using System;

using System.IO;

using System.Linq;

using System.Xml;

using System.Xml.Linq;

using System.Xml.Serialization;

using System.Text.RegularExpressions;

​

public class Program

{

    public static void Main()

    {

        PrefixValidation.IsValid("abc3").Dump();

        PrefixValidation.IsValid("abi").Dump();

        PrefixValidation.IsValid("abc").Dump();

        PrefixValidation.IsValid("DEĞ").Dump();

        PrefixValidation.IsValid("DEW").Dump();

        PrefixValidation.IsValid("VAAA").Dump();

        PrefixValidation.IsValid("VADC").Dump();

    }

}

​

public static class PrefixValidation

{

    public static readonly Regex Pattern = new("[A-Z]{3}", RegexOptions.Compiled);

    public static bool IsValid(string prefix)

    {

        if (prefix != null)

        {

            return Pattern.IsMatch(prefix);

        }

        else

        {

            return false;

        }

    }

}