String.Replace ignoring case

You could use a Regex and perform a case insensitive replace:

class Program
{
    static void Main()
    {
        string input = "hello WoRlD";
        string result = 
           Regex.Replace(input, "world", "csharp", RegexOptions.IgnoreCase);
        Console.WriteLine(result); // prints "hello csharp"
    }
}

---

var search = "world";
var replacement = "csharp";
string result = Regex.Replace(
    stringToLookInto,
    Regex.Escape(search), 
    replacement.Replace("$","$$"), 
    RegexOptions.IgnoreCase
);

The Regex.Escape is useful if you rely on user input which can contains Regex language elements

Update

Thanks to comments, you actually don't have to escape the replacement string.

Here is a small fiddle that tests the code:

using System;
using System.Text.RegularExpressions;           
public class Program
{
    public static void Main()
    {

        var tests = new[] {
            new { Input="abcdef", Search="abc", Replacement="xyz", Expected="xyzdef" },
            new { Input="ABCdef", Search="abc", Replacement="xyz", Expected="xyzdef" },
            new { Input="A*BCdef", Search="a*bc", Replacement="xyz", Expected="xyzdef" },
            new { Input="abcdef", Search="abc", Replacement="x*yz", Expected="x*yzdef" },       
            new { Input="abcdef", Search="abc", Replacement="$", Expected="$def" },
        };


        foreach(var test in tests){
            var result = ReplaceCaseInsensitive(test.Input, test.Search, test.Replacement);

            Console.WriteLine(
                "Success: {0}, Actual: {1}, {2}",
                result == test.Expected,
                result,
                test
            );

        }


    }

    private static string ReplaceCaseInsensitive(string input, string search, string replacement){
        string result = Regex.Replace(
            input,
            Regex.Escape(search), 
            replacement.Replace("$","$$"), 
            RegexOptions.IgnoreCase
        );
        return result;
    }
}

Its output is:

Success: True, Actual: xyzdef, { Input = abcdef, Search = abc, Replacement = xyz, Expected = xyzdef } 
Success: True, Actual: xyzdef, { Input = ABCdef, Search = abc, Replacement = xyz, Expected = xyzdef }
Success: True, Actual: xyzdef, { Input = A*BCdef, Search = a*bc, Replacement = xyz, Expected = xyzdef } 
Success: True, Actual: x*yzdef, { Input = abcdef, Search = abc, Replacement = x*yz, Expected = x*yzdef} 
Success: True, Actual: $def, { Input = abcdef, Search = abc, Replacement = $, Expected = $def }

---

2.5X FASTER and MOST EFFECTIVE method than other's regular expressions methods:

/// <summary>
/// Returns a new string in which all occurrences of a specified string in the current instance are replaced with another 
/// specified string according the type of search to use for the specified string.
/// </summary>
/// <param name="str">The string performing the replace method.</param>
/// <param name="oldValue">The string to be replaced.</param>
/// <param name="newValue">The string replace all occurrences of <paramref name="oldValue"/>. 
/// If value is equal to <c>null</c>, than all occurrences of <paramref name="oldValue"/> will be removed from the <paramref name="str"/>.</param>
/// <param name="comparisonType">One of the enumeration values that specifies the rules for the search.</param>
/// <returns>A string that is equivalent to the current string except that all instances of <paramref name="oldValue"/> are replaced with <paramref name="newValue"/>. 
/// If <paramref name="oldValue"/> is not found in the current instance, the method returns the current instance unchanged.</returns>
[DebuggerStepThrough]
public static string Replace(this string str,
    string oldValue, string @newValue,
    StringComparison comparisonType)
{

    // Check inputs.
    if (str == null)
    {
        // Same as original .NET C# string.Replace behavior.
        throw new ArgumentNullException(nameof(str));
    }
    if (str.Length == 0)
    {
        // Same as original .NET C# string.Replace behavior.
        return str;
    }
    if (oldValue == null)
    {
        // Same as original .NET C# string.Replace behavior.
        throw new ArgumentNullException(nameof(oldValue));
    }
    if (oldValue.Length == 0)
    {
        // Same as original .NET C# string.Replace behavior.
        throw new ArgumentException("String cannot be of zero length.");
    }


    //if (oldValue.Equals(newValue, comparisonType))
    //{
    //This condition has no sense
    //It will prevent method from replacesing: "Example", "ExAmPlE", "EXAMPLE" to "example"
    //return str;
    //}



    // Prepare string builder for storing the processed string.
    // Note: StringBuilder has a better performance than String by 30-40%.
    StringBuilder resultStringBuilder = new StringBuilder(str.Length);



    // Analyze the replacement: replace or remove.
    bool isReplacementNullOrEmpty = string.IsNullOrEmpty(@newValue);



    // Replace all values.
    const int valueNotFound = -1;
    int foundAt;
    int startSearchFromIndex = 0;
    while ((foundAt = str.IndexOf(oldValue, startSearchFromIndex, comparisonType)) != valueNotFound)
    {

        // Append all characters until the found replacement.
        int @charsUntilReplacment = foundAt - startSearchFromIndex;
        bool isNothingToAppend = @charsUntilReplacment == 0;
        if (!isNothingToAppend)
        {
            resultStringBuilder.Append(str, startSearchFromIndex, @charsUntilReplacment);
        }



        // Process the replacement.
        if (!isReplacementNullOrEmpty)
        {
            resultStringBuilder.Append(@newValue);
        }


        // Prepare start index for the next search.
        // This needed to prevent infinite loop, otherwise method always start search 
        // from the start of the string. For example: if an oldValue == "EXAMPLE", newValue == "example"
        // and comparisonType == "any ignore case" will conquer to replacing:
        // "EXAMPLE" to "example" to "example" to "example" … infinite loop.
        startSearchFromIndex = foundAt + oldValue.Length;
        if (startSearchFromIndex == str.Length)
        {
            // It is end of the input string: no more space for the next search.
            // The input string ends with a value that has already been replaced. 
            // Therefore, the string builder with the result is complete and no further action is required.
            return resultStringBuilder.ToString();
        }
    }


    // Append the last part to the result.
    int @charsUntilStringEnd = str.Length - startSearchFromIndex;
    resultStringBuilder.Append(str, startSearchFromIndex, @charsUntilStringEnd);


    return resultStringBuilder.ToString();

}

Note: ignore case == StringComparison.OrdinalIgnoreCase as parameter for StringComparison comparisonType. It is the fastest, case-insensitive way to replace all values.

Advantages of this method:

High CPU and MEMORY efficiency;

It is the fastest solution, 2.5 times faster than other's methods with regular expressions (proof in the end);

Suitable for removing parts from the input string (set newValue to null), optimized for this;

Same as original .NET C# string.Replace behavior, same exceptions;

Well commented, easy to understand;

Simpler – no regular expressions. Regular expressions are always slower because of their versatility (even compiled);

This method is well tested and there are no hidden flaws like infinite loop in other's solutions, even highly rated:

@AsValeO: Not works with Regex language elements, so it's not universal method @Mike Stillion: There is a problem with this code. If the text in new is a superset of the text in old, this can produce an endless loop.

Benchmark-proof: this solution is 2.59X times faster than regex from @Steve B., code:

// Results:
// 1/2. Regular expression solution: 4486 milliseconds
// 2/2. Current solution: 1727 milliseconds — 2.59X times FASTER! than regex!

// Notes: the test was started 5 times, the result is an average; release build.

const int benchmarkIterations = 1000000;
const string sourceString = "aaaaddsdsdsdsdsd";
const string oldValue = "D";
const string newValue = "Fod";
long totalLenght = 0;

Stopwatch regexStopwatch = Stopwatch.StartNew();
string tempString1;
for (int i = 0; i < benchmarkIterations; i++)
{
    tempString1 = sourceString;
    tempString1 = ReplaceCaseInsensitive(tempString1, oldValue, newValue);

    totalLenght = totalLenght + tempString1.Length;
}
regexStopwatch.Stop();



Stopwatch currentSolutionStopwatch = Stopwatch.StartNew();
string tempString2;
for (int i = 0; i < benchmarkIterations; i++)
{
    tempString2 = sourceString;
    tempString2 = tempString2.Replace(oldValue, newValue,
        StringComparison.OrdinalIgnoreCase);

    totalLenght = totalLenght + tempString2.Length;
}
currentSolutionStopwatch.Stop();

Original idea – @Darky711; thanks @MinerR for StringBuilder.

---

Lots of suggestions using Regex. How about this extension method without it:

public static string Replace(this string str, string old, string @new, StringComparison comparison)
{
    @new = @new ?? "";
    if (string.IsNullOrEmpty(str) || string.IsNullOrEmpty(old) || old.Equals(@new, comparison))
        return str;
    int foundAt = 0;
    while ((foundAt = str.IndexOf(old, foundAt, comparison)) != -1)
    {
        str = str.Remove(foundAt, old.Length).Insert(foundAt, @new);
        foundAt += @new.Length;
    }
    return str;
}

---

Extensions make our lives easier:

static public class StringExtensions
{
    static public string ReplaceInsensitive(this string str, string from, string to)
    {
        str = Regex.Replace(str, from, to, RegexOptions.IgnoreCase);
        return str;
    }
}

---

You can use the Microsoft.VisualBasic namespace to find this helper function:

Replace(sourceString, "replacethis", "withthis", , , CompareMethod.Text)

---

.Net Core has this method built-in: Replace(String, String, StringComparison) Doc. Now we can simply write: "...".Replace("oldValue", "newValue", StringComparison.OrdinalIgnoreCase)

---

Modified @Darky711's answer to use the passed in comparison type and match the framework replace naming and xml comments as closely as possible.

/// <summary>
/// Returns a new string in which all occurrences of a specified string in the current instance are replaced with another specified string.
/// </summary>
/// <param name="str">The string performing the replace method.</param>
/// <param name="oldValue">The string to be replaced.</param>
/// <param name="newValue">The string replace all occurrances of oldValue.</param>
/// <param name="comparisonType">Type of the comparison.</param>
/// <returns></returns>
public static string Replace(this string str, string oldValue, string @newValue, StringComparison comparisonType)
{
    @newValue = @newValue ?? string.Empty;
    if (string.IsNullOrEmpty(str) || string.IsNullOrEmpty(oldValue) || oldValue.Equals(@newValue, comparisonType))
    {
        return str;
    }
    int foundAt;
    while ((foundAt = str.IndexOf(oldValue, 0, comparisonType)) != -1)
    {
        str = str.Remove(foundAt, oldValue.Length).Insert(foundAt, @newValue);
    }
    return str;
}

---

(Edited: wasn't aware of the `naked link' problem, sorry about that)

Taken from here:

string myString = "find Me and replace ME";
string strReplace = "me";
myString = Regex.Replace(myString, "me", strReplace, RegexOptions.IgnoreCase);

Seems you are not the first to complain of the lack of case insensitive string.Replace.

---

I have wrote extension method:

public static string ReplaceIgnoreCase(this string source, string oldVale, string newVale)
    {
        if (source.IsNullOrEmpty() || oldVale.IsNullOrEmpty())
            return source;

        var stringBuilder = new StringBuilder();
        string result = source;

        int index = result.IndexOf(oldVale, StringComparison.InvariantCultureIgnoreCase);

        while (index >= 0)
        {
            if (index > 0)
                stringBuilder.Append(result.Substring(0, index));

            if (newVale.IsNullOrEmpty().IsNot())
                stringBuilder.Append(newVale);

            stringBuilder.Append(result.Substring(index + oldVale.Length));

            result = stringBuilder.ToString();

            index = result.IndexOf(oldVale, StringComparison.InvariantCultureIgnoreCase);
        }

        return result;
    }

I use two additional extension methods for previous extension method:

    public static bool IsNullOrEmpty(this string value)
    {
        return string.IsNullOrEmpty(value);
    }

    public static bool IsNot(this bool val)
    {
        return val == false;
    }

---

Doesn't this work: I cant imaging anything else being much quicker or easier.

public static class ExtensionMethodsString
{
    public static string Replace(this String thisString, string oldValue, string newValue, StringComparison stringComparison)
    {
        string working = thisString;
        int index = working.IndexOf(oldValue, stringComparison);
        while (index != -1)
        {
            working = working.Remove(index, oldValue.Length);
            working = working.Insert(index, newValue);
            index = index + newValue.Length;
            index = working.IndexOf(oldValue, index, stringComparison);
        }
        return working;
    }
}

---

Extending Petrucio's answer with Regex.Escape on the search string, and escaping matched group as suggested in Steve B's answer (and some minor changes to my taste):

public static class StringExtensions
{
    public static string ReplaceIgnoreCase(this string str, string from, string to)
    {
        return Regex.Replace(str, Regex.Escape(from), to.Replace("$", "$$"), RegexOptions.IgnoreCase);
    }
}

Which will produce the following expected results:

Console.WriteLine("(heLLo) wOrld".ReplaceIgnoreCase("(hello) world", "Hi $1 Universe")); // Hi $1 Universe
Console.WriteLine("heLLo wOrld".ReplaceIgnoreCase("(hello) world", "Hi $1 Universe"));   // heLLo wOrld

However without performing the escapes you would get the following, which is not an expected behaviour from a String.Replace that is just case-insensitive:

Console.WriteLine("(heLLo) wOrld".ReplaceIgnoreCase_NoEscaping("(hello) world", "Hi $1 Universe")); // (heLLo) wOrld
Console.WriteLine("heLLo wOrld".ReplaceIgnoreCase_NoEscaping("(hello) world", "Hi $1 Universe"));   // Hi heLLo Universe

---

Using @Georgy Batalov solution I had a problem when using the following example

string original = "blah,DC=bleh,DC=blih,DC=bloh,DC=com"; string replaced = original.ReplaceIgnoreCase(",DC=", ".")

Below is how I rewrote his extension

public static string ReplaceIgnoreCase(this string source, string oldVale, 
string newVale)
    {
        if (source.IsNullOrEmpty() || oldVale.IsNullOrEmpty())
            return source;

        var stringBuilder = new StringBuilder();
        string result = source;

        int index = result.IndexOf(oldVale, StringComparison.InvariantCultureIgnoreCase);
        bool initialRun = true;

        while (index >= 0)
        {
            string substr = result.Substring(0, index);
            substr = substr + newVale;
            result = result.Remove(0, index);
            result = result.Remove(0, oldVale.Length);

            stringBuilder.Append(substr);

            index = result.IndexOf(oldVale, StringComparison.InvariantCultureIgnoreCase);
        }

        if (result.Length > 0)
        {
            stringBuilder.Append(result);
        }

        return stringBuilder.ToString();
    }

---

My this Method could Ignore Case as well as Select Only Whole Word

public static string Replace(this string s, string word, string by, StringComparison stringComparison, bool WholeWord)
{
    s = s + " ";
    int wordSt;
    StringBuilder sb = new StringBuilder();
    while (s.IndexOf(word, stringComparison) > -1)
    {
        wordSt = s.IndexOf(word, stringComparison);
        if (!WholeWord || ((wordSt == 0 || !Char.IsLetterOrDigit(char.Parse(s.Substring(wordSt - 1, 1)))) && !Char.IsLetterOrDigit(char.Parse(s.Substring(wordSt + word.Length, 1)))))
        {
            sb.Append(s.Substring(0, wordSt) + by);
        }
        else
        {
            sb.Append(s.Substring(0, wordSt + word.Length));
        }
        s = s.Substring(wordSt + word.Length);
    }
    sb.Append(s);
    return sb.ToString().Substring(0, sb.Length - 1);
}

---

Below function is to remove all match word like (this) from the string set. By Ravikant Sonare.

private static void myfun()
{
    string mystring = "thiTHISThiss This THIS THis tThishiThiss. Box";
    var regex = new Regex("this", RegexOptions.IgnoreCase);
    mystring = regex.Replace(mystring, "");
    string[] str = mystring.Split(' ');
    for (int i = 0; i < str.Length; i++)
    {
        if (regex.IsMatch(str[i].ToString()))
        {
            mystring = mystring.Replace(str[i].ToString(), string.Empty);

        }
    }
    Console.WriteLine(mystring);
}

---

You can also try the Regex class.

var regex = new Regex( "camel", RegexOptions.IgnoreCase ); var newSentence = regex.Replace( sentence, "horse" );

---

Use this, Tested and 100% Worked!

For VB.NET

Dim myString As String
Dim oldValue As String
Dim newValue As String

myString = Form1.TextBox1.Text
oldValue = TextBox1.Text
newValue = TextBox2.Text

Dim working As String = myString
Dim index As Integer = working.IndexOf(oldValue, StringComparison.CurrentCultureIgnoreCase)

While index <> -1
    working = working.Remove(index, oldValue.Length)
    working = working.Insert(index, newValue)
    index = index + newValue.Length
    index = working.IndexOf(oldValue, index, StringComparison.CurrentCultureIgnoreCase)
    Form1.TextBox1.Text = working
End While

For C#

private void Button2_Click(System.Object sender, System.EventArgs e)
{
    string myString;
    string oldValue;
    string newValue;

    myString = Form1.TextBox1.Text;
    oldValue = TextBox1.Text;
    newValue = TextBox2.Text;

    string working = myString;
    int index = working.IndexOf(oldValue, StringComparison.CurrentCultureIgnoreCase);

    while (index != -1)
    {
        working = working.Remove(index, oldValue.Length);
        working = working.Insert(index, newValue);
        index = index + newValue.Length;
        index = working.IndexOf(oldValue, index, StringComparison.CurrentCultureIgnoreCase);
        Form1.TextBox1.Text = working;
    }
}

---

Another way is to ignore the case sensitivity in String.Replace() using the option StringComparison.CurrentCultureIgnoreCase

string.Replace("World", "csharp", StringComparison.CurrentCultureIgnoreCase)

---

I prefer this - "Hello World".ToLower().Replace( "world", "csharp" );

---