A method, computer system and computer program for optimizing the processing of a character string during execution of the program by using characteristic information that indicates a characteristic of the character string and is associated with the character string. The method includes the steps of determining, on the basis of a characteristic of a first character string and operation for the first character string, a characteristic information of at least one of the first character string and a second character string obtained as a result of the operation, and associating the characteristic information with the at least one character string.