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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";

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