1. A character input method for automatically switching an input mode in a terminal having a touch screen, the character input method comprising: assigning character groups, each of which includes at least two characters, to at least two key regions acquired by dividing a region of the touch screen and displaying the character groups in the corresponding key regions based on one-to-one correspondence; if a press event occurs in a first of the key regions, determining whether a drag event occurs prior to occurrence of a release event; executing one of a separate input mode for inputting one of characters of a corresponding character group and a text input mode for inputting a word including one of the characters of the character group according to whether the drag event occurs; and upon execution of the separate input mode, assigning each character of the character group associated with the first key region to a different one of the key regions other than the first key region and displaying the characters of the character group in the other key regions.