strsplit 与 gregexpr 不一致

一条评论 https://stackoverflow.com/questions/23961022/split-strings-on-first-and-last-commas/23964843?noredirect=1#comment36923487_23964843关于我的回答这个问题 https://stackoverflow.com/a/23964843/1478381 which should使用给出所需的结果strsplit不，尽管它seems正确匹配字符向量中的第一个和最后一个逗号。这可以证明使用gregexpr and regmatches.

那么为什么strsplit在此示例中按每个逗号进行拆分，即使regmatches只返回两个匹配项same regex?

#  We would like to split on the first comma and
#  the last comma (positions 4 and 13 in this string)
x <- "123,34,56,78,90"

#  Splits on every comma. Must be wrong.
strsplit( x , '^\\w+\\K,|,(?=\\w+$)' , perl = TRUE )[[1]]
#[1] "123" "34"  "56"  "78"  "90" 


#  Ok. Let's check the positions of matches for this regex
m <- gregexpr( '^\\w+\\K,|,(?=\\w+$)' , x , perl = TRUE )

# Matching positions are at
unlist(m)
[1]  4 13

#  And extracting them...
regmatches( x , m )
[[1]]
[1] "," ","

啊？！到底是怎么回事？

@Aprillion 的理论是准确的，来自R 文档 http://stat.ethz.ch/R-manual/R-devel/library/base/html/strsplit.html:

应用于每个输入字符串的算法是

repeat {
    if the string is empty
        break.
    if there is a match
        add the string to the left of the match to the output.
        remove the match and all to the left of it.
    else
        add the string to the output.
        break.
}

换句话说，在每次迭代时^将匹配新字符串的开头（没有前面的项目。）

为了简单地说明这种行为：

> x <- "12345"
> strsplit( x , "^." , perl = TRUE )
[[1]]
[1] "" "" "" "" ""

Here https://stackoverflow.com/a/15578980/980833，您可以使用前瞻断言作为分隔符来查看此行为的结果（感谢@JoshO'Brien 提供的链接。）