Engines

The same pattern is not the same program. An engine decides which constructs exist, what an unset group means, whether a lookbehind may vary in width, and whether a hostile input can hold your server for an hour.

Lesson	The question
Who supports what	Eleven constructs × five engines, measured by compiling each pattern — plus the three engines that refuse the whole category, and where ABAP sits.
When the engine has no lookaround	Rust's regex and Go's regexp have neither lookaround nor backreferences. What you write instead — and why one of the replacements is more correct than the lookahead it replaces.

The shortest version: there are four lineages — Perl/PCRE (which includes ABAP), the Java/Ruby/JavaScript middle ground, Python's re, and the automata engines (Go, Rust's regex). Knowing which one you are talking to predicts more than any individual fact on this page.

A caution about the table, worth reading before you trust a cell. "Yes" on the probe means the engine compiled the pattern, and compiling is not supporting. Three of the differences this library has since measured are invisible to a compile test: Rust's regex accepts a++ and reads it as (a+)+, it accepts (?R) because R is its CRLF flag, and JavaScript accepts \g<0> as the literal text g<0>. Each of those is a pattern that runs, matches something, and is not what the author wrote. Where a chapter needed a behaviour answer rather than a compile answer, it measured one — see match semantics, recursion and conditionals, and backtracking.