Why does `<input pattern="^[^ ]{2,}.*[^ ]$">` trigger "XPath processing error: Binary operator expected"?

Question

I have am XML file with XSL to process it. That works fine using an input element like <input pattern="^[^ ].*[^ ]$">. However when I changed it to <input pattern="^[^ ]{2,}.*[^ ]$">, Firefox ESR 115.3.1 complained:

Fehler beim Laden des Stylesheets: XPath-Verarbeitungsfehler: Binärer Operator erwartet:

(In English (most likely): "Error loading stylesheets: XPath processing error: Binary Operator expected:")

I completely fail to understand how to debug this, what triggers the error, and how to fix it (when I added backslashed before { and }I got a different error message, saying "Invalid character found:").

What makes things harder to understand is that any details (like line, token) are absent in the error message, and when I try to display the source, I only get an empty window.

Almost Minimal Example

<?xml version="1.0"?>
<?xml-stylesheet type="text/xsl" href="#default"?>
<xsl:stylesheet id="default" version="1.0"
        xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
        xmlns:test="test"
        exclude-result-prefixes="xsl test">
  <test:XML>
    <order>
    </order>
  </test:XML>

  <xsl:output method="html" encoding="UTF-8" indent="yes"/>

  <xsl:template match="/xsl:stylesheet">
    <xsl:apply-templates />
  </xsl:template>

  <xsl:template match="xsl:*" /><!-- ignore -->

  <xsl:template match="test:XML/order">
    <html xmlns="http://www.w3.org/1999/xhtml">
      <body>
    <form>
      <input id="customer-name" type="text" name="i.user.name"
         size="25" maxlength="40" placeholder="Name"
         pattern="^[^ ]{2,}.*[^ ]$" required="required" />
    </form>
      </body>
    </html>
  </xsl:template>

</xsl:stylesheet>

As regards your comment "What makes things harder to understand...": Running XSLT in the browser is not the ideal development environment; try an XML IDE such as Oxygen. — Michael Kay
– Michael Kay, Commented Nov 3, 2023 at 12:43

Martin Honnen · Accepted Answer · 2023-11-03 12:23:17Z

3

XSLT uses curly brackets { and } in attribute values as so called attribute value templates to delimit embedded XPath expressions. So in XSLT, in such attributes, if you want a literal { or }, you need to double them as e.g. {{ and }}, e.g. pattern="^[^ ]{{2,}}.*[^ ]$".

answered Nov 3, 2023 at 12:23

Martin Honnen

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2 Comments

U. Windl Over a year ago

Out of extra curiosity: Despite of curly brackets are there other characters I should be careful with in patterns?

Martin Honnen Over a year ago

@U.Windl, that has nothing to do with HTML or the input element and its pattern attribute, your are using XSLT and that allows attribute value templates in (literal result) element attributes, therefore you need to use the escaping mechanism (i.e. double them) to ensure they are inserted literally in the result tree. XSLT has some other, general, XML based syntax requirements (like needing to escape < as < and & as &).

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