Print an XML document without the XML header line at the top

The simplest way to get the XML for a Document without the leading "PI" (processing instruction) is to call to_s on the root element instead of the document itself:

require 'nokogiri'
doc = Nokogiri.XML('<hello world="true" />')

puts doc
#=> <?xml version="1.0"?>
#=> <hello world="true"/>

puts doc.root
#=> <hello world="true"/>

The 'correct' way to do it at the document or builder level, though, is to use SaveOptions:

formatted_no_decl = Nokogiri::XML::Node::SaveOptions::FORMAT +
                    Nokogiri::XML::Node::SaveOptions::NO_DECLARATION

puts doc.to_xml( save_with:formatted_no_decl )
#=> <hello world="true"/>

# Making your code shorter, but horribly confusing for future readers
puts doc.to_xml save_with:3
#=> <hello world="true"/>

Note that DocumentFragments do not automatically include this PI:

frag = Nokogiri::XML::DocumentFragment.parse('<hello world="true" />')
puts frag
#=> <hello world="true"/>

If you are seeing a PI in your fragment output, it means it was there when you parsed it.

xml = '<?xml version="1.0"?><hello world="true" />'
frag = Nokogiri::XML::DocumentFragment.parse(xml)
puts frag
#=> <?xml version="1.0"?><hello world="true"/>

If so, and you want to get rid of any PIs, you can do so should be able to do so with a little XPath:

frag.xpath('//processing-instruction()').remove
puts frag

…except that this does not appear to work due to oddness with XPath in DocumentFragments. To work around these bugs do this instead:

# To remove only PIs at the root level of the fragment
frag.xpath('processing-instruction()').remove
puts frag
#=> <hello world="true"/>

# Alternatively, to remove all PIs everywhere, including inside child nodes
frag.xpath('processing-instruction()|.//processing-instruction()').remove

If you have a Builder object, do either of:

builder = Nokogiri::XML::Builder.new{ |xml| xml.hello(world:"true") }

puts builder.to_xml
#=> <?xml version="1.0"?>
#=> <hello world="true"/>

puts builder.doc.root.to_xml
#=> <hello world="true"/>

formatted_no_decl = Nokogiri::XML::Node::SaveOptions::FORMAT +
                    Nokogiri::XML::Node::SaveOptions::NO_DECLARATION

puts builder.to_xml save_with:formatted_no_decl
#=> <hello world="true"/>