C# String.Replace with a start/index (Added my (slow) implementation)

Question

I'd like an efficient method that would work something like this

EDIT: Sorry I didn't put what I'd tried before. I updated the example now.

// Method signature, Only replaces first instance or how many are specified in max
public int MyReplace(ref string source,string org, string replace, int start, int max)
{
     int ret = 0;
     int len = replace.Length;
     int olen = org.Length;
     for(int i = 0; i < max; i++)
     {
          // Find the next instance of the search string
          int x = source.IndexOf(org, ret + olen);
          if(x > ret)
             ret = x;
          else
             break;

         // Insert the replacement
         source = source.Insert(x, replace);
         // And remove the original
         source = source.Remove(x + len, olen); // removes original string
     }
     return ret;
}

string source = "The cat can fly but only if he is the cat in the hat";
int i = MyReplace(ref source,"cat", "giraffe", 8, 1); 

// Results in the string "The cat can fly but only if he is the giraffe in the hat"
// i contains the index of the first letter of "giraffe" in the new string

The only reason I'm asking is because my implementation I'd imagine getting slow with 1,000s of replaces.

Answer 1

To start, try something like this:

int count = 0;
Regex.Replace(source, Regex.Escape(literal), (match) =>
{
    return (count++ > something) ? "new value" : match.Value;
});

Answer 2

How about:

public static int MyReplace(ref string source,
    string org, string replace, int start, int max)
{
    if (start < 0) throw new System.ArgumentOutOfRangeException("start");
    if (max <= 0) return 0;
    start = source.IndexOf(org, start);
    if (start < 0) return 0;
    StringBuilder sb = new StringBuilder(source, 0, start, source.Length);
    int found = 0;
    while (max-- > 0) {
        int index = source.IndexOf(org, start);
        if (index < 0) break;
        sb.Append(source, start, index - start).Append(replace);
        start = index + org.Length;
        found++;
    }
    sb.Append(source, start, source.Length - start);
    source = sb.ToString();
    return found;
}

it uses StringBuilder to avoid lots of intermediate strings; I haven't tested it rigorously, but it seems to work. It also tries to avoid an extra string when there are no matches.

Answer 3

To replace only the first match:

    private string ReplaceFirst(string source, string oldString, string newString)
    {
        var index = source.IndexOf(oldString);
        var begin = source.Substring(0, index);
        var end = source.Substring(index + oldString.Length);
        return begin + newString + end;
    }