1. A non-transitory computer-readable storage medium that stores a program making a computer realize a character string registration function, the character string registration function comprising: storing in a storage device a trie tree in which nodes corresponding to predetermined characters are connected in accordance with a tree structure; reading out, when a new character string is registered in the trie tree, characters of the new character string in sequence from a head thereof to trace nodes included in the trie tree in accordance with characters corresponding to the nodes; and adding a new node below any of the traced nodes or a leaf node arriving in accordance with the new character string to register the new character string in the node in such a manner that a single character string is registered for a single node.