Recognizing mathematical expressions from document image is a key problem in automatic conversion of scientific documents into electronic form. In this paper, we propose a simple grammar-based approach to recognize complex two- dimensional structures of printed mathematical expressions with high accuracy. The proposed technique is based on the structural information of symbols in an expression. An efficient implementation of the grammar is presented. The system generates a TEX string for the input expression. A new criterion for defining structural complexity of a mathematical expression has been formulated to measure the performance of the proposed technique. Experiment using a good representative sample of mathematical expressions shows a reasonably high efficiency of the system.