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

This is an excerpt from the latest version perlfaq6.pod, which
comes with the standard Perl distribution. These postings aim to
reduce the number of repeated questions as well as allow the community
to review and update the answers. The latest version of the complete
perlfaq is at http://faq.perl.org .


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).


The perlfaq-workers, a group of volunteers, maintain the perlfaq. They
are not necessarily experts in every domain where Perl might show up,
so please include as much information as possible and relevant in any
corrections. The perlfaq-workers also don't have access to every
operating system or platform, so please include relevant details for
corrections to examples that do not work on particular platforms.
Working code is greatly appreciated.

If you'd like to help maintain the perlfaq, see the details in

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