allow matching regex in custom mail filter

interface/lib/plugins/mail_user_filter_plugin.inc.php

interface/web/mail/form/mail_user_filter.tform.php

interface/web/mail/lib/lang/ar_mail_user_filter.lng

interface/web/mail/lib/lang/bg_mail_user_filter.lng

interface/web/mail/lib/lang/br_mail_user_filter.lng

interface/web/mail/lib/lang/ca_mail_user_filter.lng

interface/web/mail/lib/lang/cz_mail_user_filter.lng

interface/web/mail/lib/lang/de_mail_user_filter.lng

interface/web/mail/lib/lang/dk_mail_user_filter.lng

interface/web/mail/lib/lang/el_mail_user_filter.lng

interface/web/mail/lib/lang/en_mail_user_filter.lng

interface/web/mail/lib/lang/es_mail_user_filter.lng

interface/web/mail/lib/lang/fi_mail_user_filter.lng

interface/web/mail/lib/lang/fr_mail_user_filter.lng

interface/web/mail/lib/lang/hr_mail_user_filter.lng

interface/web/mail/lib/lang/hu_mail_user_filter.lng

interface/web/mail/lib/lang/id_mail_user_filter.lng

interface/web/mail/lib/lang/it_mail_user_filter.lng

interface/web/mail/lib/lang/ja_mail_user_filter.lng

interface/web/mail/lib/lang/nl_mail_user_filter.lng