Remove non-alphanumeric characters from a string in C#

This post will discuss how to remove non-alphanumeric characters from a string in C#.

If we’re to remove all non-alphanumeric characters from a string, we have to create a new string instance since strings are immutable in C#. This post provides an overview of several methods to accomplish this:

1. Using Regular Expression

The idea is to check for non-alphanumeric characters in a string and replace them with an empty string. We can use the regular expression [^a-zA-Z0-9] to identify non-alphanumeric characters in a string. Replace the regular expression [^a-zA-Z0-9] with [^a-zA-Z0-9 _] to allow spaces and underscore character.

using System;

using System.Text.RegularExpressions;

public class Example

{

public static void Main()

{

string str = "25-Dec@2019!!";

str = Regex.Replace(str, "[^a-zA-Z0-9]", String.Empty);

Console.WriteLine(str);

}

Output: 25Dec2019

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If the regular expression is frequently called, you might want to pre-compile the regular expression for a faster search on the cost of increased startup time.

using System;

using System.Text.RegularExpressions;

public class Example

{

private static readonly Regex regex = new Regex("[^a-zA-Z0-9]");

public static void Main()

{

string str = "25-Dec@2019!!";

str = regex.Replace(str, String.Empty);

Console.WriteLine(str);

}

Output: 25Dec2019

Download Run Code

In ASCII, word characters are [a-zA-Z0-9_]. The regular expression \w and \W check for the word and non-word characters, respectively. Therefore, we can use regular expression [^\w]* or [\W]* to identify non-alphanumeric characters in a string.

using System;

using System.Text.RegularExpressions;

public class Example

{

public static void Main()

{

string str = "25-Dec@2019!!";

str = Regex.Replace(str, @"[^\w]*", String.Empty);

Console.WriteLine(str);

}

Output: 25Dec2019

Download Run Code

2. Using `Array.FindAll()` method

The Array.FindAll() method returns all elements of the specified sequence which satisfies a certain condition. To filter only alphanumeric characters, pass Char.IsLetterOrDigit to the FindAll() method, as demonstrated below:

using System;

public class Example

{

public static void Main()

{

string str = "25-Dec@2019!!";

str = String.Concat(Array.FindAll(str.ToCharArray(), Char.IsLetterOrDigit));

Console.WriteLine(str);

}

Output: 25Dec2019

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Note that the IsLetterOrDigit() method does not strictly check for ASCII characters in range A-Z, a-z, and 0-9. We can write our custom logic to check characters in the desired range, as shown below:

using System;

public class Example

{

public static bool isAlphaNumeric(char c)

{

return (c >= 'A' && c <= 'Z') || (c >= 'a' && c <= 'z') ||

(c >= '0' && c <= '9');

}

public static void Main()

{

string str = "25-Dec@2019!!";

str = String.Concat(Array.FindAll(str.ToCharArray(), isAlphaNumeric));

Console.WriteLine(str);

}

Output: 25Dec2019

Download Run Code

3. Using LINQ

Another similar solution uses LINQ’s Where() method to filter elements of a sequence based on a condition. We can use this as follows:

using System;

using System.Linq;

public class Example

{

public static void Main()

{

string str = "25-Dec@2019!!";

str = String.Concat(str.Where(char.IsLetterOrDigit));

Console.WriteLine(str);

}

Output: 25Dec2019

Download Run Code

That’s all about removing all non-alphanumeric characters from a string in C#.

Remove non-alphanumeric characters from a string in C#

1. Using Regular Expression

2. Using Array.FindAll() method

3. Using LINQ

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2. Using `Array.FindAll()` method