All examples 1..4 give the same result.
Example 4 demonstrates the Mp3tag scripting function $mid(), which is one of the basic functions for editing text strings.
See Mp3tag help manual, scripting functions.
Example 3 demonstrates the Mp3tag scripting function $regexp(), which applies a so called "regular expression". A "RegEx" is build out of a mighty but cryptic "regular expression programming language".
See some examples with practical usage within Mp3tag ...
Regular Expressions
DD.20111118.1413.CET