What is a regular expression?

A regular expression (regex) is a sequence of characters that defines a search pattern. They're used in programming for string matching, validation, find/replace operations, and data extraction.

What are capture groups?

Capture groups are portions of a regex enclosed in parentheses () that capture the matched text for later reference. For example, /(\w+)@(\w+)/ captures the username and domain parts of an email address separately.

What does the global flag (g) do?

Without the g flag, a regex stops after the first match. With g, it finds all matches in the string. Always use g when you need to find or replace all occurrences.

What is a lookahead/lookbehind?

Lookahead (?=...) matches a position followed by the specified pattern without including it in the match. Lookbehind (?<=...) matches a position preceded by the pattern. They're powerful zero-width assertions.

Is regex syntax the same across all programming languages?

Core syntax is similar, but there are differences. JavaScript, Python, and PHP have slightly different flavours. This tester uses JavaScript regex syntax. Verify any patterns in your specific language before production use.

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Regex Tester

Test regular expressions with live highlighting, group matching, and a library of common regex patterns.

Pattern

Test String

What is Regex Tester?

The Regex Tester provides a live testing environment for regular expressions. Enter your pattern and test string to see all matches highlighted in real time, view capture groups, check flags (global, case-insensitive, multiline), and understand your pattern's behaviour before using it in code.

Regular expressions are powerful text matching and manipulation tools used in every programming language. They're notoriously difficult to write correctly without testing. This tool provides immediate visual feedback on what your pattern matches, making regex development and debugging significantly faster.

Why Use Regex Tester?

Regex patterns are deceptively complex — a pattern that looks correct often matches too broadly, too narrowly, or not at all. Real-time testing with match highlighting reveals exactly what your pattern matches before you use it in production code, catching errors before they cause bugs.

Key Features of Regex Tester

✓Live regex match highlighting as you type
✓Shows all match positions and lengths
✓Displays capture groups and their content
✓Supports all regex flags (g, i, m, s, u)
✓Regex reference cheat sheet

How to Use Regex Tester

1
Enter your regex pattern
Type your regular expression in the pattern field (without delimiters).
2
Select flags
Choose regex flags: g (global), i (case-insensitive), m (multiline), etc.
3
Enter test string
Paste the text you want to test your pattern against.
4
Review matches
See all matches highlighted in the test string and listed below.
5
Refine and copy
Adjust your pattern until it matches exactly what you need, then copy for use in your code.