using System;

using System.Linq;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        string unquotedPipeDelimited = """

            CONTACT_SOURCE_ID|LAST_UPDATED|LAST_UPDATED_BY|T92|D10|T94|T95|T96|T97|T98|T99|T100|T101|T124|T125|T126

            72|05/01/2006 14:48:38|13|||Mr|S|BLOGGS|1 Random Building

            Random Street|Random Town||Random County|RN1 2DM|||

            74|05/01/2006 15:48:38|31|||Dr|Delta|Who|The

            Tardis|SpaceAndTime||Universe|D1 0WW|||

            """;

​

        // Double up any quotes in the data: " becomes ""

        string search1 = "\"";

        string replace1 = "\"\"";

        string doubledQuotes = Regex.Replace(unquotedPipeDelimited, search1, replace1);

        // Quote the fields and replace the delimiter: abc|def becomes "abc","def"

        int fieldCount = 16;

        string search2 =

            "^"

            + string.Join("\\|", Enumerable.Range(1, fieldCount).Select(i => "([^|]*)"))

            + "$";

        string replace2 =

            string.Join(",", Enumerable.Range(1, fieldCount).Select(i => $"\"${i}\""));

        RegexOptions options2 = RegexOptions.Multiline;

        string csv = Regex.Replace(doubledQuotes, search2, replace2, options2);

​

        Console.WriteLine(csv);

    }

}