Regular Expressions

Regular expressions are extremely useful in extracting information from text such as code, log files, spreadsheets, or even documents. The first thing to recognize when using regular expressions is that everything is essentially a character, and we are writing patterns to match a specific sequence of characters (also known as a string).

Generally, the key part to process the text with regular expressions is regular expression engine and it is represented by Regex class in c#. The Regex class is available with System.Text.RegularExpressions namespace.

abc…	Letters
123…	Digits
\d	Any Digit
\D	Any Non-digit character
.	Any Character
\.	Period
[abc]	Only a, b, or c
[^abc]	Not a, b, nor c
[a-z]	Characters a to z
[0-9]	Numbers 0 to 9
\w	Any Alphanumeric character
\W	Any Non-alphanumeric character
{m}	m Repetitions
{m,n}	m to n Repetitions
*	Zero or more repetitions
+	One or more repetitions
?	Optional character
\s	Any Whitespace
\S	Any Non-whitespace character
^…$	Starts and ends
(…)	Capture Group
(a(bc))	Capture Sub-group
(.*)	Capture all
(abc|def)	Matches abc or def

Example

Following is the example of validating whether the given text is in proper email format or not using Regex class in C#.

using System;
using System.Text.RegularExpressions;

namespace TutlaneExamples
{
    class Program
    {
        static void Main(string[] args)
        {
            string email = "[e-mail]";
            var result = Regex.IsMatch(email, @"^[\w-\.]+@([\w-]+\.)+[\w-]{2,4}$");
            
            Console.Write("Is valid: {0} ", result);
            Console.ReadLine();
        }
    }
}

C# Regex Class Methods

Method	Description
IsMatch	It will determine whether the given input string matching with regular expression pattern or not.
Matches	It will return one or more occurrences of text that matches the regular expression pattern.
Replace	It will replace the text that matches the regular expression pattern.
Split	It will splits the string into an array of substrings at the positions that matches the regular expression pattern.