Regular Expression in C#

Introduction

This article describes Regular Expressions. In this article you will see the syntax of Regular Expressions with examples and how to create our own Regular Expression in a C# Console Application with examples of Mobile Number regular expressions.

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.

Regex 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#.