What Is a Regex Tester?
An online regex tester helps you write and debug regular expressions against sample text. You see match highlighting, capture groups, flags, and optional substitution so patterns are easier to trust before you use them in JavaScript, Python, Java, C#, or PCRE-style engines.
This free tool runs in your browser. Matching uses the JavaScript RegExp engine, with flavor notes for Python, Java, .NET, and PCRE differences such as lookbehind rules and advanced constructs.
Flags, Groups, and Replace
Common flags include g (global), i (ignore case), m (multiline anchors), and s (dotall). Capturing groups and named groups feed replace templates like $1. Use the cheat-sheet style explanations below each run to decode anchors, quantifiers, character classes, and lookaheads.
ReDoS and Catastrophic Backtracking
Nested greedy quantifiers can cause catastrophic backtracking on crafted input (ReDoS). Prefer simpler patterns for untrusted data. Engines such as RE2 take a different approach; this tester warns on suspicious patterns and limits how many matches it will list.
Regex Tester FAQs
What is an online regex tester?
A regex tester lets you write a regular expression, paste sample text, and see matches live—including capture groups, flags, and optional replace/substitution—so you can debug patterns before shipping them.
Does this support Python, Java, C#, and PCRE?
You can pick JS, Python, Java, .NET, or PCRE as the target flavor. Matching runs in the browser on the JavaScript RegExp engine, with notes about features those languages handle differently (lookbehind, possessive quantifiers, recursion, and more).
What do the g, i, m, and s flags mean?
g finds all matches, i ignores case, m makes ^ and $ work per line, and s (dotall) lets . match newlines. Sticky (y) and Unicode (u) are also available for JavaScript-style testing.
What are capture groups?
Parentheses create capture groups so you can read parts of a match (Group 1, Group 2, …) or named groups like (?<year>\d{4}). They also power $1-style replacements.
What is catastrophic backtracking / ReDoS?
Some nested quantifiers can explode in runtime on crafted input (regular expression denial of service). This tool warns on suspicious patterns and caps the number of matches. Prefer simpler patterns when testing untrusted text.
Is my text uploaded?
No. Pattern matching runs in your browser. Nothing is sent to LiveTechHacks.