Tham chiếu ký tự XML

What Are XML Character References?

XML character references are escape sequences that represent Unicode characters by code point within XML documents. The XML specification defines two forms:

• Decimal: &#N; — e.g., © for ©
• Hexadecimal: &#xH; — e.g., © for ©

Unlike HTML, XML does not support named references beyond the five predefined entities (&, <, >, ", '). All other characters must be referenced by number or defined as custom entities in the document's DTD.

XML vs. HTML Character References

HTML inherits XML-style numeric references but extends them with thousands of named references defined in the HTML specification. XML keeps a stricter, self-contained model: a parser needs no external lookup table to handle character references, only the rules for decimal and hex integers.

<!-- Valid XML character references -->
&#65;       A
&#x41;      A
&#169;      ©
&#xA9;      ©
&#x1F600;   😀

<!-- Named references in XML — only these 5 are built-in -->
&amp;   &
&lt;    <
&gt;    >
&quot;  "
&apos;  '

<!-- All others require a DTD declaration, e.g.: -->
<!DOCTYPE doc [
  <!ENTITY copy "&#xA9;">
]>
<doc>Copyright &copy; 2024</doc>

Well-Formedness Rules

For XML to be well-formed, character references must: 1. Reference a legal XML character — code points U+0009, U+000A, U+000D, U+0020–U+D7FF, U+E000–U+FFFD, and U+10000–U+10FFFF. 2. End with a semicolon ; — the semicolon is always mandatory in XML (no legacy exceptions). 3. Not reference surrogates (U+D800–U+DFFF) or the null character (U+0000).

An XML parser that encounters an invalid reference is required to raise a fatal error and halt parsing.

Encoding Interaction

XML documents declare their encoding in the XML declaration:

<?xml version="1.0" encoding="UTF-8"?>

Character references bypass encoding: € always means U+20AC (€) regardless of the document's encoding. This makes character references useful in legacy encodings that cannot natively represent the desired characters.

Parsing in Python

import xml.etree.ElementTree as ET

xml_str = "<item>Price: &#x20AC;10 &#38; shipping</item>"
root = ET.fromstring(xml_str)
print(root.text)  # "Price: €10 & shipping"

# Python's xml module decodes references automatically

SVG and XML-Based Formats

SVG, MathML, XHTML, RSS, and Atom are all XML vocabularies. Character references work identically in all of them:

<!-- SVG text with Unicode arrows -->
<text>&#x2190; Left &#x2192; Right</text>

<!-- Atom feed with special characters -->
<title>Q&#38;A: Unicode &#x2014; Explained</title>

CDATA Sections

Inside a CDATA section (<![CDATA[ ... ]]>), character references are not processed — the text is treated as raw character data. This is useful for embedding code samples in XML:

<code><![CDATA[
  if (a < b && c > d) { return "&#xA9;"; }
]]></code>
<!-- The & and < above are literal characters, NOT entities -->

Quick Facts