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/(?<!ss)/i
/(?<!ß|ss)/i
ßtabc SSTABC STABC
/(?<!s)tart/i ; /(?<![Ss])[Tt][Aa][Rr][Tt]/ ;
my $not_s= qr/(?<![Ss])/; # no explicit /i here, will never match insi +de ß my $rest= qr/abc/i; my $pattern= "$not_s$rest"; ...