Systems and methods for progressively parsing user input of a mathematical expression are provided. One disclosed method includes looping through characters in an input string, and on each loop, extracting a next token from the input string and determining a current grammar context based on the token or tokens extracted thus far. If it is determined that the current grammar context matches a predetermined condition, then the method may include modifying the tokens extracted from the input string in a predetermined manner associated with the predetermined condition. A parsing result may be obtained based on the modified tokens. The parsing result may be converted to a modified input string.