C# underscore string extension

gistfile1.cs

// Convert a string to Underscore
public static class StringEx 
{
    public static string Underscored(this string s)
    {
        var builder = new StringBuilder();

        for (var i = 0; i < s.Length; ++i)
        {
            if (ShouldUnderscore(i, s))
            {
                builder.Append('_');
            }

            builder.Append(char.ToLowerInvariant(s[i]));
        }

        return builder.ToString();
    }

    private static bool ShouldUnderscore(int i, string s)
    {
        if (i == 0 || i >= s.Length || s[i] == '_') return false;

        var curr = s[i];
        var prev = s[i - 1];
        var next = i < s.Length - 2 ? s[i + 1] : '_';

        return prev != '_' && ((char.IsUpper(curr) && (char.IsLower(prev) || char.IsLower(next))) ||
            (char.IsNumber(curr) && (!char.IsNumber(prev))));
    }
}

// Test it (requires Should)
namespace NadaissueWeb.Tests
{
    [TestClass]
    public class UnderscoredTests
    {
        [TestMethod]
        public void UnderscoredOnCaps() 
        {
            "HelloWorldAndStuff".Underscored().ShouldEqual("hello_world_and_stuff");
        }

        [TestMethod]
        public void UnderscoredOnOnlyFirstAndLastCapInRun()
        {
            "HelloABCStore".Underscored().ShouldEqual("hello_abc_store");
        }

        [TestMethod]
        public void UnderscoredHandlesEndingCaps()
        {
            "HelloABC".Underscored().ShouldEqual("hello_abc");
        }

        [TestMethod]
        public void UnderscoredHandlesEndingNums()
        {
            "Hello123".Underscored().ShouldEqual("hello_123");
        }

        [TestMethod]
        public void UnderscoredOnNumberRun() 
        {
            "Hello73Books".Underscored().ShouldEqual("hello_73_books");
        }

        [TestMethod]
        public void UnderscoredCamelCase()
        {
            "helloWorld".Underscored().ShouldEqual("hello_world");
        }

        [TestMethod]
        public void UnderscoredLastCap()
        {
            "HelloW".Underscored().ShouldEqual("hello_w");
        }

        [TestMethod]
        public void UnderscoredLastNum()
        {
            "Hello1".Underscored().ShouldEqual("hello_1");
        }

        [TestMethod]
        public void UnderscoredExistingUnderscores()
        {
            "hello_World".Underscored().ShouldEqual("hello_world");
        }
    }
}