HTML last tag conditional match

<EM>is <i>love</i></EM>,<PARTITION />

<EM>is <i>love</i>,<PARTITION />

if (Regex.IsMatch(subjectString, "</EM>")) {
    return null;
} else {
    return subjectString;
}

if (subjectString.Contains("</EM>")) {
    return null;
} else {
    return subjectString;
}

A((?!</EM>).)*Z

^<EM>(?:(?<!</EM>).)*<PARTITION />$

^<EM>(?:(?<!</EM>).)*<PARTITION />$

<EM>w*s*W*S*[^(</EM>)]<PARTITION[ ]/>