in reply to Re: Re: regex "o" modifier
in thread regex "o" modifier

nope, you wouldn't benefit from an /o in this case..

qr quotes and compiles the pattern as a regular expression. To see what it's doing, try this:

perl -e 'my $re = qr!yes|no!; print "$re\n";'
prints: (?-xism:yes|no)

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The result of qr, in this case $re, can be used on it's own, or within another regex.

my $re = qr!yes|no!;
print "yes or no!\n" if "yes" =~ $re;
print "yes or no only" if "maybe" !~ $re;
print "polite yes or no\n" if "yes thankyou" =~ /$re thankyou/;

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The first 2 uses in this code will not trigger the recompling of any regex. The 3rd will need to be compiled, although I believe you could do...

my $re = qr!yes|no!;
my $re2 = qr!\sthankyou!;
print "yes or no!\n" if "yes" =~ $re;
print "yes or no only" if "maybe" !~ $re;
print "polite yes or no\n" if "yes thankyou" =~ /$re$re2/;

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..and avoid any recompilation after the initial qr's

cheers,

J

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Re: Re: Re: Re: regex "o" modifier
by diotalevi (Canon) on May 07, 2003 at 04:18 UTC

    You'll want to revisit Re^2: meaning of /o in regexes. If you interpolate qr// compiled expressions you lose because the thing is compiled more often than it needs to be. Your "I believe you could do..." is incorrect.

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      hmm.. yup, gotcha there.. shouldn't have added that last bit.

      ok.. knew I saw something somewhere.. in "Programming Perl" it has the example:

      $regex = qr/$pattern/;
$string =~ /foo${regex}bar/; # interpolate into larger patterns

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      ..and then the comment, "This time, Perl does recompile the pattern, but you could always chain several qr// operators together into one."

      So would they maybe be suggesting something more like:

      my $re = qr!yes|no!;
my $re2 = qr!\sthankyou!;
my $bigre = $re.$re2;
print "polite yes or no\n" if "yes thankyou" =~ $bigre;

      [download]

      hmm.. on further thought, that doesn't seem right either. Would $re & $re2 each be stringified in that operation? If so that would mean that $bigre would simply be eq to the string '(?-xism:yes|no)(?-xism:\sthankyou)'

      The only other thing I can think they mean is:

      my $bigre = qr!yes|no!.qr!\sthankyou!;

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      But I really don't see the point in that... Anyone know what they mean by "chain several qr// operators together"?

      cheers,

      J

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[select]
        It says chaining, not concatenating. 
        my $re = qr!yes|no!;
my $re2 = qr!${re}\sthankyou!;
print "polite yes or no\n" if "yes thankyou" =~ $re2;

        [download]

        Makeshifts last the longest.

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[d/l]

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