Length of substring matched by culture-sensitive String.IndexOf method

Question

I tried writing a culture-aware string replacement method:

public static string Replace(string text, string oldValue, string newValue)
{
    int index = text.IndexOf(oldValue, StringComparison.CurrentCulture);
    return index >= 0
        ? text.Substring(0, index) + newValue + text.Substring(index + oldValue.Length)
        : text;
}

However, it chokes on Unicode combining characters:

// \u0301 is Combining Acute Accent
Console.WriteLine(Replace("déf", "é", "o"));       // 1. CORRECT: dof
Console.WriteLine(Replace("déf", "e\u0301", "o")); // 2. INCORRECT: do
Console.WriteLine(Replace("de\u0301f", "é", "o")); // 3. INCORRECT: dóf

To fix my code, I need to know that in the second example, String.IndexOf matched only one character (é) even though it searched for two (e\u0301). Similarly, I need to know that in the third example, String.IndexOf matched two characters (e\u0301) even though it only searched for one (é).

How can I determine the actual length of the substring matched by String.IndexOf?

NOTE: Performing Unicode normalization on text and oldValue (as suggested by James Keesey) would accommodate combining characters, but ligatures would still be a problem:

Console.WriteLine(Replace("œf", "œ", "i"));  // 4. CORRECT: if
Console.WriteLine(Replace("œf", "oe", "i")); // 5. INCORRECT: i
Console.WriteLine(Replace("oef", "œ", "i")); // 6. INCORRECT: ief

Length of substring matched by culture-sensitive String.IndexOf method

3 Answers