[BUGFIX] felogin: Unknown modifier in regular expression

A regular expression in FrontendLoginController
contains an unknown modifier. Fix it by replacing the
/ to # at the beginning and the end of the regular
expression.

Resolves: #52059
Releases: 6.2, 6.1, 6.0
Change-Id: Id4d3439c1cdbec691d977570bf76ba0c7bad493c
Reviewed-on: https://review.typo3.org/26473
Reviewed-by: Markus Klein
Tested-by: Markus Klein