Yes, it seems that you've figured it out by now, thanks to AnamolousMonk: the easiest way for achieving what you want is a negative look-ahead assertion: 
my $regex = qr/\w{3}_[tr]x_[pn]\d(?!\d)/;

[download]
But even if you did not know about zero-width assertions, you could still have obtained more or less the same resukt with an alternation: 
my $regex = qr/\w{3}_[tr]x_[pn]\d(\D|$)/;

[download]
which means: match everything to the required digit, followed by either a non-digit or the end of the string.

TIMTOWTDI.

In reply to Re^3: regular expression to match specific members of a bus_name by Laurent_R
in thread regular expression to match specific members of a bus_name by boleary

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