>Thanks, Greg. I'll review the code this weekend.
>
Nadya,

Version 3 - for lunch

Mods
- added null checking for parameters
- fast mode detection
if none of the chars in the 'from' occur in the 'to' a straight Replace() can be done
otherwise it's the long loop

If you find any bugs - please let me know

// string
namespace GregoryAdam.Base.ExtensionMethods
{
	public static partial class ExtensionMethods
	{
#region ChrTran
		public static string ChrTran(this string s, string from, string to)
		{
			// check parameters
			if( String.IsNullOrEmpty(s) )
				return s;

			if( String.IsNullOrEmpty(from) )
				return s;

	
			// (1) Split from into translate and remove

			// "1234", "56"
			// translate = "12"
			// remove = "34"
			char[] translate, remove;

			// "1234", ""
			if (String.IsNullOrEmpty(to))
			{
				translate = new char[0];

			}
				// "1234", "5678"
				// "1234", "56"
				// "1234", "A"
				// "1234", "567890"  - can happen
			else
			{
				translate = new char[Math.Min(from.Length, to.Length)];
				from.CopyTo(0, translate, 0, translate.Length);
			}

			remove = new char[from.Length - translate.Length];
			from.CopyTo(translate.Length, remove, 0, remove.Length);

			// (2) remove chars from s - but not those that will be translated
			//	avoid	StrTran("AA", "B")
			//			translate = "A", to = "B"
			//			remove = "A"
			var sb = new StringBuilder(s, s.Length);
			int i, j;

			for (i = remove.Length; --i >= 0; )
			{
				if( !translate.Contains( remove[i] ) )
					sb.Replace(remove[i].ToString(), null);
			}

			// (3) translate

			// if no char in from is in to, then a fast translate can be done
			//  if not, we'll have to loop

			if (translate.Length > 0)
			{
				bool fastTranslate = true;

				for (i = translate.Length; --i >= 0; )
				{
					if (to.Contains(from[i]))
					{
						fastTranslate = false;
						break;
					}
				}

				if (fastTranslate)
				{
					for (i = translate.Length; --i >= 0; )
					{
						sb.Replace(translate[i], to[i]);
					}
				}
				else
				{
					for (i = sb.Length; --i >= 0; )
					{
						if ((j = Array.IndexOf<char>(translate, sb[i])) >= 0)
							sb[i] = to[j];
					}
				}
			}
			return sb.ToString();
		}
#if ChrTran_TESTSUITE
		static string[,] chrtranTest = 
			{	{ "AB", "AB", "BC", "BC" } ,
				{ "AB", "AB", "B",	"B" },
				{ "0123456", "45", null, "01236"},
				{ "0123456", "456", "78", "012378"},
				{ "01234567890", "456", "", "01237890"},
				{ "02468", "02468", "1357", "1357"},
				{ "02468", "02468", "1358", "1358" }
			};

		public static void chrtran_Test()
		{
			string s;

			for (int i = -1; ++i <= chrtranTest.GetUpperBound(0); )
			{
				s = chrtranTest[i, 0].ChrTran(chrtranTest[i, 1], chrtranTest[i, 2]);

				Console.WriteLine("{0},{1},{2} >> {3} {4} : {5}",
								chrtranTest[i, 0],
								chrtranTest[i, 1],
								chrtranTest[i, 2],
								chrtranTest[i, 3],
								s,
								s == chrtranTest[i, 3]
							);
			}
		}
#endif
#endregion

	}
}