Computer-implemented systems and methods are disclosed for constructing a parser that parses complex data. In some embodiments, a method is provided for receiving a parser definition as an input to a parser generator and generating a parser at least in part from the parser definition. In some embodiments, the generated parser comprises two or more handlers forming a processing pipeline. In some embodiments, the parser receives as input a first string into the processing pipeline. In some embodiments, the parser generates a second string by a first handler and inputs the second string regeneratively into the parsing pipeline, if the first string matches an expression specified for the first handler in the parser definition.