[BUGFIX] Alphanumeric validator fails with broken PCRE

After change from #44216 the alphanumeric filter fails on
systems like debian squeeze with PHP 5.3.3 due to a broken
PCRE environment.

Change-Id: I349385c13c5b9757d879d483ec7e625501e32e88
Resolves: #44832
Related: #44216
Releases: 6.1, 6.0
Reviewed-on: https://review.typo3.org/17734
Reviewed-by: Wouter Wolters
Reviewed-by: Daniel Hürtgen
Tested-by: Daniel Hürtgen
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring