I'll ask the "dumb question" then: What is a PCRE? How is it different than a "true" regex?

PCREs are Perl Compatible Regular Expressions, they have backreferences and other constructs. Almost all languages that have Regular Expressions have similar constructs. "Computer Science" Regular Expressions are more limited and implement a "regular language" as defined in https://en.wikipedia.org/wiki/Chomsky_hierarchy. This gives some guarantees, which go out the window with PCREs.

Did you edit the comment? I cannot recall reading that at all, but I may have been in a hurry.

Nope, didn't change it!