A portable electronic device having a touch screen display performs a set of operations, including displaying a plurality of key icons, each having an adjustable size hit region, and receiving a sequence of individual touch points input by a user on the touch screen display. The operations performed by the device further include processing the received individual touch points by: forming a user-input directed graph for the sequence of individual touch points received so far, determining a character corresponding to a last received individual touch point in accordance with the adjustable hit regions of the displayed key icons, displaying a sequence of characters corresponding to the sequence of individual touch points, and updating sizes of the adjustable hit regions for a plurality of the key icons in accordance with the sequence of individual touch points input by the user.