Is Regex Tester Lite free?
Yes. Regex Tester Lite is free to use with no account, and it runs entirely in your browser.
Does it send my pattern or text to a server?
No. The regex and test text are processed locally in your browser, so nothing is uploaded and your data stays on your device.
Which regex flavor does it support?
Regex Tester Lite tests JavaScript regular expressions only. It does not support other engines like PCRE or Python in this version.
Does it support replace mode?
No. This version is match-focused: it inspects matches and capture groups but does not perform search-and-replace.
Does it show why my regex fails?
Yes. If a pattern is invalid, the tool reports a clear compile error so you can fix the syntax instead of guessing.
Which regex flavour is this?
JavaScript, running on the browser's own RegExp engine. That means lookbehind, named groups, unicode property escapes and the s and v flags all work in a current browser. It also means that a pattern using PCRE-only syntax, such as possessive quantifiers or recursion, will fail to compile.
Do I need the g flag?
Not to see your matches here. The tool enumerates all matches regardless, adding the global flag internally when your flag string does not include it. In your own code the flag matters a great deal, since a non-global regex with .exec returns only the first match, so put it in when you copy the pattern out.
Why does my pattern match an empty string everywhere?
Because it can match nothing, and something that can match nothing matches at every position. Quantifiers like * and ? allow zero repetitions, so a* matches an empty string at every index of a text with no letter a. Tighten the quantifier to + if you meant one or more.
What is the index number next to each match?
The offset of the match in the test text, counted in UTF-16 code units. For plain ASCII text that is the same as the character position. For text containing emoji or other characters outside the basic plane, each such character counts as two, so the index runs ahead of a naive character count.
Can I test a replacement?
No. This page is match-focused: it shows you what the pattern found, where it found it and what each capture group captured. Replacement introduces a second syntax with its own rules, and it is deliberately out of scope here.
Are my pattern and test text uploaded?
No. The regex is compiled and executed in the page with the browser's own engine. This is worth knowing when the text you are testing against is a real log line.