I want to remove everything contained within two HTML tags, as well as the tags themselves, using regular expressions in Ruby. Here's an example:
<tag>a bunch of stuff between the tags, no matter what it is</tag>
Basically, I want to use gsub! to filter all instances of this type out, like so:
text_file_contents.gsub!(/appropriate regex/, '')
What would be a good Ruby regular expression for doing so?
As has been said in the comments use an html parser. If, however, you just want to remove everything between two tags and don't care about nesting (e.g. if you have <tag><tag></tag></tag>) then you can simply use:
text_file_contents.gsub!(/<tag>.*?<\/tag>/, '')
But again this is flaky. Nokogiri is really easy to use and will be a lot more stable, please use that.
require 'nokogiri'
doc = Nokogiri::XML(yourfile)
doc.search('//tag').each do |node|
node.remove
end
Related
I'm trying to write a ruby script to turn a small markup language I wrote into HTML, but I can't figure out how to parse links. It's basically a trimmed down version of BBCode, so for example, if someone enters [i]{text}[/i], I use [i]{text}[/i].gsub('[i]','<i>').gsub('[/i]','</i>'. I can't figure out how to parse links, though. How would I turn [url=website.com]site[/url] into site? I'm not using a premade BBCode parser because there are a few tags that are different, and I don't want people to use some of the tags such as [img][/img].
Very naïvely:
s.gsub(/\[url=(.*?)\](.*?)\[\/url\]/) { "<a href='#{$1}'>#{$2}</a>" }
HTML injection would be quite easy. The point here (write a proper parser) still applies to what you're doing.
I agree with kch by using a regular expression but if you want to wrap your head around it using gsub() like you've been doing...
s = "[url=website.com]site[/url]"
s2 = s.gsub('[url=','<a href="').gsub('[/url]','</a>').gsub(']','">')
When I do
puts /<title>(.*?)<\/title>/.match(html)
I get
<h2>foobar</h2>
But I want just
foobar
What's the most elegant method for doing so?
The most elegant way would be to parse HTML with an HTML parser:
require 'nokogiri'
html = '<title><h2>Pancakes</h2></title>'
doc = Nokogiri::HTML(html)
title = doc.at('title').text
# title is now 'Pancakes'
If you try to do this with a regular expression, you will probably fail. For example, if you have an <h2> in your <title> what's to prevent you from having something like this:
<title><strong>Where</strong> is <span>pancakes</span> <em>house?</em></title>
Trying to handle something like that with a single regex is going to be ugly but doc.at('title').text handles that as easily as it handles <title>Pancakes</title> or <title><h2>Pancakes</h2></title>.
Regular expressions are great tools but they shouldn't be the only tool in your toolbox.
Something of this style will return just the contents of the match.
html[/<title>(.*?)<\/title>/,1]
Maybe you need to tell us more, like what html might contain, but right now, you are capturing the contents of the title block, irrespective of the internal tags. I think that is the way you should do it, rather than assuming that there is an internal tag you want to handle, especially because what would happen if you had two internal tags? This is why everyone is telling you to use an html parser, which you really should do.
I'm trying to retrieve every external link of a webpage using Ruby. I'm using String.scan with this regex:
/href="https?:[^"]*|href='https?:[^']*/i
Then, I can use gsub to remove the href part:
str.gsub(/href=['"]/)
This works fine, but I'm not sure if it's efficient in terms of performance. Is this OK to use or I should work with a more specific parser (nokogiri, for example)? Which way is better?
Thanks!
Using regular expressions is fine for a quick and dirty script, but Nokogiri is very simple to use:
require 'nokogiri'
require 'open-uri'
fail("Usage: extract_links URL [URL ...]") if ARGV.empty?
ARGV.each do |url|
doc = Nokogiri::HTML(open(url))
hrefs = doc.css("a").map do |link|
if (href = link.attr("href")) && !href.empty?
URI::join(url, href)
end
end.compact.uniq
STDOUT.puts(hrefs.join("\n"))
end
If you want just the method, refactor it a little bit to your needs:
def get_links(url)
Nokogiri::HTML(open(url).read).css("a").map do |link|
if (href = link.attr("href")) && href.match(/^https?:/)
href
end
end.compact
end
I'm a big fan of Nokogiri, but why reinvent the wheel?
Ruby's URI module already has the extract method to do this:
URI::extract(str[, schemes][,&blk])
From the docs:
Extracts URIs from a string. If block given, iterates through all matched URIs. Returns nil if block given or array with matches.
require "uri"
URI.extract("text here http://foo.example.org/bla and here mailto:test#example.com and here also.")
# => ["http://foo.example.com/bla", "mailto:test#example.com"]
You could use Nokogiri to walk the DOM and pull all the tags that have URLs, or have it retrieve just the text and pass it to URI.extract, or just let URI.extract do it all.
And, why use a parser, such as Nokogiri, instead of regex patterns? Because HTML, and XML, can be formatted in a lot of different ways and still render correctly on the page or effectively transfer the data. Browsers are very forgiving when it comes to accepting bad markup. Regex patterns, on the other hand, work in very limited ranges of "acceptability", where that range is defined by how well you anticipate the variations in the markup, or, conversely, how well you anticipate the ways your pattern can go wrong when presented with unexpected patterns.
A parser doesn't work like a regex. It builds an internal representation of the document and then walks through that. It doesn't care how the file/markup is laid out, it does its work on the internal representation of the DOM. Nokogiri relaxes its parsing to handle HTML, because HTML is notorious for being poorly written. That helps us because with most non-validating HTML Nokogiri can fix it up. Occasionally I'll encounter something that is SO badly written that Nokogiri can't fix it correctly, so I'll have to give it a minor nudge by tweaking the HTML before I pass it to Nokogiri; I'll still use the parser though, rather than try to use patterns.
Mechanize uses Nokogiri under the hood but has built-in niceties for parsing HTML, including links:
require 'mechanize'
agent = Mechanize.new
page = agent.get('http://example.com/')
page.links_with(:href => /^https?/).each do |link|
puts link.href
end
Using a parser is generally always better than using regular expressions for parsing HTML. This is an often-asked question here on Stack Overflow, with this being the most famous answer. Why is this the case? Because constructing a robust regular expression that can handle real-world variations of HTML, some valid some not, is very difficult and ultimately more complicated than a simple parsing solution that will work for just about all pages that will render in a browser.
why you dont use groups in your pattern?
e.g.
/http[s]?:\/\/(.+)/i
so the first group will already be the link you searched for.
Can you put groups in your regex? That would reduce your regular expressions to 1 instead of 2.
I have a very large xml file which I load as a string
so my XML lools like
<publication ID="7728" contentstatus="Unchanged" idID="0b000064800e9e39">
<volume contentstatus="Unchanged" idID="0b0000648151c35d">
<article ID="5756261" contentstatus="Changed" doi="10.1109/TNB.2011.2145270" idID="0b0000648151d8ca"/>
</volume>
I want to count the number of occurrences the string
article ID="5705641" contentstatus="Changed"
how can I convert the ID to a regex
Here is what I have tried doing
searchstr = 'article ID=\"/[1-9]{7}/\" contentstatus=\"Changed\"'
count = ((xml.scan(searchstr).length)).to_s
puts count
Please let me know how can I achieve this?
Thanks
I'm going to go out on a limb and guess that you're new to Ruby. First, it's not necessary to convert count into a string to puts it. Puts automatically calls to_s on anything you send to it.
Second, it's rarely a good idea to handle XML with string manipulation. I would strongly advise that you use a full fledged XML parser such as Nokogiri.
That said, you can't embed a regex in a string like that. The entire query string would need to be a regex.
Something like
/article ID="[1-9]{7}" contentstatus="Changed"/
Quotation marks aren't special characters in a regex, so you don't need to escape them.
When in doubt about regex in Ruby, I recommend checking out Rubular.com.
And once again, I can't emphasize enough that I really don't condone trying to manipulate XML via regex. Nokogiri will make dealing with XML a billion times easier and more reliable.
If XPath is an option, it is a preferred way of selecting XML elements. You can use the selector:
//article[#contentstatus="Changed"]
Or, if possible:
count(//article[#contentstatus="Changed"])
Nokogiri is my recommended Ruby XML parser. It's very robust, and is probably the standard for the language now.
I added two more "articles" to show how easily you can find and manipulate the contents, without having to rely on a regex.
require 'nokogiri'
xml =<<EOT
<publication ID="7728" contentstatus="Unchanged" idID="0b000064800e9e39">
<volume contentstatus="Unchanged" idID="0b0000648151c35d">
<article ID="5756261" contentstatus="Changed" doi="10.1109/TNB.2011.2145270" idID="0b0000648151d8ca"/>
<article ID="5756262" contentstatus="Unchanged" doi="10.1109/TNB.2011.2145270" idID="0b0000648151d8ca"/>
<article ID="5756263" contentstatus="Changed" doi="10.1109/TNB.2011.2145270" idID="0b0000648151d8ca"/>
</volume>
EOT
doc = Nokogiri::XML(xml)
puts doc.search('//article[#contentstatus="Changed"]').size.to_s + ' found'
puts doc.search('//article[#contentstatus="Changed"]').map{ |n| "#{ n['ID'] } #{ n['doi'] } #{ n['idID'] }" }
>> 2 found
>> 5756261 10.1109/TNB.2011.2145270 0b0000648151d8ca
>> 5756263 10.1109/TNB.2011.2145270 0b0000648151d8ca
The problem with using regex with HTML or XML, is they'll break really easily if the XML changes, or if your XML comes from different sources or is malformed. Regex was never designed to handle that sort of problem, but a parser was. You could have XML with line ends after every tag, or none at all, and the parser won't really care as long as the XML is well-formed. A good parser, like Nokogiri can even do fixups if the XML is broken, in order to try to make sense of it, but
Your current string looks almost perfect to me, just remove the errant / from around the numbers:
searchstr = 'article ID=\"[1-9]{7}\" contentstatus=\"Changed\"'
I'm using nokogiri to select the 'keywords' attribute like this:
puts page.parser.xpath("//meta[#name='keywords']").to_html
One of the pages I'm working with has the keywords label with a capital "K" which has motivated me to make the query case insensitive.
<meta name="keywords"> AND <meta name="Keywords">
So, my question is: What is the best way to make a nokogiri selection case insensitive?
EDIT Tomalak's suggestion below works great for this specific problem. I'd like to also use this example to help understand nokogiri better though and have a couple issues that I'm wondering about and have not been successful searching for. For example, are the regex 'pseudo classes' Nokogiri Docs appropriate for a problem like this?
I'm also curious about the matches?() method in nokogiri. I have not been able to find any clarification on the method. Does it have anything to do with the 'matches' concept in XPath 2.0 (and therefore could it be used to solve this problem)?
Thanks very much.
Nokogiri allows custom XPath functions. The nokogiri docs that you link to show an inline class definition for when you're only using it once. If you have a lot of custom functions or if you use the case-insensitive match a lot, you may want to define it in a class.
class XpathFunctions
def case_insensitive_equals(node_set, str_to_match)
node_set.find_all {|node| node.to_s.downcase == str_to_match.to_s.downcase }
end
end
Then call it like any other XPath function, passing in an instance of your class as the 2nd argument.
page.parser.xpath("//meta[case_insensitive_equals(#name,'keywords')]",
XpathFunctions.new).to_html
In your Ruby method, node_set will be bound to a Nokogiri::XML::NodeSet. In the case where you're passing in an attribute value like #name, it will be a NodeSet with a single Nokogiri::XML::Attr. So calling to_s on it gives you its value. (Alternatively, you could use node.value.)
Unlike using XPath translate where you have to specify every character, this works on all the characters and character encodings that Ruby works on.
Also, if you're interested in doing other things besides case-insensitive matching that XPath 1.0 doesn't support, it's just Ruby at this point. So this is a good starting point.
Wrapped for legibility:
puts page.parser.xpath("
//meta[
translate(
#name,
'ABCDEFGHIJKLMNOPQRSTUVWXYZ',
'abcdefghijklmnopqrstuvwxyz'
) = 'keywords'
]
").to_html
There is no "to lower case" function in XPath 1.0, so you have to use translate() for this kind of thing. Add accented letters as necessary.