Shell Programming and Scripting

I have a file with the following format

Code:

--TABLEA_START--

field1=data1;field2=data2;field3=data3

--TABLEA_END--

--TABLEB_START--

field1=data1;field2=data2;field3=data3
 
--TABLEB_END--
 
--TABLEA_START--

field1=data1;field2=data2;field3=data3

field1=data1;field2=data2;field3=data3
 
--TABLEA_END--

I have a string

Code:

lookup="TABLEA.field2,TABLEA.field3,TABLEB.field2"

I would like to construct a lookup table/something similar containing the tables and their respective fields so that I can achieve the following.

if the table fields is indicated in the string, i would like to set the the relevant field value to blank i.e field1=; and save it to the file without affecting other data. Changes to TABLEA.field1 will not affect TABLEB.field1. There can be more than one instance of the same table (like TableA shown above.

My logic is as such

Code:

IFS=";"
do
    for i in $lookup
    table_name=${i%%.*} #table name
    table_field=${i#*.} # field name
    sed -n  "/$table_START--/,/--$table_END--/p" file #extract data between start and end
    
    loop table fields
       sed "s/$table_field=[^=]*;/$table_field=;/;$//g" 

done

How can I achieve this and put all this together? Any help will be greatly appreciated.