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Re^4: Determing if this regex matched

by cormanaz (Chaplain)
on Aug 06, 2011 at 20:34 UTC ( #918995=note: print w/ replies, xml ) Need Help??


in reply to Re^3: Determing if this regex matched
in thread Determing if this regex matched

Well good point.

I still wonder why is $& always null in the original example.


Comment on Re^4: Determing if this regex matched
Re^5: Determing if this regex matched
by AnomalousMonk (Abbot) on Aug 07, 2011 at 10:22 UTC
    ... why is $& always null in the original example.

    I think the answer is that $ (in the original example) and \z (in the example below) match after the last character in the string: that's the 'position' of the end of the string. For instance, in the string 'xa' the 'a' can match and is replaced, but after this match the match position (as returned by pos) is just beyond the end of the string, where it can match once again with no 'a', no whitespace and \z — a zero-length match!

    >perl -wMstrict -le "my @strings = ('x', 'xa', 'x ', 'xa ',); ;; for my $s (@strings) { $s =~ s{ a? \s* \z } { print qq{\$& '$&' pos = }, pos($s); ''; }xmseg; print qq{\$s '$s' \n}; } " $& '' pos = 1 $s 'x' $& 'a' pos = 1 $& '' pos = 2 $s 'x' $& ' ' pos = 1 $& '' pos = 2 $s 'x' $& 'a ' pos = 1 $& '' pos = 3 $s 'x'

    Update: N.B.: The /g regex modifier does not act to 'reset' $&, $1, or other match variables.

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