prefix-from-QName()

Description

prefix-from-QName() returns the prefix component of an xs:QName value as an xs:NCName. The prefix is the part before the colon in a prefixed name. If the QName was created without a prefix — for example with QName('http://example.com', 'item') — the function returns the empty sequence, not a zero-length string.

Prefixes are not inherently meaningful in XML; what matters is the namespace URI. Consequently this function is primarily used for producing human-readable output or for round-tripping serialized QNames back to string form. Do not use prefix equality to compare namespaces; use namespace-uri-from-QName() instead.

Parameters

Return value

xs:NCName? — the prefix of the QName, or the empty sequence if the QName has no prefix or the argument is the empty sequence.

Examples

Extracting prefix from a constructed QName

Stylesheet:

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="2.0"
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xsl:output method="xml" indent="yes"/>

  <xsl:template match="/">
    <prefixes>
      <with-prefix>
        <xsl:value-of select="prefix-from-QName(QName('http://example.com', 'ex:item'))"/>
      </with-prefix>
      <without-prefix>
        <xsl:value-of select="(prefix-from-QName(QName('http://example.com', 'item')), 'none')[1]"/>
      </without-prefix>
    </prefixes>
  </xsl:template>
</xsl:stylesheet>

Output:

<prefixes>
  <with-prefix>ex</with-prefix>
  <without-prefix>none</without-prefix>
</prefixes>

Round-tripping a QName to string

Stylesheet:

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="2.0"
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xsl:output method="text"/>

  <xsl:function name="f:qname-to-string" as="xs:string"
    xmlns:f="urn:functions">
    <xsl:param name="q" as="xs:QName"/>
    <xsl:variable name="p" select="prefix-from-QName($q)"/>
    <xsl:sequence select="if ($p) then concat($p, ':', local-name-from-QName($q)) else string(local-name-from-QName($q))"/>
  </xsl:function>

  <xsl:template match="/" xmlns:f="urn:functions">
    <xsl:value-of select="f:qname-to-string(QName('http://example.com', 'ex:order'))"/>
  </xsl:template>
</xsl:stylesheet>

Output:

ex:order

Notes

• The prefix is part of the xs:QName value only if it was present when the QName was constructed; QName('http://example.com', 'item') creates a QName with no prefix.
• Avoid using prefix equality as a substitute for namespace URI equality. Two QNames with different prefixes bound to the same namespace URI are semantically identical.
• prefix-from-QName() returns the empty sequence (not an empty string) when no prefix is present. Check with exists() rather than comparing to ''.

See also

• local-name-from-QName()
• namespace-uri-from-QName()
• resolve-QName()
• QName()