RegEx - Do not allow leading and trailing whitespace

expression.re

^(?!\s)[\s\S]+(?<!\s)$

RegExTest.cs

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

namespace RegExTest
{
    static class MyExtensions
    {
        public static void PrintMatch(this Regex re, string input)
        {
            Console.WriteLine($"Math result <{input}> : {re.IsMatch(input)}");
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            const string RE = @"^(?!\s)[\s\S]+(?<!\s)$";
            Regex re = new Regex(RE);
            var inputs = new List<string>
            {
                "test",
                " test",
                " test ",
                "test ",
                "test with space",
                "test with space\n and new line",
                "тест with χαρακτήρες på olika språk"

            };
            Console.WriteLine($"RE = {re.ToString()}");
            inputs.ForEach(x => re.PrintMatch(x));
        }
        /* Output:
         * RE = ^(?!\s)[\s\S]+(?<!\s)$
         * Math result <test> : True
         * Math result < test> : False
         * Math result < test > : False
         * Math result <test > : False
         * Math result <test with space> : True
         * Math result <test with space
         * and new line> : True
         * Math result <тест with χαρακτήρες på olika språk> : True
         */
    }
}

Glad you found this useful.