ISBN2 by yowa

#!ruby -p
sub('?'){(0..9).find{|o|i=0
1>eval("-(d=$_[3+i+=1]
[i,i%2*2-3][f].*d==???o:d%48%15)".*10+3*f=$_[15]?1:0)%(11-f)}||:X}

Note that non-ascii characters in the above source code will be escaped (such as \x9f).

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