Introduction

Regular expressions (regex) are one of the most powerful tools in a developer's arsenal. Whether you're validating email addresses, extracting data from text, or performing complex search-and-replace operations, regex provides a concise and efficient solution.

In this comprehensive guide, you'll learn regex from the ground up—starting with basic patterns and progressing to advanced techniques. By the end, you'll be able to write complex regex patterns with confidence.

Practice as you learn: Use our Regex Tester & Debugger to test every example in this guide. It provides live pattern matching, syntax highlighting, and exports to multiple languages!

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What Are Regular Expressions?

Regular expressions are patterns used to match character combinations in strings. They're supported in virtually every programming language and text editor.

Common Use Cases

Validation:

• Email addresses
• Phone numbers
• Credit cards
• Passwords
• URLs and domains

Extraction:

• Log file parsing
• Data scraping
• URL parameters
• Code comments
• Numbers from text

Transformation:

• Search and replace
• Text formatting
• String sanitization
• Code refactoring

Test patterns: Regex Tester

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Basic Regex Patterns

Literal Characters

The simplest regex matches exact characters:

Try it: Regex Tester - Test: hello against "hello world"

Character Classes

Match one character from a set:

Common Shorthand Character Classes

Pattern	Equivalent	Matches
\\d	[0-9]	Any digit
\\D	[^0-9]	Any non-digit
\\w	[a-zA-Z0-9_]	Word character
\\W	[^a-zA-Z0-9_]	Non-word character
\\s	[ \\t\\n\\r]	Whitespace
\\S	[^ \\t\\n\\r]	Non-whitespace
.		Any character (except newline)

Test these patterns: Regex Tester

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Quantifiers

Quantifiers specify how many times a pattern should match.

Basic Quantifiers

Quantifier	Meaning	Example
*	0 or more	a* matches "", "a", "aa", "aaa"
+	1 or more	a+ matches "a", "aa", "aaa"
?	0 or 1 (optional)	colou?r matches "color" or "colour"
{n}	Exactly n	a{3} matches "aaa"
{n,}	n or more	a{2,} matches "aa", "aaa", "aaaa"
{n,m}	Between n and m	a{2,4} matches "aa", "aaa", "aaaa"

Greedy vs Lazy Quantifiers

Practice: Regex Tester - Try greedy vs lazy matching

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Anchors and Boundaries

Anchors match positions, not characters.

Position Anchors

Anchor	Matches
^	Start of string/line
$	End of string/line
\\b	Word boundary
\\B	Not a word boundary

Test boundaries: Regex Tester

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Groups and Capturing

Capturing Groups

Non-Capturing Groups

Backreferences

Test groups: Regex Tester

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Lookahead and Lookbehind

Positive Lookahead (?=...)

Negative Lookahead (?!...)

Positive Lookbehind (?<=...)

Negative Lookbehind (?<!...)

Practice lookarounds: Regex Tester

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Common Patterns

Email Validation

URL Matching

Phone Number Validation

Password Validation

Test all patterns: Regex Tester

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Flags and Modifiers

Common Flags

Flag	Name	Description
g	Global	Find all matches (not just first)
i	Case-insensitive	Ignore case
m	Multiline	^ and $ match line boundaries
s	Dotall	. matches newlines
u	Unicode	Full Unicode support
y	Sticky	Match from lastIndex position

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Advanced Techniques

Replace with Functions

Regex with HTML/CSS

Format code: JavaScript Formatter | HTML Formatter

Log File Parsing

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Performance Tips

Best Practices

Common Pitfalls

Debug patterns: Regex Tester - Visualize matches and catch issues

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Regex in Different Languages

JavaScript

Python

PHP

Export patterns: Regex Tester supports JavaScript, Python, PHP, Java, and Go!

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Tools and Resources

Development Tools

Our Free Tools:

• Regex Tester & Debugger - Live pattern matching with syntax highlighting
• JavaScript Formatter - Format regex code beautifully
• JSON Formatter - Format JSON data for regex processing

Regex Cheat Sheet

Quick Reference

Character Classes:

• . - Any character
• \\d - Digit [0-9]
• \\w - Word [A-Za-z0-9_]
• \\s - Whitespace

Quantifiers:

• * - 0 or more
• + - 1 or more
• ? - 0 or 1
• {n} - Exactly n
• {n,m} - n to m times

Anchors:

• ^ - Start
• $ - End
• \\b - Word boundary

Groups:

• (...) - Capturing group
• (?:...) - Non-capturing
• (?<name>...) - Named group

Lookaround:

• (?=...) - Positive lookahead
• (?!...) - Negative lookahead
• (?<=...) - Positive lookbehind
• (?<!...) - Negative lookbehind

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Conclusion

Regular expressions are an essential skill for any developer. While they can seem cryptic at first, mastering regex will make you significantly more productive when working with text processing, validation, and data extraction.

Key Takeaways:

• Start with simple patterns and build complexity gradually
• Use character classes and quantifiers for flexible matching
• Leverage capturing groups for data extraction
• Test thoroughly with various inputs
• Optimize for performance with specific patterns

Practice regex: Use our Regex Tester to test patterns in real-time with live matching, explanations, and multi-language code export!

What patterns will you build? Whether you're validating forms, parsing logs, or processing data, you now have the knowledge to write powerful regex patterns!

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Related Tools & Resources

Master text processing with these free tools:

• Regex Tester & Debugger - Test patterns with live matching and syntax highlighting
• JavaScript Formatter - Format JavaScript code with regex
• HTML Formatter - Validate and format HTML markup
• JSON Formatter - Format and validate JSON data

All tools are 100% free, require no signup, and respect your privacy.

Further Reading

• JavaScript Tips & Tricks
• Modern JavaScript ES6 Features

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Frequently Asked Questions

What's the difference between .* and .+?

.* matches zero or more characters (including empty string), while .+ matches one or more (requires at least one character). Use .*? and .+? for lazy matching.

How do I escape special characters in regex?

Use backslash \\ before special characters: . * + ? ^ $ ( ) [ ] { } | \\. For example: \\. matches a literal period. Test escaping with our Regex Tester.

What's the difference between ^ and \b?

^ matches the start of a string (or line with m flag), while \\b matches word boundaries (between \\w and \\W). ^cat matches only at string start; \\bcat\\b matches "cat" as a whole word anywhere.

Can regex parse HTML/XML reliably?

No! HTML/XML are context-free grammars that regex cannot fully parse. Use proper parsers (DOMParser, cheerio, BeautifulSoup). Use regex only for simple extraction tasks. Format HTML with our HTML Formatter.

How do I debug slow regex patterns?

Use our Regex Tester to visualize matches. Watch for catastrophic backtracking (nested quantifiers), use non-capturing groups, add anchors, and make patterns more specific.

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Happy pattern matching! 🔍