XSLT 1.0 reference

XSLT | expressions

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" "

XPath expressions often occur in XML attributes. Within expressions, literal strings are delimited by single or double quotation marks, which are also used to delimit XML attributes. To avoid a quotation mark in an expression being interpreted by the XML processor as terminating the attribute value the quotation mark can be entered as a character reference (" or '). Alternatively, the expression can use single quotation marks if the XML attribute is delimited with double quotation marks or vice-versa.

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XSLT 1.0 reference