R中的正则表达式:匹配字符序列之前的字符串 [英] Regex in R: matching the string before a sequence of characters

问题描述

我想提取某个单词之前的字符串的一部分.例如.我想在，没用"之前得到一切.

I want to extract a part of the string that comes before a certain word. E.g. I want to get everything before ", useless".

a <- "Experiment A, useless (03/25)"
b <- grep('^[^useless]+', a, perl=T)
regmatches(a,b)

这应该返回实验 A".

This should return "Experiment A".

然而，这行不通.R 给出子字符串中的错误(x[ind]，所以，eo):无效的子字符串参数".

However, this doesn't work. R gives "Error in substring(x[ind], so, eo) : invalid substring arguments".

推荐答案

我们可以使用 sub 来匹配 , 后跟零个或多个空格 (\\s*) 后跟 'useless' 和其他字符 (.*) 并将其替换为空白 ("")

We can use sub to match the , followed by zero or more spaces (\\s*) followed by 'useless' and other characters that follow (.*) and replace it with blank ("")

sub(",\\s*useless\\b.*", "", a)
#[1] "Experiment A"

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