C# program to check if a string contains any special character

To check if a string contains any special character in C#, you can use the Char.IsLetterOrDigit method. This method returns false for any character that is not a letter or digit, which means it's a special character.

Special characters include symbols like @, #, $, %, &, punctuation marks, and whitespace characters that are not alphanumeric.

Syntax

Following is the syntax for checking if a character is a letter or digit −

bool Char.IsLetterOrDigit(char c)

To check for special characters, use the negation operator −

if (!Char.IsLetterOrDigit(character)) {
   // character is a special character
}

Using Char.IsLetterOrDigit to Find Special Characters

The following example demonstrates how to identify and extract all special characters from a string −

using System;

namespace Demo {
   class myApplication {
      static void Main(string[] args) {
         string str = "Amit$#%";
         char[] one = str.ToCharArray();
         char[] two = new char[one.Length];
         int c = 0;
         
         for (int i = 0; i < one.Length; i++) {
            if (!Char.IsLetterOrDigit(one[i])) {
               two[c] = one[i];
               c++;
            }
         }
         
         Array.Resize(ref two, c);
         Console.WriteLine("Following are the special characters:");
         foreach(var items in two) {
            Console.WriteLine(items);
         }
      }
   }
}

The output of the above code is −

Following are the special characters:
$
#
%

Using LINQ to Check for Special Characters

A more concise approach using LINQ to simply check if any special characters exist −

using System;
using System.Linq;

class Program {
   static void Main() {
      string str1 = "Hello123";
      string str2 = "Hello@123";
      
      bool hasSpecialChars1 = str1.Any(c => !Char.IsLetterOrDigit(c));
      bool hasSpecialChars2 = str2.Any(c => !Char.IsLetterOrDigit(c));
      
      Console.WriteLine($"'{str1}' has special characters: {hasSpecialChars1}");
      Console.WriteLine($"'{str2}' has special characters: {hasSpecialChars2}");
   }
}

The output of the above code is −

'Hello123' has special characters: False
'Hello@123' has special characters: True

Using Regular Expressions

You can also use regular expressions to check for special characters −

using System;
using System.Text.RegularExpressions;

class Program {
   static void Main() {
      string str = "Password123!";
      
      // Pattern matches any character that is NOT alphanumeric
      bool hasSpecialChars = Regex.IsMatch(str, @"[^a-zA-Z0-9]");
      
      Console.WriteLine($"'{str}' contains special characters: {hasSpecialChars}");
      
      // Find all special characters
      MatchCollection matches = Regex.Matches(str, @"[^a-zA-Z0-9]");
      Console.WriteLine("Special characters found:");
      foreach (Match match in matches) {
         Console.WriteLine(match.Value);
      }
   }
}

The output of the above code is −

'Password123!' contains special characters: True
Special characters found:
!

Comparison of Different Approaches

Conclusion

The Char.IsLetterOrDigit method is the most efficient way to check for special characters in C#. Use LINQ's Any() method for a more concise approach when you only need to know if special characters exist, and regular expressions when you need complex pattern matching capabilities.