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php正则表达式的反向引用有关问题

WBOY
WBOYOriginal
2016-06-13 12:43:561105browse

php正则表达式的反向引用问题?
理论上的问题哈:

反向引用子表达式我知道的是: 

\1
表示引用第一个子表达式的值

后来无意看到有人这样引用: ${1} 就孤陋寡闻了,可以用,但是查不到相关文档说明,请高手教诲

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