check validity and syntax of contact entries

Expected behavior

Nextcloud/The Contacts app should contain a function to check the validity and syntax of all contact record data and should correct wrong entries.

Current behavior

Nextcloud/The contacts app doesn't contain a function to check the validity and syntax of all contact record data yet. By manually checking existing contact records I found the following list of possible entry which should be corrected:

• entries which contain masked commas, should be corrected:
  e.g. TEL;TYPE=WORK\,FAX:+49 ... -> TEL;TYPE=WORK,FAX:+49 ...

• entries which contain upper case line breaks should be corrected:
  e.g. NOTE:Line1\NLine2\NLine3 -> NOTE:Line1\nLine2\nLine3

• entries which contain lowercase labels should be converted to uppercase:
  e.g. EMAIL;TYPE=work,pref:... -> EMAIL;TYPE=WORK,PREF:...

• abandoned multi-line entries which are missing a reference to an other entry should be deleted:
  e.g. ITEM1.X-ABADR:de originally was linked to ITEM1.ADR;TYPE=WORK,PREF:;;My street 3;My City ... which doesn't exist anymore.

• entries which are not originally used by the Contacts app but exist in a record, like e.g.:
  X-CONTACTSYNC-RELATED;TYPE=sweetheart:Marry Joe -> X-CONTACTSYNC-RELATED;TYPE=SWEETHEART:Marry Joe

Steps to reproduce

1. Check your Nextcloud contact records, which might have been created with previous versions of the Contacts app (<1.5.3) and which have been synchronized with Android and iPhones over a long period if time. You will find many of strange formatted entries in it which are causing strange field labels in one or the other contacts database (Nextcloud, Android, iPhone, etc.)

Environment

Server Configuration

OS: Linux 3.2.84
Web server: Apache2 2.4.25
Database: MariaDB 5.5.54
PHP version: 5.6.29
Nextcloud version: 11.0.1
Contacts app version: 1.5.3

Client Configuration

Browser: Mozilla Firefox 51.0.1
Operating system: Windows 7

Want to back this issue? Post a bounty on it! We accept bounties via Bountysource.

The issue names no files, tests, or code entry points. Start by locating the contact-record parsing and synchronization entry points, then identify how existing vCard data is validated and stored. Done should include a defined validation and correction function covering the listed malformed entries, with tests for each case.