using System;
using System.Linq;
using System.Text.RegularExpressions;

public class Test
{
	public static void Main()
	{
		var regex = @"'[^']*'|\b([_A-Za-z]\w*)\b(?![('])";
		var _expression = @"12+x1+455+'ggg+4+rrr+tt'+3";
		var matches = Regex.Matches(_expression, regex)
       		.Cast<Match>()
       		.Select(m => m.Groups[1].Value)
       		.ToList();
       	foreach (var s in matches)
       		Console.WriteLine(s);
	}
}