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);
        }
	}
}

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