Question

filelist <- c(
  "http://content.caiso.com/green/renewrpt/20171015_DailyRenewablesWatch.txt",
  "http://content.caiso.com/green/renewrpt/20171016_DailyRenewablesWatch.txt",
  "http://content.caiso.com/green/renewrpt/20171017_DailyRenewablesWatch.txt",
  "http://content.caiso.com/green/renewrpt/20171018_DailyRenewablesWatch.txt",
  "http://content.caiso.com/green/renewrpt/20171019_DailyRenewablesWatch.txt",
  "http://content.caiso.com/green/renewrpt/20171020_DailyRenewablesWatch.txt",
  "http://content.caiso.com/green/renewrpt/20171021_DailyRenewablesWatch.txt",
  "http://content.caiso.com/green/renewrpt/20171022_DailyRenewablesWatch.txt"
)

我希望在/和_的第5次出现之间提取字符串

例如：来自"http://content.caiso.com/green/renewrpt/20171015_DailyRenewablesWatch.txt" 我想要20171015。我试过了

regmatches(filelist, regexpr("/{4}([^_]+)", filelist))

但它返回空。

Answer 1

这应该有效

gsub("(?:.*/){4}([^_]+)_.*", "\\1", filelist)
# [1] "20171015" "20171016" "20171017" "20171018" "20171019" "20171020" "20171021"
# [8] "20171022"

我们还需要匹配捕获中每个被削减前面的东西。

Answer 2

以下是一些使用正则表达式的方法：

sub(".*(\\d{8}).*", "\\1", filelist)

sub(".*/", "", sub("_.*", "", filelist))

sub("_.*", "", basename(filelist))

sapply(strsplit(filelist, "[/_]"), "[", 6)

gsub("\\D", "", filelist)

m <- gregexpr("\\d{8}", filelist)
unlist(regmatches(filelist, m))

strcapture("(\\d{8})", filelist, data.frame(character()))[[1]]

library(gsubfn)
strapplyc(filelist, "\\d{8}", simplify = TRUE)

这些解决方案根本不使用正则表达式：

substring(filelist, 41, 48)

substring(basename(filelist), 1, 8)

read.table(text = filelist, comment.char = "_", sep = "/")[[6]]

as.Date(basename(filelist), "%Y%m%d")  # returns Date class object

更新：添加了更多方法。

Answer 3

substr(x = filelist,
       start = sapply(gregexpr(pattern = "/", filelist), function(x) x[5])+1,
       stop = sapply(gregexpr(pattern = "_", filelist), function(x) x[1])-1)
#[1] "20171015" "20171016" "20171017" "20171018" "20171019" "20171020" "20171021"
#[8] "20171022"

Answer 4

有个函数可以先去掉url：

CREATE TRIGGER `lakcom_NewDB`.`Products_AFTER_UPDATE` 
    AFTER UPDATE ON `Products` 
    FOR EACH ROW
        BEGIN
            update Basket 
              set Basket.ToControl = 1 
              where Basket.Product_id=new.product_id 
              and Basket.id<>0;
        END

然后尝试使用 filelist <- basename(filelist) 包中的 str_remove 删除“_”之后的所有内容：

stringr

输出：

<块引用>

[1] "20171015" "20171016" "20171017" "20171018" "20171019" "20171020" "20171021" "20171022"

如果您想将其转换为日期，请检查 library(stringr) str_remove(filelist, "_.*") 包的 lubridate 函数。

在第n次出现的字符和另一个字符之间提取字符串

4 个答案: