import re
text = r'Codes like 123/456\n \n123/3456 \n123/23456\n etc are correct \n333.3333/333\n'
print( re.findall(r'(?<!\S(?<!\\n))\d{3}[./]\d{3,6}(?:/\d{3})?(?!(?!\\n)\S)', text) )
aW1wb3J0IHJlCnRleHQgPSByJ0NvZGVzIGxpa2UgMTIzLzQ1NlxuIFxuMTIzLzM0NTYgXG4xMjMvMjM0NTZcbiBldGMgYXJlIGNvcnJlY3QgXG4zMzMuMzMzMy8zMzNcbicKcHJpbnQoIHJlLmZpbmRhbGwocicoPzwhXFMoPzwhXFxuKSlcZHszfVsuL11cZHszLDZ9KD86L1xkezN9KT8oPyEoPyFcXG4pXFMpJywgdGV4dCkgKQo=