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

Question

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

How can I accomplish this in .NET?

Answer 1

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 2

And because there is also VB.NET to consider, I would like to offer up:

Private Function ReplaceFirst(ByVal text As String, ByVal search As String, ByVal replace As String) As String
    Dim pos As Integer = text.IndexOf(search)
    If pos >= 0 Then
        Return text.Substring(0, pos) + replace + text.Substring(pos + search.Length)
    End If
    Return text 
End Function

Answer 3

Take a look at Regex.Replace.

Answer 4

In C# syntax:

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

Answer 5

Assumes that AA only needs to be replaced if it is at the very start of the string:

var newString;
if(myString.StartsWith("AA"))
{
  newString ="XQ" + myString.Substring(2);
}

If you need to replace the first occurrence of AA, whether the string starts with it or not, go with the solution from Marc.

Answer 6

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.