Regex Python extract columns from SQL Statement

I want to extract the column names from this SQL statement using Python Regex. It can have different delimeters, such as [, CASE, (] the pattern is that we can extract columns that contain either capital letters, small letters, underscores or numbers. It must start with a letter A-Za-z.

SELECT 
    year_period
    ,month
    ,replace(source, '\n', '') source Data_source
    ,CASE 
        WHEN left(r, 2) = 'XL'
            THEN r
        ELSE CONCAT (
                'XL'
                ,r
                )
        END reg
    ,desc
    ,cc
    ,(
        CASE 
            WHEN right(full, 1) = 'I'
                THEN left(full, length(full) - 1)
            ELSE full
            END
        ) full

FROM schema1.table1

Output:

[year, month, Data_source, reg, desc, cc, full]

Our attempt with regex Python:

with open('SQL_Statement.txt') as f:
    f = f.read()
    
print(f)

pattern = r'T|\,|\s{1,}([A-Za-z]{1,}[A-Za-z0-9_]{1,})\s{1,}'

import re

re.findall(pattern, f)