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Re^2: opposite of index+rindex? How-to? Needed? (updated)

by rsFalse (Hermit)
on Sep 02, 2019 at 00:26 UTC ( #11105398=note: print w/replies, xml ) Need Help??


in reply to Re: opposite of index+rindex? How-to? Needed? (updated)
in thread opposite of index+rindex? How-to? Needed?

And alternatively with look-ahead:
;; print qq{offset of first vowel: }, $s =~ m{ (?= $vowel ) }xgms ? pos +$s : 'none'; ;; print qq{offset of last vowel: }, $s =~ m{ .* (?= $vowel ) }xgms ? po +s $s : 'none';
Note 'g' modifier.

To negate, one can use negative look-ahead (?! $vowel ). Upd. Not really! Look at the AnomalousMonk reply.

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Re^3: opposite of index+rindex? How-to? Needed? (updated)
by AnomalousMonk (Bishop) on Sep 02, 2019 at 06:44 UTC
    ... with look-ahead ...

    $+[0] also works here (note  $+[0] vice  $-[1] and no  /g modifier):

    c:\@Work\Perl\monks>perl -wMstrict -le "my $vowel = qr{ [AEIOUaeiou] }xms; ;; my $s = 'xyzzyBARxkcdFOOyz'; ;; print 'offset of first vowel: ', $s =~ m{ (?= $vowel) }xms ? $+[0] : +'none'; print 'offset of last vowel: ', $s =~ m{ .* (?= $vowel) }xms ? $+[0] +: 'none'; print 'offset of first vowel: ', 'XYZ' =~ m{ (?= $vowel) }xms ? $+[0] + : 'none'; " offset of first vowel: 6 offset of last vowel: 14 offset of first vowel: none
    I like the look-ahead idea because it avoids a capture and so may be slightly faster.

    To negate, one can use negative look-ahead (?! $vowel ).

    I think there's a problem here:

    c:\@Work\Perl\monks>perl -wMstrict -le "my $vowel = qr{ [AEIOUaeiou] }xms; ;; my $s = 'aePDQioVWXua'; ;; print 'offset of first non-vowel: ', $s =~ m{ (?! $vowel) }xmsg ? pos +($s) : 'none'; print 'offset of last non-vowel: ', $s =~ m{ .* (?! $vowel) }xmsg ? p +os($s) : 'none'; ;; $s = 'aei'; print 'offset of first non-vowel: ', $s =~ m{ (?! $vowel) }xmsg ? pos +($s) : 'none'; " offset of first non-vowel: 2 offset of last non-vowel: 12 offset of first non-vowel: 3
    The second and third cases give questionable results because there's always a place in a string where a negative look-ahead will succeed if it is true nowhere else: just beyond the end of the string. ($+[0] has the same problem in these cases as pos.)

    Update: However, a positive look-ahead to a negated char class works:

    c:\@Work\Perl\monks>perl -wMstrict -le "my $non_vowel = qr{ [^AEIOUaeiou] }xms; ;; my $s = 'aePDQioVWXua'; ;; print 'offset of first non-vowel: ', $s =~ m{ (?= $non_vowel) }xmsg ? + pos($s) : 'none'; print 'offset of last non-vowel: ', $s =~ m{ .* (?= $non_vowel) }xmsg + ? pos($s) : 'none'; ;; $s = 'aei'; print 'offset of first non-vowel: ', $s =~ m{ (?= $non_vowel) }xmsg ? + pos($s) : 'none'; " offset of first non-vowel: 2 offset of last non-vowel: 9 offset of first non-vowel: none
    The positive assertion requires that something be present, so the overall match can fail. ($+[0] works as well as pos.)


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