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...

This is the actual service worker for JS regexps: https://gist.github.com/helb/6c9edb53d73489f85390f82826c2b08...