Filter operation, limiting the set of objects. The filter expression is expected as string argument and used to reduce the context to matching elements by checking each value against the filter.

A filter expression is written in Fizzle, a grammar inspired by CSS selectors. It has the form "[" [<value>] <operator> <operand> "]" and supports the following operators:

=~ Strict equality of case-insensitive value and operand

Strict equality of value and operand !=~ Strict inequality of case-insensitive value and operand != Strict inequality of value and operand < Value is less than operand <= Value is less than or equal to operand

Value is greater than operand

Value is greater than or equal to operand $=~ Value ends with operand (string-based) or case-insensitive value's last element is equal to operand (array-based) $= Value ends with operand (string-based) or value's last element is equal to operand (array-based) ^=~ Value starts with operand (string-based) or case-insensitive value's first element is equal to operand (array-based) ^= Value starts with operand (string-based) or value's first element is equal to operand (array-based) =~ Value contains operand (string-based) or case-insensitive value contains an element that is equal to operand (array based) = Value contains operand (string-based) or value contains an element that is equal to operand (array based) instanceof Checks if the value is an instance of the operand !instanceof Checks if the value is not an instance of the operand

For the latter the behavior is as follows: if the operand is one of the strings object, array, int(eger), float, double, bool(ean) or string the value is checked for being of the specified type. For any other strings the value is used as classname with the PHP instanceof operation to check if the value matches.