如何不匹配 [英] How to match nothing

问题描述

对于某些人来说，这可能是一个简单的问题:您如何在 R 中使用正则表达式实际上不匹配任何内容?假设你有一个像这样的字符串向量:

This may be a simple question to some: how do you actually match nothing using regex in R? Suppose you have a vector of strings such as this:

q <- c("a", "12", "0", "", "300")

再假设你想匹配空字符串""，怎么做呢?似乎可以合理地匹配 "" 使用 grep 来匹配元字符 . 意思是任何字符"，无论是这样还是作为内容否定字符类，以及参数 invert = T 以匹配匹配的反面:

And suppose further you want to match the empty string "", how to go about this? It seems one can reasonably well match "" using grep to match the metacharacter . meaning 'any character', either as such or as the content of a negated character class, as well as the argument invert = T to match the opposite of the match:

grep(".", q, value = T, invert = T)
[1] ""

grep("[^.]", q, value = T, invert = T)
[1] ""

在任何一种情况下，匹配都有效.但是，当然，使用 invert 感觉像是一个方便的技巧，但不像一个严肃的正则表达式.有没有另一种方法可以匹配 R 中的任何内容?

In either case the match works. But, surely, using invert feels like a convenient trick but not like a serious regex. Is there another way of matching nothing in R?

推荐答案

一种选择是指定 ^ 作为字符串的开头，然后是 $ 作为结尾字符串

One option would be to specify the ^ for start of the string followed by $ as the end of the string

grep("^$", q)

或者用 nchar

which(nchar(q) == 0)

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