引用先前捕获的文本，并在正则表达式中重复使用它

\n \1、\2表示匹配第1、2个子表达式匹配到的内容

$n 替换函数中使用

删除重复值

dis ustrregexra("This is is is is isis a a a a cat", "(\b\w+\b )\1+", "$1")

dis ustrregexra("That is is acat", "is is ", "is ")

dis ustrregexra("That is is acat", "(is )\1", "$1")

dis ustrregexra("That is is acat", "(\b\w+\b )\1", "$1")

dis ustrregexra("That is is is acat", "(\b\w+\b )\1", "$1")

dis ustrregexra("That is is is is acat", "(\b\w+\b )\1", "$1")

dis ustrregexra("That is is is acat", "(\b\w+\b )\1\1", "$1")

dis ustrregexra("That is is is is acat", "(\b\w+\b )\1{3}", "$1")

dis ustrregexra("That is is is is isis a a a a cat", "(\b\w+\b )\1{5}","$1")

dis ustrregexra("That is a a a acat", "(\b\w+\b )\1{3}", "$1")

dis ustrregexra("That is is is is isis a a a a cat", "(\b\w+\b )\1{3,5}", "$1")