abbreviation_internal   Internal abbreviation marker, marks
                        abbreviations with an underscore behind. Useful
                        if 'parsing_option' 1 is needed, but some
                        abbreviations need 'parsing_option' 2.
caseconverter           Specific case converter shortcuts
check_design_rule       Internal helper to test the design rules for
                        any string and setting of 'to_any_case()'
preprocess_internal     Internal function that replaces regex matches
                        with underscores
relevant                Internal helper for "lower_upper",
                        "upper_lower". This helper returns a logical
                        vector with TRUE for the first and every second
                        string of those which contain an alphabetic
                        character
replace_special_characters_internal
                        Internal helper to replace special characters.
to_any_case             General case conversion
to_parsed_case_internal
                        Internal parser, which is relevant for
                        preprocessing, parsing and parsing options
