How do I replace the *first instance* of a string

Question

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

How can I accomplish this in .NET?

Answer 1

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");

Answer 2

In C# syntax:

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

Answer 3

Regex.Replace, especially RegEx.Replace(string, string, int), is probably what you're looking for. That or String.IndexOf which will give you the index and then you can cut and rebuild the string with the new text you want.

An example demonstrating the latter (as first demonstrated by @David Humpohl):

string str = "Hello WorldWorld";

str = ReplaceFirst(str, "World", "StackOverflow ");

...

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

Answer 4

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.

Answer 5

For anyone that doesn't mind a reference to Microsoft.VisualBasic, there is the Replace Method:

string result = Microsoft.VisualBasic.Strings.Replace("111", "1", "0", 2, 1); // "101"

Answer 6

Take a look at Regex.Replace.