FAQ 6.20: Are Perl regexes DFAs or NFAs? Are they POSIX compliant?

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6.20: Are Perl regexes DFAs or NFAs?  Are they POSIX compliant?

    While it's true that Perl's regular expressions resemble the DFAs
    (deterministic finite automata) of the egrep(1) program, they are in
    fact implemented as NFAs (non-deterministic finite automata) to allow
    backtracking and backreferencing. And they aren't POSIX-style either,
    because those guarantee worst-case behavior for all cases. (It seems
    that some people prefer guarantees of consistency, even when what's
    guaranteed is slowness.) See the book "Mastering Regular Expressions"
    (from O'Reilly) by Jeffrey Friedl for all the details you could ever
    hope to know on these matters (a full citation appears in perlfaq2).


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