[TASK] Consolidate regexp for utf8 and non-utf8 file systems

This patch enables the @ character to be usable now
also for non-utf8 file systems.

Resolves: #50538
Releases: 6.2, 6.1
Change-Id: I72ce24393003af8733af6fc650e69781df4a272c
Reviewed-on: https://review.typo3.org/23439
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Stefan Neufeind
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters