Systems and methods of classifying user input are disclosed. The user input can be, for example, in the form of Roman characters. An ambiguous word (e.g., a word that is a non-pinyin word written in Roman characters and a valid pinyin word) can be identified in the user input. Contextual words (e.g., words adjacent to the ambiguous word) are classified as a pinyin context or a non-pinyin context. The ambiguous word is classified based on the context of the contextual words.