在 R DataFrame 中查找 Pattern 之后的匹配字符

Question

我对字符串操作相当陌生，但我遇到了一个关于 R 数据帧中字符串和字符数据的问题。我试图从长字符串中提取数值在模式之后，然后将结果作为新列存储在我的数据框中。我有一个相当大的数据集，我试图找出一些有用的信息，这些信息存储在一个名为“notes”的列中。

例如，我感兴趣的字符串总是遵循这种模式（任务没有什么意义）：

df$notes[1] <- "On 5 June, some people walked down the street in this area. [size=around 5]" 
df$notes[2] <- "On 6 June, some people rode bikes down the street in this area. [size= nearly 4]" 
df$notes[3] <- "On 7 June, some people walked into a grocery store in this area. [size= about 100]"

在某些列中，我们没有得到数值，这是我在得到解决方案后可以处理的问题。这些行遵循与此类似的内容：

df$notes[4] <- "On 10 July, an hundreds of people drank water from this fountain [size=hundreds]"
df$notes[5] <- "on 15 August, an unreported amount of people drove their cars down the street. [size= no report]" 

我试图在“size= (some quantifier)”之后提取整个匹配项，并将值存储到我的数据框的附加列中。

最后，我需要在我的数据框中编写一个遍历该列（称为“notes”）的循环，并将值“5、4、100”存储到一个新列中（称为“est_size”）。

理想情况下，我的新专栏将如下所示：

df$est_size[1] <- "around 5"
df$est_size[2] <- "nearly 4"
df$est_size[3] <- "about 100"
df$est_size[4] <- "hundreds"
df$est_size[5] <- "no report"

我尝试过/坚持的代码：

stringr::str_extract(notes[1], \w[size=]\d"

但我得到的只是 "size=" 而不是 after

的值

提前感谢您的帮助！

Answer 1

我们可以使用正则表达式环视来匹配size= 之后的一个或多个不是右方括号] 的字符

library(dplyr)
library(stringr)
df <- df %>%
  mutate(est_size = trimws(str_extract(notes, '(?<=size=)[^\\]]+')))

-输出

df                                                                                           #notes  est_size
#1                      On 5 June, some people walked down the street in this area. [size=around 5]  around 5
#2                 On 6 June, some people rode bikes down the street in this area. [size= nearly 4]  nearly 4
#3               On 7 June, some people walked into a grocery store in this area. [size= about 100] about 100
#4                 On 10 July, an hundreds of people drank water from this fountain [size=hundreds]  hundreds
#5 on 15 August, an unreported amount of people drove their cars down the street. [size= no report] no report

数据

df <- structure(list(notes = c("On 5 June, some people walked down the street in this area. [size=around 5]", 
"On 6 June, some people rode bikes down the street in this area. [size= nearly 4]", 
"On 7 June, some people walked into a grocery store in this area. [size= about 100]", 
"On 10 July, an hundreds of people drank water from this fountain [size=hundreds]", 
"on 15 August, an unreported amount of people drove their cars down the street. [size= no report]"
)), class = "data.frame", row.names = c(NA, -5L))

Answer 2

使用str_extract：

library(stringr)
trimws(str_extract(df$notes, "(?<=size=)[\\w\\s]+"))
[1] "around 5"  "nearly 4"  "about 100" "hundreds"  "no report"

在这里，我们使用正向后向 (?<=...) 来断言我们要提取的内容的伴随模式：我们要提取 size= 之后的字母数字字符串，因此我们将 size= 放入后向表达式并提取其后出现的任何字母数字字符 (\\w) 和空白字符 (\\s)（但不是特殊字符，例如 ]!）。