Regex Tester

Features

• Live match highlighting as you type
• Numbered and named capture group inspector
• All standard flags with one-click toggles
• Replacement preview with $1 backreferences
• Library of twelve common patterns

How to use it

1. Type your pattern in the top field.
2. Paste sample text into the box below.
3. Matches highlight immediately; groups appear in the table.
4. Add a replacement string to preview a substitution.

Flags, greediness and catastrophic backtracking

The flags change behaviour more than newcomers expect. Without g, only the first match is returned. The m flag makes ^ and $ match at line boundaries rather than only at the start and end of the whole string. The s flag lets . match newlines, which it otherwise never does, the single most common reason a pattern fails on multi-line input.

Quantifiers are greedy by default, meaning <.+> against <a><b> matches the entire string rather than just <a>. Adding ? makes them lazy, so <.+?> stops at the first closing bracket. Knowing which you want is usually the difference between a pattern that works and one that almost works.

The genuine hazard is catastrophic backtracking. Nested quantifiers such as (a+)+b can take exponential time on input that nearly matches, a few dozen characters can hang the engine for minutes. This has taken down production services; Cloudflare's 2019 global outage was caused by exactly this. If a pattern will ever see untrusted input, avoid nesting quantifiers, and prefer atomic constructs or a hard input length limit. This tool caps execution to keep the tab responsive if you hit one.