in jekyll/plugins/markdown_converter.rb [274:290]
def parse_codeblock_fenced
if @src.check(self.class::FENCED_CODEBLOCK_MATCH)
start_line_number = @src.current_line_number
@src.pos += @src.matched_size
el = new_block_el(:codeblock, unindent(@src[5]), nil, :location => start_line_number)
lang = @src[3].to_s.strip
unless lang.empty?
el.options[:lang] = lang
el.attr['class'] = "language-#{@src[4]}"
end
@tree.children << el
true
else
false
end
end