C# Online Compiler | .NET Fiddle

using System;
public class Program {
  public static void Main() {

    string str1 = "Welcome ";
    string str2 = "to ";
    string str3 = "CSharp Course";
    string result = str1 + str2 + str3;

    Console.WriteLine(result);
  }
}

CheckIfRegexAcceptsLowercase:
abc IgnoreCase => True == True
ABC IgnoreCase => True == True
[ABC] None => False == False
[abc] None => True == True
[abc^ABC] None => True == True
[ABC^abc] None => False == False
\[abc^ABC\] None => True == True
\[ABC^abc\] None => True == True
[aBC^Abc] None => True == True
[Abc^aBC] None => True == True
\p{Ll} None => True == True
\\p{Ll} None => True == True
\P{Ll} None => False == False
\p{Lu} None => False == False
\\p{Lu} None => True == True
\P{Lu} None => True == True

CheckIfRegexAcceptsUppercase:
abc IgnoreCase => True == True
ABC IgnoreCase => True == True
[ABC] None => True == True
[abc] None => False == False
[abc^ABC] None => False == False
[ABC^abc] None => True == True
\[abc^ABC\] None => True == True
\[ABC^abc\] None => True == True
[aBC^Abc] None => True == True
[Abc^aBC] None => True == True
\p{Ll} None => False == False
\\p{Ll} None => True == True
\P{Ll} None => True == True
\p{Lu} None => True == True
\\p{Lu} None => True == True
\P{Lu} None => False == False