7. The computer-readable recording medium according to claim 1 , wherein the determining evaluates validity of the extracted characters of each combination in terms of character arrangement, and determines the combinations having verified validity as the possible strings.
9. The computer-readable recording medium according to claim 7 , wherein the determining judges that a character group forming a combination is inappropriate as a possible string if there is another character between two characters of the character group corresponding to two characters which are next to each other in the form logical definitions.