<TopicSet FormalName="PARENT1">
    <Topic>
      <TopicType FormalName="Child1" />
    </Topic>
    <Topic>
      <TopicType FormalName="Child2" />
    </Topic>
    <Topic>
      <TopicType FormalName="Child3" />
    </Topic>
</TopicSet>
<TopicSet FormalName="PARENT2">
    <Topic>
      <TopicType FormalName="Child1" />
    </Topic>
    <Topic>
      <TopicType FormalName="Child2" />
    </Topic>
    <Topic>
      <TopicType FormalName="Child3" />
    </Topic>
</TopicSet>

(<TopicSet.*?>)([sS]*?)(</TopicSet>)

(<TopicSet FormalName="PARENT1">)([sS]*?)(</TopicSet>)