regex - strsplit 与 gregexpr 不一致

Question

A comment关于我对 this question 的回答这应该使用 strsplit 给出所需的结果不会，即使它似乎正确匹配字符向量中的第一个和最后一个逗号。这可以使用 gregexpr 证明和 regmatches .

那么为什么strsplit在此示例中，在每个逗号上拆分，即使 regmatches只为同一个正则表达式返回两个匹配项？

#  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] "," ","

嗯？!到底是怎么回事？

Answer 1

@Aprillion 的理论是准确的，来自 R documentation :

The algorithm applied to each input string is

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，在每次迭代中 ^将匹配一个新字符串的开头(没有前面的项目。)

简单说明这种行为:

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

Here ，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 提供链接。)