How can I validate the string can be up to 30 characters long of letters, numbers, and/or spaces in C# .NET

Why not just write a string extension method that uses regular .Net classes/methods. it would be much more efficient than using Regex.

public static bool ValidString(this string text)
{
    bool result = true;
    if (text.Length <= 30)
    {
        string lowCaseText = text.ToLower();
        string validChars = "abcdefghijklmnopqrstuvwxyz 1234567890";
        for (int i = 0; i < text.Length; i++)
        {
            if (!validChars.Contains(text[i]))
            {
                result = false;
                break;
            }
        }
    }
    else
    {
        result = false;
    }
    return result;
}

Here is a few links that can help to use RegEx
Here is a link to RegEx documentation:
perlre - perldoc.perl.org[^]
Here is links to tools to help build RegEx and debug them:
.NET Regex Tester - Regex Storm[^]
Expresso Regular Expression Tool[^]
This one show you the RegEx as a nice graph which is really helpful to understand what is doing a RegEx:
Debuggex: Online visual regex tester. JavaScript, Python, and PCRE.[^]