A grammar can be written to describe the syntax of the language of balanced parentheses:
S &rarr ( S )
S &rarr [ S ]
S &rarr { S }
S &rarr S S
S &rarr &epsilon
This grammar describes a balanced parenthesis string as a tree. As we check the syntax of a string, the stack always contains the unbalanced symbols between the current symbol and the root of the tree.