I find it helpful to distinguish between "regular expressions" as something mathematical and "regexes" as nebulous more-than an NFA and less than a Turing complete grammar...DFA regex engines being the Platypus that proves the rule.
As soon as you have backtracking and capture groups it's not a regular expression in the mathematical sense. These features make regexes more useful in many simple cases at the cost of making them harder to reason about in more difficult cases.
As soon as you have backtracking and capture groups it's not a regular expression in the mathematical sense. These features make regexes more useful in many simple cases at the cost of making them harder to reason about in more difficult cases.