I want to write a regular expression that will accept all the words that follow this rules:

1. The word will only contain capital letters and numbers;
2. The numbers will always be after the letters;
3. There will be an even pairs of capital letters followed by a number;
4. A digit can only appear once in the word.

These are "good" words:

```
T3L9A8L4
A1B2
Q2W4E6R8A1B3D5F7

```

These are "bad" words:

```
T3
TT33
Y3Y4Y6
T1A2B3T1

```

How do I write the regex for this?
Sorry for not adding any code - don't know how to start writing it.