stringr str_extract 捕获组捕获所有内容 [英] stringr str_extract capture group capturing everything

问题描述

我想从字符串中提取年份.这总是在X"之后和."之前.然后是一串其他字符.

I'm looking to extract the year from a string. This always comes after an 'X' and before "." then a string of other characters.

使用 stringr 的 str_extract 我正在尝试以下操作:

Using stringr's str_extract I'm trying the following:

year = str_extract(string = 'X2015.XML.Outgoing.pounds..millions.'
                 , pattern = 'X(\\d{4})\\.')

我以为括号会定义捕获组，返回2015，但实际上我得到了完整的匹配X2015.

I thought the brackets would define the capture group, returning 2015, but I actually get the complete match X2015.

我这样做正确吗?为什么我不修剪X"和."?

Am I doing this correctly? Why am i not trimming "X" and "."?

推荐答案

在这种情况下，捕获组无关紧要.函数 str_extract 将返回整个匹配，包括捕获组前后的字符.

The capture group is irrelevant in this case. The function str_extract will return the whole match including characters before and after the capture group.

您必须改为使用后视和前视.它们的长度为零.

You have to work with lookbehind and lookahead instead. Their length is zero.

library(stringr)
str_extract(string = 'X2015.XML.Outgoing.pounds..millions.',
            pattern = '(?<=X)\\d{4}(?=\\.)')
# [1] "2015"

此正则表达式匹配以 X 开头且后跟 . 的四个连续数字.

This regex matches four consecutive digits that are preceded by an X and followed by a ..

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