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s/^\s+(.*?)\s+$/$1/;   # fails sometimes
s/^\s*(.*?)\s*$/$1/;   # succeeds, but slow
s/^\s*(.*\S)\s*$/$1/;  # fails sometimes
s/^\s*//, s/\s*$//;    # succeeds, but WHY use * ?
s/^\s+//, s/\s+$//;    # succeeds, but is it good?

$_ = "a b  c   d    ";  # 1, 2, 3, 4 spaces
s/\s+$//;
...
    AAX
       AAAX
           AAAA

($_ = reverse) =~ s/^\s+//; $_ = reverse;

1 while s/\s$//;