What is Regex?

Regular Expressions, or regex, are patterns used to match and manipulate text. They are a powerful tool for text processing tasks such as validation, searching, extracting, and replacing data.

Regex syntax can appear intimidating at first, but once you understand the basics, it becomes a versatile tool for various text-processing tasks.

Why Learn Regex?

• Validation: Check if a string matches a specific format (e.g., email, phone number).
• Searching: Find specific text patterns in large datasets.
• Text Manipulation: Extract, replace, or split text based on patterns.

Regex Basics

1. Literals

Literals match the exact text you specify. For example:

• cat matches cat in the text.
• hello matches hello.

2. Metacharacters

Metacharacters are special symbols with specific meanings:

Character	Description	Example
.	Matches any single character	c.t matches cat, cut
^	Matches the start of a string	^Hello matches Hello world only at the beginning
$	Matches the end of a string	world$ matches Hello world
*	Matches 0 or more repetitions	ca*t matches ct, cat, caaat
+	Matches 1 or more repetitions	ca+t matches cat, caaat but not ct
?	Matches 0 or 1 repetitions	ca?t matches cat or ct
\	Escapes a special character	\. matches a literal period .

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Regex Character Classes

Character classes let you define a set of characters to match.

Class	Description	Example
[abc]	Matches a, b, or c	[aeiou] matches vowels
[^abc]	Matches anything but a, b, or c	[^aeiou] matches consonants
[a-z]	Matches any lowercase letter	[0-9] matches any digit
\d	Matches any digit (0-9)	\d+ matches 123
\D	Matches any non-digit	\D+ matches abc
\w	Matches any word character (letters, digits, underscores)	\w+ matches hello123
\W	Matches any non-word character	\W+ matches !@#
\s	Matches any whitespace	\s+ matches spaces, tabs
\S	Matches any non-whitespace	\S+ matches hello

Quantifiers

Quantifiers define how many times a character or group should be matched.

Quantifier	Description	Example
{n}	Matches exactly n times	\d{3} matches 123
{n,}	Matches n or more times	\d{3,} matches 12345
{n,m}	Matches between n and m times	\d{2,4} matches 12, 123, 1234

Grouping and Capturing

Parentheses () group patterns and capture matched text.

• (ab)+ matches one or more repetitions of ab.
• Captured groups can be referred to using backreferences (\1, \2).

Example:
Regex: (cat|dog)
Input: I have a cat and a dog.
Matches: cat, dog

Anchors

Anchors define the position in the text:

Anchor	Description	Example
^	Start of a string	^Hello matches Hello
$	End of a string	world$ matches world
\b	Word boundary	\bcat\b matches cat but not cats
\B	Non-word boundary	\Bcat matches scatter

Common Regex Patterns

Here are some common regex examples:

Task	Regex	Explanation
Match a valid email	[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}	Email validation
Match a phone number	\+?\d{1,3}[-.\s]?\(?\d{1,4}\)?[-.\s]?\d{1,4}[-.\s]?\d{1,9}	Phone numbers in various formats
Match a URL	https?://[^\s/$.?#].[^\s]*	URL matching
Match digits only	^\d+$	Entire string is digits
Match a date (DD/MM/YYYY)	\b\d{2}/\d{2}/\d{4}\b	Matches dates in this format

Using Regex in Code

Here’s an example of using regex in C#:

using System;
using System.Text.RegularExpressions;

class Program
{
    static void Main()
    {
        string input = "My email is example@domain.com";
        string pattern = @"[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}";

        Match match = Regex.Match(input, pattern);
        if (match.Success)
        {
            Console.WriteLine($"Found email: {match.Value}");
        }
    }
}

Tools for Practicing Regex

1. Regex101 – Interactive regex testing tool with explanations.
2. RegExr – Test regex with examples and a cheat sheet.
3. Regex Crossword – Fun regex puzzles to practice.

Tips for Mastering Regex

• Start Simple: Begin with basic patterns and gradually use advanced features.
• Use Tools: Visual tools like Regex101 can help you debug and understand patterns.
• Practice Often: Experiment with different patterns to get comfortable.
• Break Down Patterns: Analyze complex regex step-by-step.

Regex is a skill that improves with practice. Once mastered, it becomes an indispensable tool in your programming toolkit.