Regular Expression In C#

Shaili Dashora
Updated date Mar 11, 2020

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A C# regular expression is used in pattern matching. 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.

Regular Expression

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.

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 classe
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").

create new project

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 = { "+91-9678967101", "9678967101", "+91-9678-967101", "+91-96789-67101", "+919678967101"};
//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.
}
}
}
}