perlmeditation
toolic
<p>
I had a bug in my code which was a result of insufficient validation of a string passed to the built-in [doc://hex] function.
<p>
In my opinion, [doc://hex] is much too forgiving. As shown in its
documentation, it allows for a string to have a leading <c>0x</c>; for
example, it converts <c>0xC</c> to <c>12</c>. Although <c>x</c> is not
a legal hexadecimal character, it is customary to denote a hex value
with the <c>0xC</c> prefix. This is quite reasonable.
<p>
Although not mentioned in the documentation, it also allows for a string
to have a leading <c>x</c>; for example, it converts <c>xC</c> to <c>12</c>.
I guess that's reasonable, too.
<p>
My problem occurred when I inadvertently passed a lone <c>x</c> to
[doc://hex]. The function returned 0. Even with [doc://warnings] enabled,
I did not get a warning message (perl v5.12.2, linux):
<c>
perl -w -E "say hex(q{x})"
0
</c>
<p>
Passing it an illegal hex string like <c>t</c> does generate a warning, as desired:
<c>
perl -w -E "say hex(q{t})"
Illegal hexadecimal digit 't' ignored at -e line 1.
0
</c>
<p>I think passing a lone <c>x</c> should generate a warning message (with [doc://warnings] enabled), but I'm not sure it's worth submitting a
[doc://perlbug] since I really need to do some checking before passing
a string to [doc://hex] anyway.
<p>Here is a wrapper function I decided to use. It calls [doc://hex]
after performing some input validation:
<c>
sub hex2 {
my $str = shift;
$str =~ s/^(0x|x)//;
if (length $str) {
if ($str =~ /([^0-9a-f])/i) {
die "hex2: Illegal hexadecimal digit found: '$1'";
}
else {
return hex $str;
}
}
else {
die "hex2: No chars found after stripping leading 0x or x";
}
}
</c>
<p>
Another approach is to
[http://perldoc.perl.org/CORE.html#OVERRIDING-CORE-FUNCTIONS|override hex].