希望 match 到某几个特定单词的所有首次出现，比如下面这段文字（随便 copy 来的）：
Mr. Obama also used his own remarks to try to drive a wedge between Mr. Trump’ s campaign and Republican voters. “ It wasn ’ t particularly Republican and it sure wasn ’ t conservative,” he said of last week ’ s Republican convention. “ There were no serious solutions to pressing problems. Just the fanning of resentments and blame and hate and anger.” The president ’ s contempt for Mr. Trump took on a personal dimension as well when he recalled his grandparents from Kansas and said, “ I don ’ t know if they had their birth certificates ” — a reference to Mr. Trump’ s leadership of the so-called birther movement that raised questions about Mr. Obama’ s citizenship.
正则 (Obama|Trump) 会匹配到所有的 Obama 和 Trump （粗体的），现在希望仅匹配首次出现，即粗斜体的两个，研究了半天 lookbehind/back reference 啥的也没试出怎么写，求指导。
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