[This is the spec](https://www.irs.gov/pub/irs-utl/giin_composition.pdf) for the FATCA GIIN.

19 digits, alphanumeric and only uppercase, in 4 groups, each separated by a compulsory dot/period:

XXXXXX.XXXXX.XX.XXX

First two groups are alphanumeric uppercase, but never include uppercase O.

3rd group seems to only allow strict combination of 2 uppercase letters:

```
LE|SL|ME|BR|SP

```

4th group is only digits.

I have this so far, but am stuck on how to add that 'never allow O' rule:

```
[A-Z0-9]){6}\.[A-Z0-9]){5}\.(LE|SL|ME|BR|SP)\.\d{3}

```

So how can I restrict those first 2 groups to now allow the O?

NOTE - this is for **Ruby**