http://www.perlmonks.org?node_id=255049


in reply to A Perl aptitude test

Just a nit, but regular expressions don't change strings, they match portions of strings. The s/// operator employs a regular expression in the left portion to match, and then modifies the matched substring according to the right portion.

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Re: Re: A Perl aptitude test
by TheDamian (Vicar) on May 02, 2003 at 23:01 UTC
    regular expressions don't change strings
    <yoda>
    Hmmmmmmmm. Regular expressions change no string? So sure you are. But for a Japhi, always more to learn there is. Use the Perl!
    </yoda> 
    $_ = "the end is nigh!\t\n    \n";

m/(\s+)$(?{substr($_,-length$^N)=""})/;

print "[$_]";

    [download]
    ;-)
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      I contend that (?{ }) is not a regular expression, but a hook (a place for a Perl expression) to be evaluated by the regex engine during its matching work.

      The phrase "Regular Expression" is a mathematical concept predating perl and emacs and sed. Quoting from Freidl's Mastering Regular Expressions:

        The Origins of Regular Expressions

        The seeds of regular expressions were planted in the early 1940s... Through the 1950s and 1960s, regular expressions enjoyed a rich study in theoretical mathematical circles... Ken Thompson's 1968 article Regular Expression Search Algorithm... led to his work on qed, an editor that formed the basis for the Unix editor, ed.

      Regular expressions are expressions that regulate a sequence as belonging to a set, or not belonging to a set. (What, you thought 'regular' as in 'everyday' or 'fixed interval'?)

      Since (?{}) doesn't fail and cause the engine to backtrack or bail, it's not regulating anything. The regex syntax handles the wrapping stuff but the contents are what's doing the string-modifying, and the results have no direct bearing on the regulation. That's what I mean when I say that (?{}) is not a regular expression.

      (I know, I'm forever editing and extending my comments with afterthoughts rather than creating new followups.)
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        Well, even the regulating pieces of Perl's "regular expressions" are far from regular in any sense of the precise definition. The name just (incorrectly) stuck. (Recent Camels talk of "pattern matching"; I prefer that term too.) Finally, you also forget (??{ code }), which can both alter the string as well as regulate the match in procession.

        Makeshifts last the longest.

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Re: Re: A Perl aptitude test
by Jonathan (Curate) on May 02, 2003 at 15:58 UTC
    Of course and apologies, I transcribed the question incorrectly.
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