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


I want to replace the first occurrence in a given string.

How can I accomplish this in .NET?

9/15/2018 9:21:30 PM

Exper Answer

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);


string output = input.ReplaceFirstInstance("AA", "XQ");
11/3/2010 12:00:11 PM

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);


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");

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