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Using curly brace symbols in a simpleType - legal and illegal usage

• From: Roger L Costello <costello@mitre.org>
• To: "xml-dev@lists.xml.org" <xml-dev@lists.xml.org>
• Date: Wed, 8 Dec 2021 19:13:37 +0000

Hi Folks,

This XML Schema simpleType specifies that the allowable values are: one or more lower- and upper-case letters of the alphabet and left/right curly braces.

    <xs:simpleType name="Test">
        <xs:restriction base="xs:string">
            <xs:pattern value="[a-zA-Z\{\}]+" />
        </xs:restriction>
    </xs:simpleType>

Since the curly braces are regex meta-symbols, they are escaped.

That simpleType was then placed inside an XSLT template:

    <xsl:template match="/">
        <xs:simpleType name="Test">
            <xs:restriction base="xs:string">
                <xs:pattern value="[a-zA-Z\{\}]+" />
            </xs:restriction>
        </xs:simpleType>
    </xsl:template>

The XSLT processor flags an error: Invalid character '\' in expression.

The curly braces are also meta-symbols in XSLT. However, to escape the curly braces in XSLT use double curly braces:

    <xsl:template match="/">
        <xs:simpleType name="Test">
            <xs:restriction base="xs:string">
                <xs:pattern value="[a-zA-Z{{}}]+" />
            </xs:restriction>
        </xs:simpleType>
    </xsl:template>

 If the XSLT program is generating an XML Schema, then you must escape the double curly braces:

    <xsl:template match="/">
        <xs:simpleType name="Test">
            <xs:restriction base="xs:string">
                <xs:pattern value="[a-zA-Z\{{\}}]+" />
            </xs:restriction>
        </xs:simpleType>
    </xsl:template>

/Roger