[C#][netcore] fix regular expression when it contains double quotes (#…

…7147)

* escape special characters in regular expression in csharp netcore client

* escape double quote

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/CSharpNetCoreClientCodegen.java

@@ -476,7 +476,7 @@ public void postProcessPattern(String pattern, Map<String, Object> vendorExtensi
                         + "/pattern/modifiers convention. " + pattern + " is not valid.");
             }
 
-            String regex = pattern.substring(1, i).replace("'", "\'");
+            String regex = pattern.substring(1, i).replace("'", "\'").replace("\"", "\"\"");
             List<String> modifiers = new ArrayList<String>();
 
             // perl requires an explicit modifier to be culture specific and .NET is the reverse.