Lookbehinds on debuggex (PCRE, Javascript and Python) cannot be of variable width, meaning that you can use (?<=<a>)
which has a fixed width (3 characters) but not something that can vary in length (?<=<a.*>)
(can have 3 characters, or 4, or 5, etc).
The regex simply is not valid but debuggex tells you that there is no match.