I was trying to find the answer on the Net but I was not able to. Does the third section on this page applies to PHP / PCRE or not?
https://www.regular-expressions.info/recursebackref.html
the "Backreferences to other recursion levels part". I am only interested in PHP, so if this does not apply to the PHP language (or possibly JavaScript), this is all that I need to know.
Thanks.
Backreferences with recursion level are not supported by PCRE, the information pertains to Ruby Onigmo regex engine.
See Onigmo reference:
See a Rubular demo.
As for PCRE, the only syntax backreferences allow is