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

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

Quick solution:

int n = 2;
string text = "ABCD";
string replacement = "xy";
string result = replacement + text[n..];

Console.WriteLine(result);  // xyCD

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

1. Practical examples

1.1 With + and range operator

In this example, we will take the last n 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)
    {
        int n = 2;
        string text = "ABCD";
        string replacement = "xy";
        string result = replacement + text[n..];

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

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)
    {
        int n = 2;
        string text = "ABCD";
        string replacement = "xy";
        string result = string.Concat(replacement, text[n..]);

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

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

In this example, we use Regex.Replace() with "^.{n}" regex to replace the first 2 characters in the text string.

Regex explanation:

• ^ - matches the beginning of the string,
• . - matches any character except linebreaks,
• {n} - 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)
    {
        int n = 2;
        string text = "ABCD";
        string replacement = "xy";
        string pattern = @"^.{" + n + "}";

        string result = Regex.Replace(text, pattern, replacement);

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