using System;

using System.Text.RegularExpressions;

​

public class Program

{

    public static void Main()

    {

        var string1 = "500g Flour 14g Salt 7g Dry yeast 45ml Olive oil 309ml Water";

        var result = Regex.Replace(string1, @"\b(\d+)", Doubler, RegexOptions.Multiline);

        Console.WriteLine(result);

    }

​

    private static string Doubler(Match match)

    {

        return (Convert.ToInt32(match.Value) * 2).ToString();

    }

}