You aren't matching what you think you are. The [^ ]+ greedily matches all the letters and numbers but one, as this example will show:

$_ = "\@abcde99\@";
if (/^([^ ]+)([a-z0-9]+)([^ ]+)$/) {
  # prints: *@abcde9*9*@*
  print "*$1*$2*$3*\n";
}
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A better regex might be this one:

$_ = "\@abcde99\@";
if (/^([^ ]+?)([a-z0-9]{5,12})([^ a-z0-9]+)$/) {
  print "*$1*$2*$3*\n";
}
[download]