using System;

using System.Text.RegularExpressions;

​

public class Program

{

    public static void Main()

    {

        string[] inputTexts = {

            "$18.20 to 20.20 per hour",

            "18.20 to 20.20 per hour",

            "18.20 to $20.20 per hour",

            "$18.20 to $20.20 per hour"

        };

​

        foreach (string inputText in inputTexts)

        {

            string pattern = @"(?<!\$)\b(\d+\.\d+)\b"; // Match numeric values with decimals not preceded by "$"

            string result = Regex.Replace(inputText, pattern, m => "$" + m.Value);

​

            Console.WriteLine(result);

        }

    }

}