searchable.rb error

[Luciano Soares]

Hi folks!

I have an only-5-lines code above project,

require ‘Nokogiri’
require ‘open-uri’
cagometro = Nokogiri::HTML(URI.open(“http://www.cagometro.com/”))
cagadas = cagometro.xpath("")
puts cagadas

And I’m getting the error

Traceback (most recent call last):
4: from parser.rb:4:in <main>' 3: from C:/Ruby26-x64/lib/ruby/gems/2.6.0/gems/nokogiri-1.10.10-x64-mingw32/lib/nokogiri/xml/searchable.rb:154:in xpath’
2: from C:/Ruby26-x64/lib/ruby/gems/2.6.0/gems/nokogiri-1.10.10-x64-mingw32/lib/nokogiri/xml/searchable.rb:179:in xpath_internal' 1: from C:/Ruby26-x64/lib/ruby/gems/2.6.0/gems/nokogiri-1.10.10-x64-mingw32/lib/nokogiri/xml/searchable.rb:198:in xpath_impl’
C:/Ruby26-x64/lib/ruby/gems/2.6.0/gems/nokogiri-1.10.10-x64-mingw32/lib/nokogiri/xml/searchable.rb:198:in `evaluate’: ERROR: Invalid expression: (Nokogiri::XML::XPath::SyntaxError)

Since the error is mentioning searchable.rb in nokogiri project, and is not mentioning any line of my own code, I’m thinking that this can be a nokogiri’s bug?

There is something that I can do by my side to avoid this?

Thanks in advance!

[Mike Dalessio]

Hi! Thanks for asking this question.

If you read the error carefully, you'll see that the library is raising an exception:

> ERROR: Invalid expression: (Nokogiri::XML::XPath::SyntaxError)

What this means is that the expression you passed into the `xpath` method is not a valid query. If you're unfamiliar with XPath queries, you may want to read a tutorial or the spec. Alternatively, if you're more familiar with CSS queries, you could try using the `css` method instead.

To get this to run without error, try passing in a string like "//a" which will find all anchor tags.

Hope this helps!

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[Wayne Brissette]

To add a bit more context around what Mike stated.

Right now you have an invalid xpath. Here is how you would parse all the divs on that page both using CSS and XPATH.

require "nokogiri"

require "open-uri"

doc = Nokogiri::HTML(URI.open("http://www.cagometro.com/"))


doc.css('div').each do |node|
  puts node.text
end


doc.xpath('.//div').each do |node|
  puts node.text
end

This walks through each div on the page and print out the text contents of it (there are 12 div nodes on that page). Nothing fancy. Now, let's say we want only H2s that fall within a div do this:

require "nokogiri"

require "open-uri"

doc = Nokogiri::HTML(URI.open("http://www.cagometro.com/"))

puts 'From css:'

doc.css('div > h2').each do |node|
  puts node.text
end

puts 'From xpath:'

doc.xpath('.//div/h2').each do |node|
  puts node.text
end

Results from both:

>From css:
O contador de cagadas oficial do Governo Bolsonaro
Confira todas as cagadas mês a mês:
>From xpath:
O contador de cagadas oficial do Governo Bolsonaro
Confira todas as cagadas mês a mês:


2 nodes total.

Finally, if you want to try to find an attribute just use:

require "nokogiri"


require "open-uri"

doc = Nokogiri::HTML(URI.open("http://www.cagometro.com/"))


puts 'From css:'

doc.css('div > h2[@class="site-description"]').each do |node|
  puts node.text
end

puts 'From xpath:'

doc.xpath('.//div/h2[@class="site-description"]').each do |node|
  puts node.text
end

Again nothing fancy here, and the results are:

>From css:
O contador de cagadas oficial do Governo Bolsonaro
>From xpath:
O contador de cagadas oficial do Governo Bolsonaro


There are a lot of various sites including the Nokogiri site which walks you through a bunch of examples similar to this and I highly recommend you spend some time walking through their examples. I find for most things CSS to be easier, but there are times xpath works better for me personally. I will warn you though, xpath support in everything short of saxon, is limited to version 1.0 of the language. One of my colleagues thinks it's possible to hook in the 2.0 or 3.0 support, but it requires some major surgery and we've not needed it since Ruby can do many of the things already that XSLT 2.0 and 3.0 added.

-Wayne