Ciao. The Camel Book (Wall, L, Christiansen, T, and Schwartz, R, Programming Perl, 2/ed, O'Reilly & Associates, Sebastapol, CA: 1996 has some explanation of the regexp engine. For detailed information about the internal workings of the regexp engine you could read the code, or, probably a trifle less painful, ask Mr Schwartz or Mr Wall very nicely. Before doing either, I'd do a super search.