using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        Program p = new Program();

        Console.WriteLine(p.ValidateString(",;helloworld", 5, 30));

    }

    public string ValidateString(string stringValue, int minLength, int maxLength)

    {

        if (string.IsNullOrEmpty(stringValue))

        {

            return null;

        }

​

        Regex rgx = new Regex($"^[,;a-zA-Z0-9._-]{{{minLength},{maxLength}}}$");

​

        if (rgx.IsMatch(stringValue))

        {

            return stringValue;

        }

​

        return null;

    }   

}