Validate C# identifier name

Identifier.cs

using System;
using System.Linq;
using System.Text.RegularExpressions;

public static class IdentifierExtensions
{
    // definition of a valid C# identifier: http://msdn.microsoft.com/en-us/library/aa664670(v=vs.71).aspx
    private const string FORMATTING_CHARACTER = @"\p{Cf}";
    private const string CONNECTING_CHARACTER = @"\p{Pc}";
    private const string DECIMAL_DIGIT_CHARACTER = @"\p{Nd}";
    private const string COMBINING_CHARACTER = @"\p{Mn}|\p{Mc}";
    private const string LETTER_CHARACTER = @"\p{Lu}|\p{Ll}|\p{Lt}|\p{Lm}|\p{Lo}|\p{Nl}";

    private const string IDENTIFIER_PART_CHARACTER = LETTER_CHARACTER + "|" +
                                                     DECIMAL_DIGIT_CHARACTER + "|" +
                                                     CONNECTING_CHARACTER + "|" +
                                                     COMBINING_CHARACTER + "|" +
                                                     FORMATTING_CHARACTER;

    private const string IDENTIFIER_PART_CHARACTERS = "(" + IDENTIFIER_PART_CHARACTER + ")+";
    private const string IDENTIFIER_START_CHARACTER = "(" + LETTER_CHARACTER + "|_)";

    private const string IDENTIFIER_OR_KEYWORD = IDENTIFIER_START_CHARACTER + "(" +
                                                 IDENTIFIER_PART_CHARACTERS + ")*";

    // C# keywords: http://msdn.microsoft.com/en-us/library/x53a06bb(v=vs.71).aspx
    private static readonly HashSet<string> _keywords = new HashSet<string>
    {
        "__arglist",    "__makeref",    "__reftype",    "__refvalue",
        "abstract",     "as",           "base",         "bool",
        "break",        "byte",         "case",         "catch",
        "char",         "checked",      "class",        "const",
        "continue",     "decimal",      "default",      "delegate",
        "do",           "double",       "else",         "enum",
        "event",        "explicit",     "extern",       "false",
        "finally",      "fixed",        "float",        "for",
        "foreach",      "goto",         "if",           "implicit",
        "in",           "int",          "interface",    "internal",
        "is",           "lock",         "long",         "namespace",
        "new",          "null",         "object",       "operator",
        "out",          "override",     "params",       "private",
        "protected",    "public",       "readonly",     "ref",
        "return",       "sbyte",        "sealed",       "short",
        "sizeof",       "stackalloc",   "static",       "string",
        "struct",       "switch",       "this",         "throw",
        "true",         "try",          "typeof",       "uint",
        "ulong",        "unchecked",    "unsafe",       "ushort",
        "using",        "virtual",      "volatile",     "void",
        "while"
    };

    private static readonly Regex _validIdentifierRegex = new Regex("^" + IDENTIFIER_OR_KEYWORD + "$", RegexOptions.Compiled);

    public static bool IsValidIdentifier(this string identifier)
    {
        if (String.IsNullOrWhiteSpace(identifier)) return false;

        var normalizedIdentifier = identifier.Normalize();

        // 1. check that the identifier match the validIdentifer regex and it's not a C# keyword
        if (_validIdentifierRegex.IsMatch(normalizedIdentifier) && !_keywords.Contains(normalizedIdentifier))
        {
            return true;
        }

        // 2. check if the identifier starts with @
        if (normalizedIdentifier.StartsWith("@") && _validIdentifierRegex.IsMatch(normalizedIdentifier.Substring(1)))
        {
            return true;
        }

        // 3. it's not a valid identifier
        return false;
    }
}

Thanks @bpierson. I've update the gist.
Of course, the list of keywords should also be updated but I've currently not the free time to do it.

Your hashset is missing four keywords: __arglist, __makeref, __reftype and __refvalue. Though they are not mentioned in the documentation, they exist and are not valid identifiers.

Thanks @piotrstenke. I've updated the list of identifiers.
I also took the opportunity to rearrange the list of identifiers and to format the code using the dotnet-format tool.