using System;
using System.Text.RegularExpressions;

public class Test
{
	public static void Main()
	{
    	var text = "field1   :    value1a  value1b   field2   : value2a value2b";
    	var matches = Regex.Matches(text, @"(\w+)\s*:\s*(.*?)(?=\s*\w+\s*:|$)");
		foreach (Match m in matches)
		{
		    Console.WriteLine("-- MATCH FOUND --\nKey: {0}, Value: {1}", 
		    	m.Groups[1].Value, m.Groups[2].Value);
		}
	}
}