Small LINQ to JSON Library

using System.Text.RegularExpressions;
using System;

namespace Ranslant.JSON.Linq
{
    public class JString:IJValue
    {
        private const string AllowedEscapeCharacters = "\"\\/bfnrtu";
        private const string EscapedUFourDigits = "[0-9a-fA-F]{4}";
        /*
         this regular expression needs to be checked:
         1. does it accurately describe a JSON string (without start and end delimiters)
         2. does it make sense to use it?
         */
        //private const string Pattern = @"^([^\\""]|\\([\\""/bfnrt]|(u[0-9a-fA-F]{4})))*$";

        private const string InvalidString = "invalid string: ";
        
        private string _content;
        public string Content
        {
            get
            {
                return this._content;
            }
        }

        /// <summary>
        /// Content is checked for validity. An excpetion is thrown if an error is found.
        /// </summary>
        /// <param name="content"> the string that is contained withing the JString value; this string is validated</param>
        public JString(string content)
        {
            if(CheckEscapedCharacters(content))
                _content = content;
        }

        /// <summary>
        /// throws JsonExceptions if something is wrong with the strings
        /// </summary>
        /// <param name="content"></param>
        private bool CheckEscapedCharacters(string content)
        {
            bool nexCharIsEscaped = false;

            for (int i = 0; i < content.Length; i++ )
            {
                char charToTest = content[i];

                if (nexCharIsEscaped)
                {
                    // TODO use a regexp for the whole string?

                    if (JString.AllowedEscapeCharacters.IndexOf(charToTest) < 0)
                        throw new JsonException("character (" + charToTest + ") cannot be escaped");

                    if (charToTest == 'u')
                    {
                        // after a \u, we expect 4 hexa digits ( http://www.json.org/ )
                        if (i + 1 + 4 > content.Length)
                            throw new JsonException(InvalidString + content);

                        if (!Regex.IsMatch(content.Substring(i + 1, 4), EscapedUFourDigits))
                            throw new JsonException(InvalidString + content);
                    }

                    nexCharIsEscaped = false;
                }
                else
                {
                    if (charToTest == '\\')
                        nexCharIsEscaped = true;
                    if (charToTest == '"' && i < content.Length - 1)
                        throw new JsonException(InvalidString + content);
                }

             }
            return true;
        }

        #region IJValue Members

        public string ToString(int indentLevel)
        {
            return this.ToString();
        }

        public new string ToString()
        {
            return JToken.StringDelimiter + _content + JToken.StringDelimiter;
        }

        #endregion
    }
}