What is a regular expression (regex)?

A regular expression (regex) is a sequence of characters that defines a search pattern. Regex is used for pattern matching in strings—finding, validating, extracting, or replacing text. It's supported in virtually every programming language and text editor.

How do I match an email address with regex?

A basic email regex is: ^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$. This matches standard email formats but note that fully validating emails per RFC 5322 is extremely complex. For production, use established email validation libraries.

What do the regex flags g, i, and m mean?

g (global) finds all matches instead of stopping at the first. i (case-insensitive) ignores letter case. m (multiline) makes ^ and $ match line starts/ends instead of string starts/ends. Combine them as needed: 'gi' for global case-insensitive matching.

Why isn't my regex matching anything?

Common issues: forgetting to escape special characters (. \ * + ? [ ] etc.), not using the global flag for multiple matches, incorrect character class syntax, or greedy vs. lazy quantifier confusion. Test incrementally and build complex patterns step by step.

What's the difference between .* and .*? in regex?

.* is 'greedy'—it matches as many characters as possible. .*? is 'lazy' (or 'reluctant')—it matches as few as possible. For example, matching HTML tags: matches from first , while matches individual tags.

Regex Tester

Test and debug regular expressions in real-time

Regular Expression

Test String

Matches (0)

Enter a pattern to test

About the Regex Tester

Regular expressions (regex) are powerful pattern-matching tools used throughout software development for text search, validation, extraction, and manipulation. The Regex Tester on UniversalDevTools.in is a professional-grade utility that allows developers to test, debug, and refine regular expressions in real-time with instant visual feedback. This tool is essential for anyone working with pattern matching, form validation, data extraction, log parsing, or text processing.

Regular expressions use a specialized syntax to define search patterns. For example, the pattern ^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$ validates email addresses, while \d{3}-\d{3}-\d{4} matches US phone numbers in the format 123-456-7890. Learning and debugging regex patterns can be challenging, but this tool provides instant feedback showing which parts of your test string match, where matches occur, and what groups are captured.

The tool supports all standard regex flags: global (g) for finding all matches, case-insensitive (i) for ignoring letter case, and multiline (m) for matching across line boundaries. These flags dramatically change regex behavior—for instance, without the global flag, a pattern only matches the first occurrence, while with it, all occurrences are found. Understanding flag behavior is crucial for writing correct regex patterns.

Real-time pattern matching updates as you type, making it easy to iterate and refine patterns. Match highlighting shows exactly which parts of your test string match the pattern, while index positions help you understand where matches occur in the string. This visual feedback is invaluable for debugging complex patterns, understanding regex behavior, and learning how different patterns work.

All regex testing happens entirely in your browser using JavaScript's native RegExp engine. Your patterns, test strings, and data never leave your device or get transmitted to external servers. This privacy-first approach ensures complete confidentiality for proprietary patterns, sensitive test data, and any regex work requiring secure processing.

features

Real-time pattern matching that updates as you type
Flag customization supporting global (g), case-insensitive (i), and multiline (m) flags
Match highlighting showing exactly which parts match your pattern
Index position display for understanding match locations
Support for all JavaScript regex features including lookaheads and groups
Multiple match display for patterns with global flag
Capture group extraction and display
100% client-side processing—your patterns never leave your browser

use cases

Email validation patterns for form inputs and APIs
Phone number formatting and validation across different countries
Data extraction scripts for parsing structured text
Form input validation for web applications
Log file parsing to extract specific events and errors
URL pattern matching for routing and validation
Password strength validation with complexity requirements
Text search and replace operations in code editors
CSV and data file parsing with pattern-based extraction
Natural language processing and text analysis

why use it

Instant match results with zero network latency
Complete privacy—no data sent to servers
Free unlimited usage with no registration required
Works with complex patterns including nested groups and lookaheads
Cross-platform compatibility on all devices and browsers
Educational resource for learning regex syntax and behavior
Saves time compared to writing test scripts for each pattern
Visual feedback helps debug and refine regex patterns quickly

How to use the Regex Tester

01Enter or paste your input
02Adjust the options if you need to
03Run the tool
04Copy the result

Questions

What is a regular expression (regex)?: A regular expression (regex) is a sequence of characters that defines a search pattern. Regex is used for pattern matching in strings—finding, validating, extracting, or replacing text. It's supported in virtually every programming language and text editor.
How do I match an email address with regex?: A basic email regex is: ^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$. This matches standard email formats but note that fully validating emails per RFC 5322 is extremely complex. For production, use established email validation libraries.
What do the regex flags g, i, and m mean?: g (global) finds all matches instead of stopping at the first. i (case-insensitive) ignores letter case. m (multiline) makes ^ and $ match line starts/ends instead of string starts/ends. Combine them as needed: 'gi' for global case-insensitive matching.
Why isn't my regex matching anything?: Common issues: forgetting to escape special characters (. \ * + ? [ ] etc.), not using the global flag for multiple matches, incorrect character class syntax, or greedy vs. lazy quantifier confusion. Test incrementally and build complex patterns step by step.
What's the difference between .* and .*? in regex?: .* is 'greedy'—it matches as many characters as possible. .*? is 'lazy' (or 'reluctant')—it matches as few as possible. For example, matching HTML tags: <.*> matches from first < to last >, while <.*?> matches individual tags.

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