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 List<string> { " [ f ] s = t", "s = t"};
    	var pattern = @"(?:^\p{Zs}*|])([^]=]*)=";
        foreach (var s in strs)
        {
        	var match = Regex.Match(s, pattern);
        	if (match.Success)
        		Console.WriteLine(match.Groups[1].Value);
        }
	}
}