What is a Regular Expression in C#?

A regular expression, often abbreviated as regex, is a sequence of characters that forms a search pattern. It is a powerful tool used for pattern matching and manipulating strings of text. Regular expressions are widely used in various programming languages, text editors, and command-line tools. With regular expressions, you can define patterns that describe specific sets of strings. These patterns can be used to search, validate, or transform text data.

A regular expression in C# is represented by the C# Regex class. This article teaches using the Regex class to work with regular expressions in C#. If you are looking for some C# Regex code examples, here is are the top C# Regex Examples (2023).

C# Regex class syntax

The following basic syntax is used for regular expressions, 

Quantifiers

The most important quantifiers are *?+.

1. * - Matches the preceding character zero or more times.

   Example

2. + - Matches the preceding character 1 or more times.

   Example

3. ? - Matches the preceding char zero or one time.

   Example

Special characters

Many special characters are available for regex building. Here are some of the more usual ones.

1. ^ - It is used to match the beginning of a string.

   Example

2. $ - It is used to match the end of a string.

   Example

3. (Dot) - Matches any character only once.

   Example

4. \d - It is used to match a digit character.

   Example

5. \D - It is used to match any non-digit character.

   Example

6. \w - It matches an alphanumeric character plus "_".

   Example

7. \W - It is used to match any non-word character.

   Example

8. \s - Matches white space characters.

   Example

9. \S - Matches a non-whitespace character.

   Example

10. \n - Matches a newline character.

Character classes

You can group characters by putting them between square brackets. This way, any character in the class will match one character in the input.

1. [ ] - It is used to match a range of characters.

   Example

Grouping and alternatives

It's often necessary to group things with parentheses ( and ).

1. () - It is used to group expressions.

   Example

   In the preceding image, the | operator is the Or operator that takes any of the alternatives.

2. {} - It matches the preceding character a specified number of times.

   i) {n} - Matches the previous element exactly n times.

   Example

   ii) {n,m} - Matches the previous element at least n times, but no more than m times.

   Example

   Example- Regex for Mobile Number Validation

The following procedure will explain how to create our regular expression for Mobile Number validation in a C# console application.

Step-by-step creation of a Regular Expression

Step 1

Open Visual Studio 2013.

Step 2

Then click on "File" > "New" > "Project..." ( or press "Ctrl +Shift + N").

Step 3

Then select Console Application, provide the application's name, "RegularExpression1," and click OK.

Step 4

After adding the project "RegularExpression1", you will see the following code in the "Program.cs" file, and for the creation of the regex, add the namespace.

using System.Text.RegularExpressions;

Step 5

Now write the following code in the program.cs file to create the regex.

using System;    
using System.Collections.Generic;    
using System.Linq;    
using System.Text;    
using System.Text.RegularExpressions;    
    
namespace RegularExpression1    
{    
    class Program    
    {    
        static void Main(string[] args)    
        {       
            Regex r = new Regex(@"^\+?\d{0,2}\-?\d{4,5}\-?\d{5,6}");    
            //class Regex Repesents an immutable regular expression.    
    
            string[] str = { "[phone]", "9678967101", "[phone]", "[phone]", "[phone]"};    
            //Input strings for Match valid mobile number.    
    
            foreach(string s in str)    
            {    
                Console.WriteLine("{0} {1} a valid mobile number.", s,    
                    r.IsMatch(s) ? "is":"is not");    
            //The IsMatch method is used to validate a string or     
           //to ensure that a string conforms to a particular pattern.    
            }    
        }    
    }    
} 

Step 6

After writing the code, build the program, then run it and get the following output,

Explanation of Regular Expression Pattern

It is one way to create a Mobile Number validation RegularExpression.

Conclusion

It is all about how to create the RegularExpression in C#. Learn more here C# Regex Examples (2023).