Selecting elements with specific namespaced attributes - ruby

Question

My problem is specifically about some trouble I'm having parsing an Inkscape (XML) file, but it's solution should be applicable to any XML doc, so I feel it's Stackoverflow relevant.
I'm trying to use the Nokogiri CSS selectors to get all the <g> elements that have the attribute inkscape:groupmode="layer". But the colon is causing the error:
Nokogiri::CSS::SyntaxError: unexpected ':' after 'inkscape'
My XML document looks like:
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!-- Created with Inkscape (http://www.inkscape.org/) -->
<svg xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:cc="http://creativecommons.org/ns#" xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:svg="http://www.w3.org/2000/svg" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:sodipodi="http://sodipodi.sourceforge.net/DTD/sodipodi-0.dtd" xmlns:inkscape="http://www.inkscape.org/namespaces/inkscape" width="744.09448819" height="1052.3622047" id="svg3720" version="1.1" inkscape:version="0.48.1 r9760" sodipodi:docname="test.svg">
<defs id="defs3722">
<inkscape:perspective sodipodi:type="inkscape:persp3d" inkscape:vp_x="0 : 526.18109 : 1" inkscape:vp_y="0 : 1000 : 0" inkscape:vp_z="744.09448 : 526.18109 : 1" inkscape:persp3d-origin="372.04724 : 350.78739 : 1" id="perspective3728"/>
</defs>
<sodipodi:namedview id="base" pagecolor="#ffffff" bordercolor="#666666" borderopacity="1.0" inkscape:pageopacity="0.0" inkscape:pageshadow="2" inkscape:zoom="0.35" inkscape:cx="375" inkscape:cy="634.28571" inkscape:document-units="px" inkscape:current-layer="g2818" showgrid="false" inkscape:window-width="550" inkscape:window-height="483" inkscape:window-x="66" inkscape:window-y="471" inkscape:window-maximized="0"/>
<metadata id="metadata3725">
<rdf:RDF>
<cc:Work rdf:about="">
<dc:format>image/svg+xml</dc:format>
<dc:type rdf:resource="http://purl.org/dc/dcmitype/StillImage"/>
<dc:title/>
</cc:Work>
</rdf:RDF>
</metadata>
<g inkscape:label="Layer 1" inkscape:groupmode="layer" id="layer1">
<rect style="fill:#d2e149;fill-opacity:1;stroke:none" id="rect2812" width="211.42857" height="128.57143" x="168.57143" y="215.21933" ry="64.285713"/>
</g>
<g inkscape:label="Layer 1 copy copy" inkscape:groupmode="layer" id="g2818">
<rect style="fill:#d2e149;fill-opacity:1;stroke:none" id="rect2820" width="211.42857" height="128.57143" x="145.71428" y="615.2193" ry="64.285713"/>
</g>
</svg>
My selector looks like:
nokogiri_document.css('[inkscape:groupmode="layer"]').to_html
I also tried replacing the colon with a pipe
How do I write the CSS selector to work on the inkscape:groupmode attribute...or for that matter any foo:bar attribute?

Answer 1

Use XPath, specifying the namespace for the g elements. Since your root element declares the xmlns:svg to be the same as the new default namespace (xmlns) you can use svg as your prefix:
require 'nokogiri'
doc = Nokogiri.XML(IO.read('contents.xml'))
layers = doc.xpath('//svg:g[#inkscape:groupmode="layer"]')
p layers.map{ |layer| layer['id'] }
#=> ["layer1", "g2818"]
Decoded, the above XPath says:
// - At any level of the document
svg:g - …find g elements with a namespace matching the svg namespace
[…] - …but only if the contents of this are met
#inkscape:groupmode - …there is an attribute (#) named groupmode with a namespace matching inkscape
="layer" - and the intrinsic value of this attribute is the text layer.
Alternatively, if you're just trying to read this file (and not manipulate and re-save it) you can use the gross-but-simplifying hack of removing all namespaces. In this case, your original code works simply:
doc.remove_namespaces!
p doc.css('g[groupmode="layer"]').map{ |g| g['id'] }
#=> ["layer1", "g2818"]

Answer 2

I suggest that you try to use XPath. Look at this snippet:
require 'nokogiri'
doc = Nokogiri::XML(File.read('your_file.xml'))
doc.xpath('//xmlns:g[starts-with(#inkscape:label, "Layer")]').size # => 2
Please pay attention at xmlns in XPath expression. Because XPath query looks for elements that are not in any namespace, you need to tell your XPath processor that you are looking for elements in given namespace. You can do this by several ways. I use most simple case - the using the default namespace in XPath query. Also you can define a custom namespace in the second argument of the XPath method call and use that in the query.