How can I efficiently trim selected characters from middle of a string?

Question

Can anyone think of an efficient way (time wise) to trim a few selected characters from the middle of a string?

Best I came up with was:

public static string Trim(this string word, IEnumerable<char> selectedChars)
{
    string result = word;
    foreach (char c in selectedChars)
        result = result.Replace(c.ToString(), "");
    return result;
}

But it is still too slow.

Answer 1

Two options spring to mind:

• Use a StringBuilder
• Use a regular expression

Here's the StringBuilder version:

public static string Trim(this string word, IEnumerable<char> selectedChars)
{
    // The best form for this will depend largely on the size of selectedChars
    // If you can change how you call the method, there are optimisations you
    // could do here
    HashSet<char> charSet = new HashSet<char>(selectedChars);

    // Give enough capacity for the whole word. Could be too much,
    // but definitely won't be too little
    StringBuilder builder = new StringBuilder(word.Length);

    foreach (char c in word)
    {
        if (!charSet.Contains(c))
        {
            builder.Append(c);
        }
    }
    return builder.ToString();
}

The regular expression option could be very efficient if you have a fixed set of chars you want to trim, and can build the regex once.

Something like:

// Put this statically somewhere
Regex unwantedChars = new Regex("[def]", RegexOptions.Compiled);

// Then do this every time you need to use it:
word = unwantedChars.Replace(word, "");

Answer 2

start by using StringBuilder not string for your replacements...
see http://blogs.msdn.com/charlie/archive/2006/10/11/Optimizing-C_2300_-String-Performance.aspx