What do you mean by "returns something"? I'm going to assume you mean "returns something defined". Adjust as needed. 

foreach my $range (values %Build) {
   $message =~ s{ (\[ ([$range]+) \]) }{
      my $val = return_number("$2");
      defined($val) ? $val : $1
   }exig
}

[download]

If return_number is costly, you can memoize the returned value. 

my %memoize;
foreach my $range (values %Build) {
   $message =~ s{ (\[ ([$range]+) \]) }{
      my $val = exists($memoize{$2}) ? $memoize{$2} : return_number("$
+2");
      $memoize{$2} = $val;
      defined($val) ? $val : $1
   }exig
}

[download]

Why is Build a hash instead of an array? You don't use the keys. Also, Build is not a particularly good name.

In reply to Re^3: Regex (?{ code }) and use re 'eval' by ikegami
in thread Regex (?{ code }) and use re 'eval' by SFLEX

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