Regex Tester
Test a regular expression against sample text with live match highlighting and capture groups.
Regular expressions are how you find a needle in a log file, and in security work you write them constantly — for grepping evidence, for detection rules, and occasionally for input validation, where they are also a classic source of bugs.
How to use it
- Type a pattern into Pattern.
- Paste sample text into Test text.
- Watch matches highlight live as you refine the expression.
What to take away
Build patterns incrementally
Start with something that matches too much, then tighten. Writing a long expression in one go and debugging it afterwards is how you lose an afternoon.
Anchors matter for validation
A pattern without ^ and $ matches anywhere inside the string. A validator checking for a safe value without anchoring can be defeated by putting the safe value next to a dangerous one.
Regex is the wrong tool for HTML and for blocklists
Filtering XSS or SQL injection by pattern-matching bad strings fails, because the space of encodings is effectively infinite. Parameterised queries and proper encoding fix those; regex does not.
Frequently asked questions
What is catastrophic backtracking?
Some patterns with nested quantifiers take exponential time on crafted input, so a short string can hang a server. That is a denial-of-service bug known as ReDoS, and it is a real finding in real assessments.
What are the most useful pieces to learn first?
\d for digits, \w for word characters, . for anything, + and * for repetition, ^ and $ for anchors, and parentheses for capture groups. That handful covers most log-hunting work.
Do all tools use the same regex syntax?
No. This tester uses JavaScript regex. grep, Python and PCRE differ in the details — lookbehind support and escaping especially — so test in the environment you will actually run in.
Where this fits in the course
This security tool pairs with Path 04 — Python — see all 26 learning paths.
It runs entirely in your browser: nothing is uploaded, no account is needed, and it is free.
Related
- Log Grep / Filter Playground — Filter a block of log lines with a regex — the exact "find the signal in the noise" skill IR work depends on.
- Simplified YARA Rule Tester — Test a small set of required strings against sample content and see whether your rule matches.
- Log Analysis / IR Lab — A messy auth.log dump from a compromised box. Reconstruct the timeline and find the breach — blue-team skills, not exploitation.
- Secure Code Review Toy — Paste a code snippet and get flagged patterns — eval, string-built SQL, hardcoded secrets. Not a real SAST tool, but trains the eye.