ChrissieRegex

using NUnit.Framework;
using System.Text.RegularExpressions;

namespace blah
{
    [TestFixture]
    public class RegexTests
    {
        [TestCase("111111", true, Description = "Exact match")]
        [TestCase("a11111", false, Description = "Letter present")]
        [TestCase("99999a", false, Description = "Letter present at end")]
        [TestCase(".11111", false, Description = "Symbol present")]
        [TestCase("⁹11111", false, Description = "Extended character present")]
        [TestCase("⁹111111", false, Description = "Extra extended character present at start")]
        [TestCase("111111⁹", false, Description = "Extra extended character present at end")]
        [TestCase("22222", false, Description = "Too short")]
        [TestCase("333333\n", false, Description = "line feed at end")]
        [TestCase("333333\r", false, Description = "cr at end")]
        [TestCase("333333\n\n", false, Description = "double line feed at end")]
        [TestCase("333333\r\n", false, Description = "crln at end")]
        [TestCase("\n222222", false, Description = "line feed at start")]
        [TestCase("222\n222", false, Description = "line feed in middle")]
        [TestCase("   444444", false, Description = "space at start")]
        [TestCase("444444    ", false, Description = "space at end")]
        [TestCase("111111-222222", false, Description = "longer string")]
        [TestCase("111111 and 222222 and 333333", false, Description = "also longer string")]
        [TestCase("1E02", false)]
        [TestCase("NaN", false)]
        [TestCase("Infinity", false)]
        [TestCase("111,111", false)]
        [TestCase("€111111", false)]
        [TestCase("€11111", false)]
        [TestCase("(╯°□°）╯︵ ┻━┻)", false)]
        [TestCase("１１１１１１", false)]
        [TestCase("₁₁₁₁₁₁", false)]
        [TestCase("1️⃣1️⃣1️⃣1️⃣1️⃣1️⃣", false)]
        [TestCase("1️⃣111111", false)]
        [TestCase("111111️⃣", false)]
        [TestCase("111111🌈", false)]
        [TestCase("🌈111111", false)]
        [TestCase("1🌈11111", false)]
        [TestCase("Ⅳ11111", false)]
        [TestCase("Ⅳ111111", false)]
        public void Basic(string testString, bool expectMatch)
        {
            var regex = new Regex("^\\d{6}(?!\\n)$");
            var isMatch = regex.IsMatch(testString);
            Assert.AreEqual(expectMatch, isMatch);
        }
    }
}

Some more, just because:

        [TestCase($"111111{"\x7"}", false, Description = "bell at end")]
        [TestCase($"{"\x7"}111111", false, Description = "bell at beginning")]
        [TestCase($"111111{"\x3"}1", false, Description = "false report of end of string via text end ascii")]
        [TestCase($"111111{"\x8"}", false, Description = "backspace ascii at end of string")]
        [TestCase($"111111{"\x8"}1", false, Description = "backspace ascii of extra character")]
        [TestCase($"111111{"\x127"}", false, Description = "delete ascii at end of string")]
        [TestCase($"111111{"\x0"}", false, Description = "null ascii at end of string")]
        [TestCase($"{"\x0"}111111", false, Description = "null ascii at start of string")]