Regular expression for validating names and surnames?

I sympathize with the need to constrain input in this situation, but I don't believe it is possible - Unicode is vast, expanding, and so is the subset used in names throughout the world.

Unlike email, there's no universally agreed-upon standard for the names people may use, or even which representations they may register as official with their respective governments. I suspect that any regex will eventually fail to pass a name considered valid by someone, somewhere in the world.

Of course, you do need to sanitize or escape input, to avoid the Little Bobby Tables problem. And there may be other constraints on which input you allow as well, such as the underlying systems used to store, render or manipulate names. As such, I recommend that you determine first the restrictions necessitated by the system your validation belongs to, and create a validation expression based on those alone. This may still cause inconvenience in some scenarios, but they should be rare.

I would think you would be better off excluding the characters you don't want with a regex. Trying to get every umlaut, accented e, hyphen, etc. will be pretty insane. Just exclude digits (but then what about a guy named "George Forman the 4th") and symbols you know you don't want like @#$%^ or what have you. But even then, using a regex will only guarantee that the input matches the regex, it will not tell you that it is a valid name.

EDIT after clarifying that this is trying to prevent XSS: A regex on a name field is obviously not going to stop XSS on its own. However, this article has a section on filtering that is a starting point if you want to go that route:

s/[\<\>\"\'\%\;\(\)\&\+]//g;

"Secure Programming for Linux and Unix HOWTO" by David A. Wheeler, v3.010 Edition (2003)

v3.72, 2015-09-19 is a more recent version.

I would just allow everything (except an empty string) and assume the user knows what his name is.

There are 2 common cases:

1. You care that the name is accurate and are validating against a real paper passport or other identity document, or against a credit card.
2. You don't care that much and the user will be able to register as "Fred Smith" (or "Jane Doe") anyway.

In case (1), you can allow all characters because you're checking against a paper document.

In case (2), you may as well allow all characters because "123 456" is really no worse a pseudonym than "Abc Def".

I'll try to give a proper answer myself:

The only punctuations that should be allowed in a name are full stop, apostrophe and hyphen. I haven't seen any other case in the list of corner cases.

Regarding numbers, there's only one case with an 8. I think I can safely disallow that.

Regarding letters, any letter is valid.

I also want to include space.

This would sum up to this regex:

^[\p{L} \.'\-]+$

This presents one problem, i.e. the apostrophe can be used as an attack vector. It should be encoded.

So the validation code should be something like this (untested):

var name = nameParam.Trim();
if (!Regex.IsMatch(name, "^[\p{L} \.\-]+$")) 
    throw new ArgumentException("nameParam");
name = name.Replace("'", "'");  //' does not work in IE

Can anyone think of a reason why a name should not pass this test or a XSS or SQL Injection that could pass?

---

complete tested solution

using System;
using System.Text.RegularExpressions;

namespace test
{
    class MainClass
    {
        public static void Main(string[] args)
        {
            var names = new string[]{"Hello World", 
                "John",
                "João",
                "タロウ",
                "やまだ",
                "山田",
                "先生",
                "мыхаыл",
                "Θεοκλεια",
                "आकाङ्क्षा",
                "علاء الدين",
                "אַבְרָהָם",
                "മലയാളം",
                "상",
                "D'Addario",
                "John-Doe",
                "P.A.M.",
                "' --",
                "<xss>",
                "\""
            };
            foreach (var nameParam in names)
            {
                Console.Write(nameParam+" ");
                var name = nameParam.Trim();
                if (!Regex.IsMatch(name, @"^[\p{L}\p{M}' \.\-]+$"))
                {
                    Console.WriteLine("fail");
                    continue;
                }
                name = name.Replace("'", "&#39;");
                Console.WriteLine(name);
            }
        }
    }
}