Regular Expressions in C#

Regular expressions denote different patterns like literals, constructs, operators etc. that can be matched with the input.

Regex Class

The immutable regular expressions are represented using a Regex Class. Details about the Regex class is given as follows:

Constructors in Regex Class

The different constructors and their description is given as follows:

Table: Constructors in Regex Class in C#

Source: MSDN

Fields in Regex Classz

The different fields and their description is given as follows:

Table: Fields in Regex Class in C#

Properties in Regex Class

The different properties and their description is given as follows:

Table: Properties in Regex Class in C#

Source: MSDN

Methods in Regex Class

The different methods and their description is given as follows:

Table: Methods in Regex Class in C#

Source: MSDN

Program 1

A program that displays all the patterns of one or more digits is given as follows:

Source Code: Program that displays all the patterns of one or more digits using regular expressions in C#

using System;
using System.Text.RegularExpressions;
class Example
{
   static void Main()
   {
       Regex regex = new Regex(@"\d+");
       MatchCollection match = regex.Matches("The numbers 3 and 5 are prime");
       foreach (Match m in match)
       {
           Console.WriteLine(m);
        }
   }
}

The output of the above program is as follows:

3
5

Now let us understand the above program.

The Regex class constructor is used in the above program and it uses a pattern that specifies one or more digits. The code snippet for this is given as follows:

Regex regex = new Regex(@"\d+");

Then the matches method is used that specifies all the occurences of digits in the given string. Finally these occurrences are printed using the foreach loop. The code snippet for this is given as follows:

MatchCollection match = regex.Matches("The numbers 3 and 5 are prime");
foreach (Match m in match)
{
     Console.WriteLine(m);
}

Program 2

A program that substitutes all the white spaces with underscore is given as follows:

Source Code: Program that substitutes all the white spaces with underscore using regular expressions in C#

using System;
using System.Text.RegularExpressions;
class Example
{
   static void Main()
   {
       string original_string = "C# is fun";
       string replace = "_";
        Regex regex = new Regex(@"\s+");
        string modified_string = regex.Replace(original_string, replace);
        Console.WriteLine("Original String: {0}", original_string);
        Console.WriteLine("Modified String: {0}", modified_string);    
   }
}

The output of the above program is as follows:

Original String: C# is fun
Modified String: C#_is_fun

Now let us understand the above program.

The original string is “C# is fun” and all the white spaces in the string need to be replaced with underscore. Then the Regex class constructor is used and it uses a pattern that specifies one or more whitespace characters. The code snippet for this is given as follows:

string original_string = "C# is fun";
       string replace = "_";
        Regex regex = new Regex(@"\s+");

Then the modified string is created where all the whitespace characters in the original string are replaced by underscore using the Replace() method. Finally, the original string and modified string are printed. The code snippet for this is given as follows:

string modified_string = regex.Replace(original_string, replace);
        Console.WriteLine("Original String: {0}", original_string);
        Console.WriteLine("Modified String: {0}", modified_string);