[RFC] Support for IDN (IRI)

[rxu]

I didn't find any similar RFC, but just a discussion topic of Älphäbet url support for 4.0.
Since October, 2009 ICANN has approved the creation of internationalized country code top-level domains (IDN ccTLDs), and first of then were installed in the DNS root zone on May, 2010. Now IDNs are actually in use already.

Thus, support for IDN (which is a part of IRI - International resource identifier) seems to be a mandatory for any web application, including bulletin board.
It would be nice to implement it as soon as possible to make it included into 3.1a.

Some useful links:
Internationalized domain name - Wikipedia
Internationalized Resource Identifiers (IRIs) - W3
rfc5891 - Internationalized Domain Names in Applications (IDNA): Protocol
rfc5892 - The Unicode Code Points and Internationalized Domain Names for Applications (IDNA)

EDIT: The derivation of valid characters in terms of Unicode properties for IDNA2008 - Internationalized Domain Names (IDN) FAQ

EDIT2:
Unicode Technical Standard #46. Unicode IDNA Compatibility Processing
Unicode Character Database (txt file)

[Oleg]

We should support IDN I suppose.

[ecwpa]

+1

It was weird from the begining not being able to use iconic characters like Ñ in urls for languages like Spanish, the name itself, España wasn't even posible within urls. Now that this finally changed phpBB should suppport it.

[imkingdavid]

+1 from me as well

[bantu]

We should support IDN I suppose.

+1

[rxu]

Hmm. Valid IDN characters should be NFKC Case folded, while phpBB3 messages are normalized by NFC.
So we 'd need to add NFKC casefolded message content to the parser to process IDNs, unless I've missed the way of derivation of IDN valid characters.