XML

## Namespaces - XPath `/root/child` fails if document has default namespace—use `//*[local-name()='child']` or register prefix - Default namespace (`xmlns="..."`) applies to elements, not attributes—attributes need explicit prefix - Namespace prefix is arbitrary—`<foo:element>` and `<bar:element>` are identical if both prefixes map to same URI - Child elements don't inherit parent's prefixed namespace—each must declare or use prefix explicitly ## Encoding - `<?xml version="1.0" encoding="UTF-8"?>` must match actual file encoding—mismatch corrupts non-ASCII - Encoding declaration must be first thing in file—no whitespace or BOM before it (except UTF-8 BOM allowed) - Default encoding is UTF-8 if declaration omitted—but explicit is safer across parsers ## Escaping & CDATA - Five entities always escape in text: `&amp;` `&lt;` `&gt;` `&quot;` `&apos;` - CDATA sections `<![CDATA[...]]>` for blocks with many special chars—but `]]>` inside CDATA breaks it - Attribute values: use `&quot;` if delimited by `"`, or `&apos;` if delimited by `'` - Numeric entities `&#60;` and `&#x3C;` work everywhere—useful for edge cases ## Whitespace - Whitespace between elements is preserved by default—pretty-printing adds nodes that may break processing - `xml:space="preserve"` attribute signals whitespace significance—but not all parsers respect it - Normalize-space in XPath: `normalize-space(text())` trims and collapses internal whitespace ## XPath Pitfalls - `//element` is expensive—traverses entire document; use specific paths when structure is known - Position is 1-indexed: `[1]` is first, not `[0]` - `text()` returns direct text children only—use `string()` or `.` for concatenated descendant text - Boolean in predicates: `[@attr]` tests existence, `[@attr='']` tests empty value—different results ## Structure - Self-closing `<tag/>` and empty `<tag></tag>` are semantically identical—but some legacy systems choke on self-closing - Comments cannot contain `--`—will break parser even inside string content - Processing instructions `<?target data?>` cannot have `?>` in data - Root element required—document with only comments/PIs and no element is invalid ## Validation - Well-formed ≠ valid—parser may accept structure but fail against schema - DTD validates but can't express complex constraints—prefer XSD or RelaxNG for new projects - XSD namespace `xmlns:xs="http://www.w3.org/2001/XMLSchema"` commonly confused with instance namespace