This question already has an answer here:
Computer science has automata theory with lessons on regular expressions and FSM. How are these different from regex engines used in programming such as C++, Perl, PHP etc.?
I would like to know some differences i.e. stuff that can only be done with regexps in programming but cannot be done in formal regexps in computer science.
I heard there are a lot are differences but what are some major ones? Let's take Perl Compatible Regular Expressions (PCRE) or Boost.Regex for comparsion.