XPath 1.0 reference

XPaths | expressions

!= | " " | > | >= | < | <= | " | () | * | + | − | ⁄ | :: | = | @* | [] | and | asix nodetest predicate | div | func() | mod | name | or& | | |

" "

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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XPath 1.0 reference