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

you'd need to find the first occurrence, remove that substring and replace it with the new string.

Answer 2

Take a look at Regex.Replace.

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

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;
  }
  return text.Substring(0, pos) + replace + text.Substring(pos + search.Length);
}

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: As this is such a common task you might want to make the method an extension method for all strings:

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

... and use it like this:

string str = "Hello WorldWorld";

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

Answer 5

In C# syntax:

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

Answer 6

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);    
    } 
}

Enjoy

Answer 7

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 8

There are many ways of doing this operation as given in above answers. But string concetenations are not that efficient. Use string.format, it is more efficient than using +.

int i = 90;
string.Format("Count is {0}", i.ToString());

Output : Count is 90

Answer 9

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.

Answer 10

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.