QGIS Regex expression label

Question

I'm trying to use regex expression to label my layer in order to show the parcel number. Here is some examples of what i'm looking for :

From this "53042A0027/00M002", to "A27M/2"
From this "53042A2000/02A003", to "A2000A2/3"
From this "53042C0002/00_000", to "A2"
From this "53042G1257/00X000", to "G1257X"

EDIT (rules to consider) :

Copy the location 6 ==> "53042**G**1257/00X000" 
Then for location 7 to 10, add numbers after the first not null number ==> "53042A**2000**/02A003", "53042A00**27**/00M002"
Then copy location 14 if not equal to "_" ==> "53042A0027/00**M**002"
Then for location 12 and 13, addnumbers after the first not null number ==> "53042A2000/0**2**A003", "53042A0027/00M002"
Then for location 15 to 17, add "/" and all numbers after the first not null number ==> "53042A2000/02A00**3**", "53042A0027/00M00**2**"

Answer 1

This function work like expected (at least for the 4 examples) :

regexp_replace(
    regexp_replace(
        "Test", 
        '[0-9]{5}([A-Z])0*([1-9][0-9]*)/(0|([1-9]))*(_|([A-Z]))(0|([1-9])){3}', 
        '\\1\\2\\6\\4/\\8'),
     '/$', 
     ''
)

The second 'regexp_replace' function (the one with the big regexp) defines eight groups (the values in brackets) that you can use to generate a new string, like explain by the next figure.

The first 'regexp_replace' remove the slash / character when the new string when it is the last character (I can't do it in the same function).