FAQ 6.21 Are Perl regexes DFAs or NFAs? Are they POSIX compliant?

Discussion in 'Perl Misc' started by PerlFAQ Server, Feb 14, 2011.

  1. 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 .

    --------------------------------------------------------------------

    6.21: 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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    so please include as much information as possible and relevant in any
    corrections. The perlfaq-workers also don't have access to every
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    perlfaq.pod.
    PerlFAQ Server, Feb 14, 2011
    #1
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