my $reg1 = qr{((?:(?<=<)ab(?=>))|(?:(?<!<)ab(?!>)))};
while(<DATA>) {
    chomp;
    print $_, "\t", m/$reg1/?"yes $1":"no", "\n";
}
__DATA__
<ab
ab>
<>
<ab>
ab
__END__
Output:
<ab     no
ab>     no
<>      no
<ab>    yes ab
ab      yes ab