# Filter expressions

The notation `E[P]` is used to select items from the sequence obtained by evaluating `E`. If the predicate `P` is numeric, the predicate selects an item if its position (counting from 1) is equal to `P`; otherwise, the effective boolean value of `P` determines whether an item is selected or not. The effective boolean value of a sequence is false if the sequence is empty, or if it contains a single item that is one of: the boolean value false, the zero-length string, or a numeric zero or NaN value. If the first item of the sequence is a node, or if the sequence is a singleton boolean, number or string other than those listed above, the effective boolean value is true. In other cases (for example, if the sequence contains two booleans or a date), evaluating the effective boolean value causes an error. (This has changed in Saxon 8.3.)

In XPath 2.0, `E` may be any sequence, it is not restricted to a node sequence. Within the predicate, the expression `.` (dot) refers to the context item, that is, the item currently being tested. The XPath 1.0 concept of context node has thus been generalized, for example `.` can refer to a string or a number.

Generally the order of items in the result preserves the order of items in `E`. As a special case, however, if `E` is a step using a reverse axis (e.g. preceding-sibling), the position of nodes for the purpose of evaluating the predicate is in reverse document order, but the result of the filter expression is in forwards document order.

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