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

public class Test
{
	public static void Main()
	{
		var text = "(String1) | (string2 string3)";
    	var result = Regex.Match(text, @"\|\s*\(([^()]*)\)")?.Groups[1].Value;
        Console.WriteLine(result);
        
        var result2 = text.Split(new[] {" | "}, StringSplitOptions.None)[1].Trim(new[] {'(', ')'});
        Console.WriteLine(result2);
	}
}