What is an alternative to string.Replace that is case-insensitive in C#?

The standard string.Replace() method in C# is case-sensitive by default. When you need case-insensitive string replacement, there are several alternative approaches you can use. The most common methods include using regular expressions, the StringComparison parameter (available in .NET 5+), or creating custom replacement methods.

Using Regular Expressions for Case-Insensitive Replace

Regular expressions provide the most flexible approach for case-insensitive string replacement using RegexOptions.IgnoreCase −

using System;
using System.Text.RegularExpressions;

class Program {
   static void Main(string[] args) {
      string str = "Cricket Team";
      Console.WriteLine("Original String: " + str);
      
      string result = CaseInsensitiveReplace(str, "TEAM", "Squad");
      Console.WriteLine("After Replace: " + result);
      
      result = CaseInsensitiveReplace(str, "cricket", "Football");
      Console.WriteLine("After Replace: " + result);
   }
   
   static string CaseInsensitiveReplace(string originalString, string oldValue, string newValue) {
      Regex regex = new Regex(Regex.Escape(oldValue), RegexOptions.IgnoreCase);
      return regex.Replace(originalString, newValue);
   }
}

The output of the above code is −

Original String: Cricket Team
After Replace: Cricket Squad
After Replace: Football Team

Using StringComparison (Available in .NET 5+)

Starting with .NET 5, the string.Replace() method supports a StringComparison parameter for case-insensitive operations −

using System;

class Program {
   static void Main(string[] args) {
      string str = "Hello World";
      Console.WriteLine("Original: " + str);
      
      // Case-insensitive replace using StringComparison
      string result = str.Replace("HELLO", "Hi", StringComparison.OrdinalIgnoreCase);
      Console.WriteLine("After Replace: " + result);
      
      result = str.Replace("world", "Universe", StringComparison.OrdinalIgnoreCase);
      Console.WriteLine("After Replace: " + result);
   }
}

The output of the above code is −

Original: Hello World
After Replace: Hi World
After Replace: Hello Universe

Custom Case-Insensitive Replace Method

For compatibility with older .NET versions, you can create a custom method using IndexOf() with StringComparison −

using System;

class Program {
   static void Main(string[] args) {
      string str = "The Cat in the Hat";
      Console.WriteLine("Original: " + str);
      
      string result = ReplaceIgnoreCase(str, "cat", "Dog");
      Console.WriteLine("After Replace: " + result);
      
      result = ReplaceIgnoreCase(str, "THE", "A");
      Console.WriteLine("After Replace: " + result);
   }
   
   static string ReplaceIgnoreCase(string source, string oldValue, string newValue) {
      int index = source.IndexOf(oldValue, StringComparison.OrdinalIgnoreCase);
      
      while (index >= 0) {
         source = source.Remove(index, oldValue.Length);
         source = source.Insert(index, newValue);
         index = source.IndexOf(oldValue, index + newValue.Length, StringComparison.OrdinalIgnoreCase);
      }
      
      return source;
   }
}

The output of the above code is −

Original: The Cat in the Hat
After Replace: A Dog in A Hat
After Replace: A Cat in A Hat

Comparison of Methods

Conclusion

For case-insensitive string replacement in C#, use the StringComparison.OrdinalIgnoreCase parameter with Replace() in .NET 5+, or regular expressions with RegexOptions.IgnoreCase for older versions. Choose the method based on your .NET version and performance requirements.