In the last two decades, several studies have been carried out to translate Behavior Trees (BTs) into other formal languages. However, as BTs are usually drawn directly from natural languages, there is no formal grammar to define what is a valid BT. In this research, we first propose a normal form for requirement BT as a building block for a valid BT, and then design a context-free grammar that can generate and verify all valid BTs. This work provides a solid foundation for BT research and will improve the quality of requirements modeling by identifying some common requirement defects.