How to Use Regex.Replace in C# for Text Replacement

Use Regex.Replace in C# to find text matching a pattern and replace it with a new string. It takes the input text, a regular expression pattern, and a replacement string, returning the modified text.

Syntax

The Regex.Replace method has this basic syntax:

• input: The original string where you want to replace text.
• pattern: The regular expression pattern to find matches.
• replacement: The string to replace each match with.

It returns a new string with replacements applied.

csharp

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

Example

This example shows how to replace all digits in a string with the '#' character using Regex.Replace.

csharp

using System;
using System.Text.RegularExpressions;

class Program
{
    static void Main()
    {
        string input = "My phone number is 123-456-7890.";
        string pattern = "\\d"; // Matches any digit
        string replacement = "#";

        string result = Regex.Replace(input, pattern, replacement);
        Console.WriteLine(result);
    }
}

Output

My phone number is ###-###-####.

Common Pitfalls

Common mistakes when using Regex.Replace include:

• Forgetting to escape special characters in the pattern (e.g., use \\d for digits, not \d in strings).
• Using the wrong pattern that matches unintended text.
• Not importing System.Text.RegularExpressions namespace.
• Assuming Regex.Replace changes the original string (strings are immutable; it returns a new string).

csharp

using System;
using System.Text.RegularExpressions;

class Program
{
    static void Main()
    {
        string input = "Price: $100";
        // Wrong pattern: missing escape for $ (special char)
        string wrongPattern = "$\d+";
        string replacement = "USD";

        // This will throw an error or behave unexpectedly
        // string wrongResult = Regex.Replace(input, wrongPattern, replacement);

        // Correct pattern with escape
        string correctPattern = "\$\d+";
        string correctResult = Regex.Replace(input, correctPattern, replacement);
        Console.WriteLine(correctResult); // Output: Price: USD
    }
}

Output

Price: USD

Quick Reference

Tips for using Regex.Replace effectively:

• Always escape special regex characters in patterns.
• Use verbatim strings @"pattern" to reduce escaping.
• Remember it returns a new string; original is unchanged.
• You can use RegexOptions overloads for case-insensitive or multiline replacements.

Key Takeaways

Regex.Replace finds text matching a pattern and returns a new string with replacements.

Escape special characters in patterns to avoid errors or unexpected matches.

Use verbatim strings (@"pattern") to simplify regex patterns in C#.

Regex.Replace does not change the original string; it returns a modified copy.

You can customize behavior with RegexOptions for case or culture sensitivity.