenAPIClient-unityWebRequest/docs/Apple.md

@@ -6,6 +6,7 @@ Name | Type | Description | Notes
 ------------ | ------------- | ------------- | -------------
 **Cultivar** | **string** |  | [optional] 
 **Origin** | **string** |  | [optional] 
+**ColorCode** | **string** |  | [optional] 
 
 [[Back to Model list]](../README.md#documentation-for-models) [[Back to API list]](../README.md#documentation-for-api-endpoints) [[Back to README]](../README.md)
 

samples/client/petstore/csharp/OpenAPIClient-unityWebRequest/docs/Fruit.md

@@ -7,6 +7,7 @@ Name | Type | Description | Notes
 **Color** | **string** |  | [optional] 
 **Cultivar** | **string** |  | [optional] 
 **Origin** | **string** |  | [optional] 
+**ColorCode** | **string** |  | [optional] 
 **LengthCm** | **decimal** |  | [optional] 
 
 [[Back to Model list]](../README.md#documentation-for-models) [[Back to API list]](../README.md#documentation-for-api-endpoints) [[Back to README]](../README.md)

samples/client/petstore/csharp/OpenAPIClient-unityWebRequest/docs/GmFruit.md

@@ -7,6 +7,7 @@ Name | Type | Description | Notes
 **Color** | **string** |  | [optional] 
 **Cultivar** | **string** |  | [optional] 
 **Origin** | **string** |  | [optional] 
+**ColorCode** | **string** |  | [optional] 
 **LengthCm** | **decimal** |  | [optional] 
 
 [[Back to Model list]](../README.md#documentation-for-models) [[Back to API list]](../README.md#documentation-for-api-endpoints) [[Back to README]](../README.md)