regex - Why does adding a character class to the end of a regex fail to match hex numbers?

Question

Perl accepts:

my @resultado = $texto =~ /[^g-zG-z]$RE{num}{real}{-base=>16}/g

but doesn't accept:

my @resultado = $texto =~ /[^g-zG-z]$RE{num}{real}{-base=>16}[^g-zG-z]/g

I just add [^g-zG-z] at the end; at the beginning it works but not at the end. Why? I want to print hexadecimal numbers but, for example, in cases where there is a word like 'call' it should not say that 'ca' is a hexadecimal number.