using System;

using System.Text.RegularExpressions;

​

public class Program

{

    public static void Main()

    {

        string config = "maria";

        string sql = "SELECT TOP 150 * FROM tbl WHERE XYZ = 123";

        if ("maria".Equals(config)) { 

            Regex regex = new Regex(@"(SELECT.*?)TOP (\d+)(.*)");

            string output = regex.Replace(sql, "$1 $3 LIMIT $2");

            Console.WriteLine(output);

        }

    }

}