Columbia Bank's expression validation utility receives strings containing parentheses, braces, and square brackets. Determine whether every opening bracket has the correct closing bracket in the correct order.
Implement is_balanced(expression) and return True when the expression is balanced, or False otherwise. An empty expression is considered balanced. The input contains only the six bracket characters: (, ), {, }, [, and ].
expression containing zero or more bracket characters.def is_balanced(expression):