There are many different synonyms for the phrase "by regular stages". Some of the most common include "gradually", "bit by bit", "step by step", "in stages", "progressively", and "systematically". Each of these words or phrases conveys a sense of progression or movement over time, with a focus on steady, gradual progress rather than sudden leaps or changes. Depending on the context in which the phrase is used, different synonyms may be more appropriate or effective, but all of them are useful for describing processes or actions that occur over an extended period, with a clear sense of direction and purpose.