The goal of this diploma work is the suggestion and the implementation of the application, which allows adding / removing of diacritics into / from Czech written text. Retrieval "trie" structure is described along with its relation to finite state automata. Further, algorithm for minimization of finite state automata is described and various methods for adding diacritics are discussed. In practical part the implementation in Java programming language with usage of object-oriented approach is given. Achieved results are evaluated and analysed in the conclusion.