Luxembourg postal code regex rejects valid L-XXXX postal codes

[jcglombard]

Hi,
The Luxembourg postal code validation pattern appears to be incorrect because it does not accept valid Luxembourg postal codes in the official L-XXXX format.

For example:

L-8211

is a valid Luxembourg postal code.

The official documentation from POST Luxembourg explicitly shows Luxembourg addresses using the L-XXXX format, for example:

L-1234 DUDELANGE
L-9874 REMICH
L-2998 LUXEMBOURG

The official postal code database also contains 8211 as a valid postal code for Mamer.

Current behavior

The current Luxembourg regex appears to accept only four numeric digits:

^\d{4}$

As a consequence:

8211 ✅
L-8211 ❌

However, L-8211 is a valid representation of a Luxembourg postal code.

Expected behavior

The validator should accept both representations:

8211 ✅
L-8211 ✅

Ideally, the regex should therefore support the optional L- prefix, for example:

^(?:L-)?\d{4}$

If the validator is intended to be case-insensitive, the corresponding implementation could also allow l-8211.

References

Official POST Luxembourg documentation:

POST Luxembourg — Comment bien rédiger une adresse
POST Luxembourg — Codes postaux (édition du 27 octobre 2025)

The official POST Luxembourg address documentation explicitly uses the L-XXXX format, and the official postal-code list identifies 8211 as a valid postal code for Mamer.

Suggested test cases

I would suggest adding at least the following test cases:

8211 → valid
L-8211 → valid
1234 → valid
L-1234 → valid
123 → invalid
12345 → invalid
L1234 → invalid

Thank you for maintaining this package and for considering this correction.