Regular Expression In C#

C# regular expression is used for pattern matching in C# applications. This article explains the syntax of C# regular expressions with examples and how to create our own C# regular expression in a C# console application with examples of mobile number regular expressions. Also learn how to use Regex in C#.

 

Regular Expression in C#

 

C# Regex class is used for creating a C# regular expression. Regular expressions are a pattern matching standard for string parsing and replacement and is a way for a computer user to express how a computer program should look for a specified pattern in text and then what the program is to do when each pattern match is found.

 

Sometimes it is abbreviated "regex". They are a powerful way to find and replace strings that take a defined format.

C# Regex class syntax

The following basic syntax are 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 is used to match 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-white space 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 together 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 is used to match the preceding character for 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

Now in the following procedure, I will explain how to create our own 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 name of the application "RegularExpression1" and click on OK.

Step 4

 

After adding the project "RegularExpression1" you will see the following code in the "Program.cs" file and for 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 program then run it and get the following output,

 

 

Explanation of Regular Expression Pattern

It is one way for creation of a Mobile Number validation RegularExpression.

 

It is all about how to create a RegularExpression in C#.

 

Learn more, here Regular Expressions and Top 7 Examples of C# Regex.