1. An image processing apparatus, comprising: a document input unit that inputs document data of a document; a first identifying unit that identifies a position of a string in the document; a second identifying unit that identifies a mark handwritten from a highlighter and is non-textual, the mark being separate from the string, in the document; and an orientation detection unit that detects an orientation of the mark, wherein the mark is defined by a range from a start position and an end position based on the detected orientation; and a character string extracting unit that extracts a new string, based on the position of the string identified by the first identifying unit, and the mark identified by the second identifying unit and defined based on the detected orientation.