在特定类之后从href标签获取文本

Question

我正在尝试抓取网页

library(RCurl)
webpage <- getURL("https://somewebpage.com")

webpage

<div class='CredibilityFacts'><span id='qZyoLu'><a class='answer_permalink'
action_mousedown='AnswerPermalinkClickthrough' href='/someurl/answer/my_id' 
id ='__w2_yeSWotR_link'>
<a class='another_class' action_mousedown='AnswerPermalinkClickthrough' 
href='/ignore_url/answer/some_id' id='__w2_ksTVShJ_link'>
<a class='answer_permalink' action_mousedown='AnswerPermalinkClickthrough' 
href='/another_url/answer/new_id' id='__w2_ksTVShJ_link'>

class(webpage)
[1] "character"

我正在尝试提取所有 href 值，但前提是它前面带有 answer_permalink 类。

这个的输出应该是

[1] "/someurl/answer/my_id"  "/another_url/answer/new_id"

/ignore_url/answer/some_id 应该被忽略，因为它前面是 another_class 而不是 answer_permalink 类。

现在，我正在考虑使用正则表达式的方法。我认为这样的东西可以用于stri_extract_all中的正则表达式

class='answer_permalink'.*href='

但这并不是我想要的。

我可以通过什么方式实现这一目标？此外，除了正则表达式之外，R 中还有一个函数可以像 Javascript 中那样按类提取元素吗？

Answer 1

您可以使用rvest 或xml2 之类的包来代替字符串解析：

library(xml2)
xml <- read_html(webpage)
l <- as_list(xml)[[1]][[1]][[1]][[1]]  #not sure why you need to go this deep.

l2 <- l[sapply(l, attr, ".class") == "answer_permalink"]
sapply(l2, attr, "href")

                       a                            a 
 "/someurl/answer/my_id" "/another_url/answer/new_id"

Answer 2

有了dplyr 和rvest，我们可以这样做：

library(rvest)
library(dplyr)

"https://www.quora.com/profile/Ronak-Shah-96" %>% 
  read_html() %>% 
  html_nodes("[class='answer_permalink']") %>% 
  html_attr("href")

[1] "/How-can-we-adjust-in-engineering-if-we-are-not-in-IITs-or-NITs-How-can-we-enjoy-engineering-if-we-are-pursuing-it-from-a-local-private-college/answer/Ronak-Shah-96"                                                                        
[2] "/Do-you-think-it-is-worth-it-to-change-my-career-path-For-the-past-2-years-I-was-pursuing-a-career-in-tax-advisory-in-a-BIG4-company-I-just-got-a-job-offer-that-will-allow-me-to-learn-coding-It-is-not-that-well-paid/answer/Ronak-Shah-96"
[3] "/Why-cant-India-opt-for-40-hours-work-a-week-for-all-professions-when-it-is-proved-and-working-well-in-terms-of-efficiency/answer/Ronak-Shah-96"

[4] "/Why-am-I-still-confused-and-thinking-about-my-career-after-working-more-than-one-year-in-software-engineering/answer/Ronak-Shah-96"

[5] "/Would-you-rather-be-a-jack-of-all-trades-or-the-master-of-one-trade/answer/Ronak-Shah-96"

Answer 3

require(XML)
require(RCurl)

doc <- getURL("https://www.quora.com/profile/Ronak-Shah-96" )
html <- htmlTreeParse(doc, useInternalNodes = TRUE)
nodes <- getNodeSet(html, "//a[@class='answer_permalink']")
sapply(nodes, function(x) x <- xmlAttrs(x)[["href"]])

[1] "/Do-you-think-it-is-worth-it-to-change-my-career-path-For-the-past-2-years-I-was-pursuing-a-career-in-tax-advisory-in-a-BIG4-company-I-just-got-a-job-offer-that-will-allow-me-to-learn-coding-It-is-not-that-well-paid/answer/Ronak-Shah-96"
[2] "/Why-cant-India-opt-for-40-hours-work-a-week-for-all-professions-when-it-is-proved-and-working-well-in-terms-of-efficiency/answer/Ronak-Shah-96"                                                                                             
[3] "/Why-am-I-still-confused-and-thinking-about-my-career-after-working-more-than-one-year-in-software-engineering/answer/Ronak-Shah-96"                                                                                                         
[4] "/Would-you-rather-be-a-jack-of-all-trades-or-the-master-of-one-trade/answer/Ronak-Shah-96"                                                                                                                                                   
[5] "/Is-software-engineering-a-good-career-choice-I-know-it-pays-well-initially-but-if-you-look-at-the-managing-directors-of-most-companies-they-are-people-with-MBAs/answer/Ronak-Shah-96"