C# Regex Examples and Basics

In this article, I will explain, what is Regular expression in C# and how we can use take benefit of using Regular Expressions (Regex) in C# by providing various C# Regex examples using Console application.

What is Regular Expression (Regex) in C#?

So, before we proceed to check various Regex examples, let's understand what is Regular expressions and how we can use them in C#.

Regular Expression is a pattern using which we can parse and confirm if a given text contains sequence of characters required by us (by using pattern) or not. To accomplish this, C# Regex Class offer methods and properties which we can use to parse the text. So, C# Regex or also known as C# Regular expression is sequence of characters that defines a pattern. A pattern may consist of literals, numbers, characters, operators, or constructs.

Namespace to use Regex or Regular expressions in C# is "System.Text.RegularExpressions".

Regex Class

Regex class is used to represent the regular expression in C#, so for example, if you want to check for zero of more white spaces, you can have regex as below

   // Create a pattern for zero or more whitespace sequence
   // * = watch 0 or more precedence
   // \s = to find any whitespace
    string pattern = @"\s*";  
    // Create a Regex  
    Regex rg = new Regex(pattern);  

Regex Class Methods

Regex class also contains few methods, which can be useful

• IsMatch: Indicates whether the regular expression finds a match in the input string.
• Matches: Searches an input string for all occurrences of a regular expression and returns all the matches.
• Replace: replaces strings that match a regular expression pattern with a specified replacement string.
• Split:Splits an input string into an array of substrings at the positions defined by a regular expression match.

Understanding Regex pattern using Cheatsheet

Let's understand basics for regex characters using the below table

Regex Examples in C#

Let's try to make some examples using above details about Regular expressions.

So open your Visual Studio and Navigate to "File"-> "New" -> "Project" -> Select "Visual C#" from left-pane and "Console Application" from right-pane -> name your Project and click "Ok"

Now, let's create Regex examples one by one

Simple Regex Example in C#

Let's start with basic Regex, to find if given string is found within string or not.

using System;
using System.Text.RegularExpressions;

namespace RegexExamples
{
    public class Program
    {
        public static void Main(string[] args)
        {
			 string patternText = "Captain America";
            // Define a regular expression          
            Regex reg = new Regex(patternText);
 
            //IsMatch(string input)
            Console.WriteLine(reg.IsMatch("Captain America")); //'true'
			Console.WriteLine(reg.IsMatch("America")); // 'false' at it checks complete string
 
            //IsMatch(string input, int index)
            Console.WriteLine(reg.IsMatch("Captain America", 0));
 
        }
    }
}

Output:

True
False
True

In the above example, instead of defining any Regex patterns, we have used simple 'string' as a pattern, and then used "IsMatch" to check if string is found within strings.

Let's move on to move complex example, which will have Regex patterns.

Find duplicate words from a string using C# Regex

using System;
using System.Text.RegularExpressions;

namespace RegexExamples
{
    class Program
    {
        static void Main(string[] args)
        {
            // Define a regular expression for repeated words.
            Regex rx = new Regex(@"\b(?<word>\w+)\s+(\k<word>)\b");

            // Define a test string.
            string text = "the the quick brown fox  fox jumps over the lazy dog dog.";

            // Find matches.
            MatchCollection matches = rx.Matches(text);

            // Report the number of matches found.
            Console.WriteLine("{0} matches found in:  {1}",
                              matches.Count,
                              text);

            // Report on each match.
            foreach (Match match in matches)
            {
                GroupCollection groups = match.Groups;
                Console.WriteLine("'{0}' repeated at positions {1} and {2}",
                                  groups["word"].Value,
                                  groups[0].Index,
                                  groups[1].Index);
            }
            Console.ReadKey();
        }
    }
}

Output:

3 matches found in:   the the quick brown fox  fox jumps over the lazy dog dog.
'the' repeated at positions 0 and 4
'fox' repeated at positions 20 and 25
'dog' repeated at positions 49 and 53

Validate email address using Regex in C#

We can also validate email address using regex pattern in C#

using System;
using System.Text.RegularExpressions;

namespace RegexExamples
{
    class Program
    {
        static void Main(string[] args)
        {
            var EmailId1 = "test@gmail.com";
            var EmailId2 = "test1212@com";

            string pattern = @"\A(?:[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*@(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\.)+[a-z0-9](?:[a-z0-9-]*[a-z0-9])?)\Z";

            ValidateEmail(EmailId1,pattern);
            ValidateEmail(EmailId2, pattern);

            Console.ReadKey();
        }

        public static void ValidateEmail(string Email, string pattern)
        {
            //check second string
            if (Regex.IsMatch(Email, pattern))
            {
                //print if email id is valid
                Console.WriteLine(Email + " is Valid Email address ");
            }
            else
            {
                //print if email id is not valid
                Console.WriteLine(Email + " is Not a Valid Email address");
            }
        }
    }
}

Output:

test@gmail.com is Valid Email address
test1212@com is Not a Valid Email address

Replace Special Characters from String using Regex in C#

Considering the code in the answer for a question, remove special characters from string using Regex in C#

using System;
using System.Text.RegularExpressions;

namespace RegexExamples
{
    class Program
    {
        static void Main(string[] args)
        {
            var StringToBeRepalced = "test@ Suite #218 ";

            var StringWithoutSpclCharac = Regex.Replace(StringToBeRepalced, @"[^0-9a-zA-Z\._]", " ");

            Console.WriteLine(StringWithoutSpclCharac);

            Console.ReadKey();
        }

       
    }
}

Output:

test  Suite  218

You may also like to read:

C# Collections

C# List

Convert List to Dictionary in C# (Various ways)

Remove All WhiteSpace from string in C# (Multiple Methods)

Convert String to List in C# and string array to list

Convert List to string in C# (Comma-separated, New-Line)