1. A computer-implemented method for text-to-speech (TTS) synthesis, comprising: in response to a word of a text sequence, generating a first part-of-speech POS tag using a statistical POS tagger based on a corpus of trained text sequences, each representing a likely POS of a word for a given text sequence, wherein the first POS tag is selected from a first POS tag set; generating a second POS tag using a rule-based POS tagger based on a set of one or more rules associated with a type of an application associated with the text sequence, wherein the second POS tag is selected from a second POS tag set that is different from the first POS tag set; calculating a first confidence score for the second POS tag based on a statistic data of applying a rule associated with the second POS tag, wherein the first confidence score is calculated based on a percentage of successful applications of the rule in previous TTS synthesis; designating the second POS tag as the final POS tag if the first confidence score is greater than or equal to a first predetermined threshold; designating the first POS tag as the final POS tag if the first confidence score is less than the first predetermined threshold; assigning a final POS tag to the word of the text sequence for TTS synthesis based on the first POS tag and the second POS tag; adjusting the first confidence score for the rule for future TTS synthesis based on whether the second POS tag has been selected as the final POS tag; and removing the rule from the set of one or more rules if the first confidence score is below a second predetermined threshold.