MySQL string splitting on delimiters

Question

Based on https://stackoverflow.com/a/59666211/4250302 I created the stored function get_enum_item for future processing the lists of possible values in the ENUM() type fields.

It works fine enough, but... but I can't determine what to do if the delimiter itself is the part of a string being split. For example: (square brackets are for readability)

mysql> set @q=",v1,',v2'" --empty string, "v1", "comma-v2";

mysql> select concat('[',get_enum_item(@q,',',0),']') as item;
+------+
| item |
+------+
| []   |
+------+

it is OK

mysql> select concat('[',get_enum_item(@q,',',1),']') as item;
+------+
| item |
+------+
| [v1] |
+------+

it is also OK

mysql> select concat('[',get_enum_item(@q,',',2),']') as item;
+------+
| item |
+------+
| [']  |
+------+

It is not OK

the @q contains 3 commas, the first two of these are real delimiters, while the last one is the part of the third possible value: "comma-v-two". And I have no idea how to avoid confusion of splitting function. MySQL WorkBench in the "form editor" mode solves this trouble somehow, but how can I solve this with MySQL's code?

Well, I can rely on the fact that the show_columns-like queries show the enums in "hardcoded" manner:

select column_name,column_type
  from information_schema.columns
  where data_type='enum' and table_name='assemblies';
+--------------+------------------------------------------------------------------+
| COLUMN_NAME  | COLUMN_TYPE                                                      |
+--------------+------------------------------------------------------------------+
| AssetTagType | enum('','И/Н','Н/Н',',fgg')                                      |
| PCTagType    | enum('','И/Н','Н/Н')                                             |
| MonTagType   | enum('','И/Н','Н/Н')                                             |
| UPSTagType   | enum('','И/Н','Н/Н')                                             |
| OtherTagType | enum('','И/Н','Н/Н')                                             |
| state        | enum('в работе','на списание','списано')                         |
+--------------+------------------------------------------------------------------+

Thus I can try to use ',' as a delimiter, but this will not save me from the case if the "comma-apostrophe" combination is the part of possible value... :-( The only thing I can imagine is to count apostrophes and if the delimiting comma is after the even number of ''s, then it is the delimiter, while if it follows an odd number of ''s, it is the part of the value. And I can't invent anything except for dumb scanning the input string inside the loop. But maybe there are some other suggestions to get the values split correctly?

Please, don't suggest use PHP, Python, AWK, and so on. The query will be executed from the Pascal (Lazarus, CodeTyphoon) application, and calling external processors is highly unsafe. As a last resort, I can process the column_type with Pascal's code, but at first, I must make myself sure that the task is not solvable by MySQL's features.

edit:

select column_type from information_schema.columns
where column_name='assettagtype' and table_name='assemblies';

+------------------------------------------+
| COLUMN_TYPE                              |
+------------------------------------------+
| enum('','И/Н','Н/Н',''''',fgg','''')     |
+------------------------------------------+
1 row in set (0.00 sec)

Fourth field: '',fgg, fifth field: '

Answer 1

set @q="'в работе','на списание','списано'";
WITH RECURSIVE cte as (
   select 1 as a union all
   select a+1 from cte where a<35
   )
select distinct  regexp_substr(@q,'''[^,]*''',a) as E from cte;

• Too high values for 35 raise an error ERROR 3686 (HY000): Index out of bounds in regular expression search.. (I created a bug for this)
• The null value should be filtered out...

output:

E
'в работе'
'на списание'
'списано'
null

EDIT: With some effort, this also works for a more complex example (not for every "staged" example!)

set @q="'в работе','на списание','списано',''',fgg'";
select @q;

WITH RECURSIVE cte as (
   select 1 as a union all
   select a+1 from cte where a<35
   )
select distinct regexp_substr(@q,'(''([^,]|[^''][^''])*'')',a) E  from cte;

output:

E
'в работе'
'на списание'
'списано'
''',fgg'