Regex Tester - Test Regular Expressions Online

How to Use the Regex Tester

Enter your regular expression pattern in the top field, select your flags (g for global, i for case-insensitive, m for multiline), then type or paste a test string in the text area below. Matches are highlighted in real time as you type. A results table shows each match with its index position, matched text, and any capture group values.

Understanding Regular Expressions

Regular expressions (regex) are patterns that describe sets of strings. They are one of the most powerful tools in a developer’s toolkit, used for validation, search, text extraction, and data transformation across virtually every programming language.

Despite their power, regex patterns can be difficult to write correctly. A small mistake in the pattern can cause it to match too much, too little, or nothing at all. This tester gives you instant visual feedback so you can iterate on your pattern without running code or writing test scripts.

Common Regex Patterns

Here are frequently needed patterns that developers search for regularly:

Pattern	Matches	Example
^\S+@\S+\.\S+$	Email (basic)	user@example.com
^https?://	URL protocol	https://example.com
\b\d{3}-\d{3}-\d{4}\b	US phone	555-123-4567
^\d{4}-\d{2}-\d{2}$	ISO date	2026-04-06
^#?([0-9a-fA-F]{6})$	Hex color	#2563eb
\b\d{1,3}(\.\d{1,3}){3}\b	IPv4 address	192.168.1.1

Regex Flags Explained

Flags modify how the regex engine processes your pattern. Choosing the right flags is as important as writing the pattern itself.

The global (g) flag is needed when you want to find all occurrences in the text, not just the first match. The case-insensitive (i) flag is useful for matching text regardless of capitalization, such as finding “Error”, “error”, and “ERROR” with a single pattern. The multiline (m) flag changes the behavior of ^ and $ so they match line boundaries rather than the start and end of the entire string.

Debugging Regex Patterns

When your pattern does not match as expected, start by simplifying it. Remove parts of the pattern until it matches, then add complexity back gradually. Check for common mistakes like unescaped special characters (dots, brackets, parentheses), incorrect quantifiers, and missing anchors.

Greedy vs. lazy quantifiers are a frequent source of bugs. By default, * and + are greedy and match as much text as possible. Adding ? makes them lazy, matching as little as possible. For example, <.*> matches from the first < to the last > in the string, while <.*?> matches individual tags.

Using Capture Groups for Data Extraction

Capture groups let you extract specific portions of matched text. They are essential for parsing structured data like log files, CSV rows, and configuration values. Named capture groups using (?<name>pattern) make your regex more readable and your code easier to maintain.

This tool displays capture group values in the results table, so you can verify that each group captures exactly the text you expect before using the pattern in your application code.

For testing patterns against structured data, you may also find the Base64 Encoder useful when working with encoded strings, or the URL Encoder when building patterns for URL validation.

Related Tools

• Text Diff Checker - Compare text before and after regex replacements
• Base64 Encoder/Decoder - Encode and decode Base64 strings
• URL Encoder/Decoder - Encode special characters in URLs