expression

The expression to be evaluated. The expression can refer to numbers and node-sets as well as booleans.

Boolean true or false after evaluating expression.

• A number evaluates to false if it is positive or negative zero or NaN. Otherwise, it evaluates true.
• A node-set evaluates to true if it is non-empty.
• A string evaluates to false if it an empty string. Otherwise, it evaluates to true.
• An object of a type other than the four basic types is converted to a boolean in a way that is dependent on that type.

XPath 1.0 4.3

Supported.