How do I replace the *first instance* of a string in .NET?

Question

string ReplaceFirst(string text, string search, string replace)
{
  int pos = text.IndexOf(search);
  if (pos < 0)
  {
    return text;
  }
  return text.Substring(0, pos) + replace + text.Substring(pos + search.Length);
}

Example:

string str = "The brown brown fox jumps over the lazy dog";

str = ReplaceFirst(str, "brown", "quick");

EDIT: As @itsmatt mentioned, there's also Regex.Replace(String, String, Int32), which can do the same, but is probably more expensive at runtime, since it's utilizing a full featured parser where my method does one find and three string concatenations.

EDIT2: If this is a common task, you might want to make the method an extension method:

public static class StringExtension
{
  public static string ReplaceFirst(this string text, string search, string replace)
  {
     // ...same as above...
  }
}

Using the above example it's now possible to write:

str = str.ReplaceFirst("brown", "quick");

As itsmatt said Regex.Replace is a good choice for this however to make his answer more complete I will fill it in with a code sample:

using System.Text.RegularExpressions;
...
Regex regex = new Regex("foo");
string result = regex.Replace("foo1 foo2 foo3 foo4", "bar", 1);             
// result = "bar1 foo2 foo3 foo4"

The third parameter, set to 1 in this case, is the number of occurrences of the regex pattern that you want to replace in the input string from the beginning of the string.

I was hoping this could be done with a static Regex.Replace overload but unfortunately it appears you need a Regex instance to accomplish it.

Take a look at Regex.Replace.

Taking the "first only" into account, perhaps:

int index = input.IndexOf("AA");
if (index >= 0) output = input.Substring(0, index) + "XQ" +
     input.Substring(index + 2);

?

Or more generally:

public static string ReplaceFirstInstance(this string source,
    string find, string replace)
{
    int index = source.IndexOf(find);
    return index < 0 ? source : source.Substring(0, index) + replace +
         source.Substring(index + find.Length);
}

Then:

string output = input.ReplaceFirstInstance("AA", "XQ");

using System.Text.RegularExpressions;

RegEx MyRegEx = new RegEx("F");
string result = MyRegex.Replace(InputString, "R", 1);

will find first F in InputString and replace it with R.

In C# syntax:

int loc = original.IndexOf(oldValue);
if( loc < 0 ) {
    return original;
}
return original.Remove(loc, oldValue.Length).Insert(loc, newValue);

C# extension method that will do this:

public static class StringExt
{
    public static string ReplaceFirstOccurrence(this string s, string oldValue, string newValue)
    {
         int i = s.IndexOf(oldValue);
         return s.Remove(i, oldValue.Length).Insert(i, newValue);    
    } 
}