C# / .NET - replace first 3 characters in string (range operator .. )

In this article, we would like to show you how to replace the first 3 characters in a string in C# / .NET.

Note: range operator .. was introduced in C# 8.0.

Quick solution:

string text = "ABCD";
string replacement = "xyz";
string result = replacement + text[3..];

Console.WriteLine(result);  // xyzD

1. Practical examples

1.1 With + and range operator

In this example, we will take the last 3 characters from the original string and add them to the end of the replacement.

using System;

public class StringUtils
{
    public static void Main(string[] args)
    {
        string text = "ABCD";
        string replacement = "xyz";
        string result = replacement + text[3..];

        Console.WriteLine(result);  // xyzD
    }
}

1.2 With string.Concat() method and range operator

This approach is equivalent to the above one. Instead of + operator we concatenate the strings using string.Concat() method.

using System;

public class StringUtils
{
    public static void Main(string[] args)
    {
        string text = "ABCD";
        string replacement = "xyz";
        string result = string.Concat(replacement, text[3..]);

        Console.WriteLine(result);  // xyzD
    }
}

2. Practical example using string Regex.Replace() with regex pattern

In this example, we use Regex.Replace() with "^.{3}" regex to replace the first 3 characters in the text string.

Regex explanation:

• ^ - matches the beginning of the string,
• . - matches any character except linebreaks,
• {3} - matches the specified quantity of the previous token (in our case the .).

using System;
using System.Text.RegularExpressions;

public class StringUtils
{
    public static void Main(string[] args)
    {
        string text = "ABCD";
        string replacement = "xyz";
        string pattern = @"^.{3}";
        string result = Regex.Replace(text, pattern, replacement);

        Console.WriteLine(result);  // xyzD
    }
}

Note:

Regular expressions are slower than Substring() method.