Regex Tester

Welcome to our Regex Tester, a powerful and free online tool for testing, debugging, and learning regular expressions. Whether you are a developer validating input patterns, a data analyst extracting structured data, or a student learning regex syntax, this tool provides instant visual feedback with match highlighting, capture group colors, and plain-English explanations of your patterns.

What Is a Regular Expression?

A regular expression (often shortened to regex or regexp) is a compact pattern language for describing search patterns within text. Originally developed in the 1950s by mathematician Stephen Kleene, regex has become an essential tool in computing — used in virtually every programming language, text editor, and command-line utility. A single regex pattern can replace dozens of lines of procedural string-processing code.

Key Features

How to Use This Tool

1. Enter your regex pattern: Type or paste your regular expression in the Pattern field. Select flags (g, i, m, s, u) as needed.
2. Provide a test string: Enter or paste the text you want to test against in the Test String area.
3. View live results: Matches are highlighted instantly with color-coded capture groups. Review match details, group values, and character indices in the Match Details panel.
4. Read the explanation: Check the Pattern Explanation panel to understand what each part of your regex does in plain English.
5. Try replace mode: Toggle to Replace mode, enter a replacement string with backreferences, and preview the result.

Understanding Regex Flags

Global (g)

By default, a regex stops after the first match. The g flag finds all matches in the string instead of stopping at the first one. This is the most commonly used flag.

Case-Insensitive (i)

Makes the pattern match regardless of letter case. For example, /hello/i matches "Hello", "HELLO", and "hElLo".

Multiline (m)

Changes the behavior of ^ and $ anchors. Without this flag, they match the start/end of the entire string. With m, they also match the start/end of each line.

DotAll (s)

By default, the dot . matches any character except newlines. The s flag makes . match newline characters as well.

Unicode (u)

Enables full Unicode matching, including support for Unicode property escapes like \p{Letter}. Important for patterns that work with non-ASCII text.

Common Regex Patterns

Email Validation

The pattern [a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,} matches standard email addresses by checking for a local part, @ symbol, domain, and TLD of at least 2 characters.

URL Matching

Use https?://[^\s/$.?#].[^\s]* to match HTTP and HTTPS URLs. The s? makes the "s" optional, matching both http and https protocols.

Date Formats

The pattern (\d{4})[-/](\d{1,2})[-/](\d{1,2}) matches dates like 2026-03-07 or 2026/3/7, capturing year, month, and day in separate groups.

IP Address

Match IPv4 addresses with \b\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}\b. The \b word boundaries prevent matching numbers embedded in longer strings.

Regex Cheat Sheet

Character Classes

. matches any character (except newline). \d matches a digit (0-9). \w matches a word character (letter, digit, underscore). \s matches whitespace. Use uppercase versions (\D, \W, \S) for the inverse.

Quantifiers

* means zero or more, + means one or more, ? means zero or one. Use {n} for exactly n, {n,m} for between n and m, and {n,} for n or more repetitions.

Anchors and Boundaries

^ matches the start of a string (or line with m flag). $ matches the end. \b matches a word boundary — the position between a word character and a non-word character.

Groups and Alternation

Parentheses () create capture groups. Use (?:) for non-capturing groups. The pipe | means "or" — cat|dog matches either "cat" or "dog". Named groups use (?<name>...) syntax.

Tips and Tricks

Start Simple

Build your regex incrementally. Start with a simple pattern that matches part of what you need, then add complexity step by step. Test after each change.

Use the Global Flag

When debugging, always enable the g flag to see all matches. This helps you catch unexpected matches or misses across the entire test string.

Escape Special Characters

Characters like ., *, +, ?, (, ), [, ], {, }, \, ^, $, and | have special meanings. To match them literally, prefix with a backslash: \. matches an actual period.

Prefer Specific Patterns

Use \d instead of [0-9] for digits, and \w instead of [a-zA-Z0-9_] for word characters. They are equivalent but more readable and less error-prone.

Frequently Asked Questions

What is a regular expression (regex)?

A regular expression (regex) is a sequence of characters that defines a search pattern. It is used for pattern matching within strings, enabling tasks like validation, searching, and text manipulation. For example, the pattern \d{3}-\d{4} matches phone number formats like 555-1234.

What regex flags are available?

This tool supports five regex flags: 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; s (dotAll) makes the dot match newline characters; and u (unicode) enables full Unicode matching.

How do capture groups work in regex?

Capture groups are defined by parentheses () in a regex pattern. They capture the matched text so you can extract or reference specific parts of a match. For example, in the pattern (\d{4})-(\d{2})-(\d{2}), group 1 captures the year, group 2 the month, and group 3 the day from a date string.

Is my data processed on the server?

No. All regex processing happens entirely in your browser using JavaScript. No data is sent to any server. Your patterns and test strings remain completely private and are never stored or transmitted.

Can I use this tool for regex replacement?

Yes. Switch to Replace mode using the toggle, enter your replacement string (supporting backreferences like $1, $2 for capture groups), and see a live preview of the replaced text alongside the original.

Additional Resources

• MDN: Regular Expressions Guide
• Regular-Expressions.info — Comprehensive Regex Tutorial
• Wikipedia: Regular Expression