using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text.RegularExpressions;

public class Test
{
	public static void Main()
	{
		var strs = new String[] {"GMT-05:00 Eastern Time(Toronto)","(GMT - 06:00) Central Time(US, Canada)","GMT-10:00 Hawaii - Aleutian Standard Time(Honolulu)"};
		foreach (var s in strs)
		{
		    var result = Regex.Match(s, @"\bGMT\s*([-+]?)\s*(\d+:\d+)");
		    if (result.Success) {
		        Console.WriteLine($"Parsing '{s}'\nResult: {result.Groups[1].Value}{result.Groups[2].Value}");
		    }
		}
	}
}