Test a regular expression against your text with live highlighting, groups, replace, a plain-English explanation and ready-made code – on the real JavaScript, PHP (PCRE2) or .NET engine.
Most online regex testers run everything in JavaScript and imitate other languages. This one doesn't: pick JavaScript and the pattern runs in your browser, pick PCRE2 and it runs through PHP's own preg_* functions, pick .NET and it runs on .NET 10's Regex class. Differences that bite in practice – group numbering, replacement syntax, possessive quantifiers – show up exactly as in your code.
Matches are highlighted as you type, each with its capture groups. Try a replacement, read the pattern explained token by token, and copy a snippet for JavaScript, PHP or C#. Runaway patterns such as (a+)+$ are stopped after two seconds instead of freezing the page, and the Common patterns menu has practical versions for e-mail, IPv4, dates, UUIDs and more.