From their FAQ: For JavaScript, regex101 uses the browser's regex engine, so you may see different results on different browsers for the same test case. FAQ l

From their FAQ: For JavaScript, regex101 uses the browser's regex engine, so you may see different results on different browsers for the same test case. FAQ link: https://github.com/firasdib/Regex101/wiki/FAQ#how-close-does-regex101-emulates-the-engines https://github.com/firasdib/Regex101/wiki/FAQ#how-close-does... This is the actual service worker for JS regexps: https://gist.github.com/helb/6c9edb53d73489f85390f82826c2b084#file-javascriptworker-js-L36-L67 https://gist.github.com/helb/6c9edb53d73489f85390f82826c2b08...