# 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#.

```csharp
using System;
using System.Text.RegularExpressions;

namespace TutlaneExamples
{
    class Program
    {
        static void Main(string[] args)
        {
            string email = "support@tutlane.com";
            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. |


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