Put in match groups, i.e., replace 
my $p = join '|', map { "$pre[$_]\\0{@{[($N-$_)>>3]}}$suf[($N-$_)&0x7]
+" } 0..7;

[download]
with 
my $p = join '|', map { "$pre[$_](\\0{@{[($N-$_)>>3]}}$suf[($N-$_)&0x7
+])" } 0..7;

[download]
By construction, only one will match and $#- will tell you which one. We place the leftparen so that the group always starts at the first byte that's beginning with a zero bit. Then you just have to subtract the number of zero bits at the end of the byte that $pre[$_] is matching, but that's just $#- - 1. So the answer is 
$-[$#-]*8 - $#- + 1

[download]
Still needs a bit more work to handle the $N < 8 case.

In reply to Re^5: Locating a specified number of contiguous 0 bits within a large bitstring efficiently. by wrog
in thread Locating a specified number of contiguous 0 bits within a large bitstring efficiently. by BrowserUk

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