XPath nic nevrací: výchozí jmenný prostor

<?xml version="1.0" encoding="UTF-8"?>
<feed xmlns="http://www.w3.org/2005/Atom">
  <title>Novinky XML.cz</title>
  <entry>
    <title>JSON v PostgreSQL</title>
    <link href="https://xml.cz/programovani/postgresql/"/>
  </entry>
  <entry>
    <title>Billion laughs</title>
    <link href="https://xml.cz/xxe/"/>
  </entry>
</feed>

//entry/title

//atom:entry/atom:title      (prefix atom → http://www.w3.org/2005/Atom)

const doc = new DOMParser().parseFromString(xml, "application/xml");
const resolver = prefix => ({ atom: "http://www.w3.org/2005/Atom" })[prefix] ?? null;

const r = doc.evaluate("//atom:entry/atom:title", doc, resolver,
  XPathResult.ORDERED_NODE_SNAPSHOT_TYPE, null);
for (let i = 0; i < r.snapshotLength; i++) console.log(r.snapshotItem(i).textContent);

doc.evaluate("//atom:entry", doc, null, XPathResult.ANY_TYPE, null);
// NamespaceError: … '//atom:entry' contains unresolvable namespaces.

var doc = new XmlDocument();
doc.Load("feed.xml");
doc.SelectNodes("//entry/title")!.Count;                 // 0

var nsm = new XmlNamespaceManager(doc.NameTable);
nsm.AddNamespace("atom", "http://www.w3.org/2005/Atom");
foreach (XmlNode t in doc.SelectNodes("//atom:entry/atom:title", nsm)!)
    Console.WriteLine(t.InnerText);

// LINQ to XML bez XPath: jméno = XNamespace + lokální jméno
XNamespace a = "http://www.w3.org/2005/Atom";
var hrefs = XDocument.Load("feed.xml")
    .Descendants(a + "entry").Elements(a + "link").Attributes("href");

DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
dbf.setNamespaceAware(true);           // výchozí je false
Document doc = dbf.newDocumentBuilder().parse(new File("feed.xml"));

XPath xp = XPathFactory.newInstance().newXPath();
xp.setNamespaceContext(new NamespaceContext() {
  public String getNamespaceURI(String prefix) {
    return "atom".equals(prefix) ? "http://www.w3.org/2005/Atom" : XMLConstants.NULL_NS_URI;
  }
  public String getPrefix(String uri) { return null; }
  public Iterator<String> getPrefixes(String uri) { return null; }
});
NodeList tituly = (NodeList) xp.evaluate("//atom:entry/atom:title", doc, XPathConstants.NODESET);

import xml.etree.ElementTree as ET   # cizí vstup: defusedxml.ElementTree, stejné API

strom = ET.parse("feed.xml")
strom.findall(".//entry/title")                                   # []

ns = {"atom": "http://www.w3.org/2005/Atom"}
for t in strom.findall(".//atom:entry/atom:title", ns):
    print(t.text)

# Python 3.8+: klíč "" nastaví výchozí namespace pro jména bez prefixu
strom.findall(".//entry/title", {"": "http://www.w3.org/2005/Atom"})

# lxml (plné XPath 1.0): root.xpath("//atom:entry/atom:title", namespaces=ns)

$doc = new DOMDocument();
$doc->load('feed.xml');
$xp = new DOMXPath($doc);
$xp->query('//entry/title')->length;                       // 0

$xp->registerNamespace('atom', 'http://www.w3.org/2005/Atom');
foreach ($xp->query('//atom:entry/atom:title') as $t) echo $t->textContent, "\n";

$sx = simplexml_load_file('feed.xml');
$sx->registerXPathNamespace('a', 'http://www.w3.org/2005/Atom');
foreach ($sx->xpath('//a:entry/a:link/@href') as $href) echo $href, "\n";

SELECT xpath('//a:entry/a:title/text()', doc,
             ARRAY[ARRAY['a', 'http://www.w3.org/2005/Atom']])
FROM feed;

//*:entry/*:title

//Q{http://www.w3.org/2005/Atom}entry/Q{http://www.w3.org/2005/Atom}title

//*[local-name()='entry']/*[local-name()='title']

//*:entry/*:link/@href