在R中使用Perl正则表达式进行刮擦

Question

当你刮掉R中的链接时使用rvest或RSelenium，你可以通过定义HTML代码的开头部分来实现，例如：给定节点内的href。如果我面对以下链接怎么办？

<a href="www.website.com" data-tracking="click_body" data-tracking- 
data='{"touch_point_button":"photo"}' data-featured-name="listing_no_promo" >

如果我想不抓取促销链接，那么我将使用（来自XML和httr包）以下代码：

library(XML)
library(httr)
response <- GET(yourLink)
parsedoc <- htmlParse(response)
xpathSApply(parsedoc, "//a[@data-featured-tracking='listing_no_promo']", 
xmlGetAttr, "href")

如果我想获得以xpath的'photo'部分结尾的链接，我该怎么办？

data-tracking- data='{"touch_point_button":"photo"}'

不关心促销或没有促销部分？我的猜测是，花括号在这里会产生一些噪音。

Answer 1

我假设你的示例链接结构实际上如下（其中data-tracking-data是实际属性：

<a href="www.website.com" data-tracking="click_body" data-tracking-data=\'{"touch_point_button":"photo"}\' data-featured-name="listing_no_promo">link</a>

由于我不知道您正在使用哪个网站，因此我通过将链接添加到此页面的正文来重新创建了一个html文档：

# I'm going to use the jsonlite and xml2 packages

library(jsonlite)
library(xml2)

# This page
stack_url <- "https://stackoverflow.com/questions/40934644/xpath-for-element-whose-attribute-value-ends-with-a-specific-string"

# Your html element example
test_a <- '<a href="www.website.com" data-tracking="click_body" data-tracking-data=\'{"touch_point_button":"photo"}\' data-featured-name="listing_no_promo" >link</a>'

# read in stackoverflow page
raw_page <- read_html(stack_url)
# read in the element a
raw_a <- read_html(test_a)

# add the link element from example to raw_page
xml_add_child(raw_page, raw_a)
# This is just to show that the tag you provided is mixed in with multiple link elements... since this would be the case in your actual use i assume
xml_find_all(raw_page,".//a") %>% tail()

{xml_nodeset (6)}
[1] <a href="https://www.facebook.com/officialstackoverflow/" class="-link">Facebook</a>
[2] <a href="https://twitter.com/stackoverflow" class="-link">Twitter</a>
[3] <a href="https://linkedin.com/company/stack-overflow" class="-link">LinkedIn</a>
[4] <a href="https://creativecommons.org/licenses/by-sa/3.0/" rel="license">cc by-sa 3.0</a>
[5] <a href="https://stackoverflow.blog/2009/06/25/attribution-required/" rel="license">attribution required</a>
[6] <a href="www.website.com" data-tracking="click_body" data-tracking-data='{"touch_point_button":"photo"}' data-f ...

所以我们的xml_document现在存储到raw_page，我们将使用xpath找到我们想要的东西

.//a[attribute::*[contains(.,'{') or contains(.,'photo')] and @data-tracking]

# Our xpath pattern reads as:
# 
# - .//a[ -> find all 'a' html elements where
# - attribute::*[contains(.,'{') or contains(.,'photo')] -> any(*) attribute containing either a '{' OR the string 'photo'
# - and @data-tracking -> and the element must have the attribute data-tracking, but it doesn't matter what the value is
# - ] -> end

在短期内： 查找具有数据跟踪属性且具有包含单词photo或字符{的属性的所有链接。

our_xpath <- ".//a[attribute::*[contains(.,'{') or contains(.,'photo')] and @data-tracking]"
# Extract all of the matching elements using our xpath
# Get all the attribute values for data-tracking-data
# Parse from JSON
xml_find_all(raw_page,our_xpath) %>% xml_attr("data-tracking-data") %>% fromJSON()

结果如下：

$touch_point_button
[1] "photo"

我无法对你的页面进行测试......但是如果你发布了网址，我会很乐意确保它能够正常运行。

Answer 2

//*[ends-with(@data-tracking-data, '"photo"}')]/@href

从您的示例中，如果数据固定数据以字符串"photo"}结尾，则此xpath将为您提供href属性