[GENERAL] What is the difference between these queries

[salah jubeh]

Query1
-- the first select return 10 rows
SELECT a, b 
FROM table1 LEFT JOIN table2 on (table1_id = tabl2_id)
Where table1_id NOT IN (SELECT DISTINCT table1_id FROM table3)
EXCEPT 
-- this select return 5 rows 
SELECT a, b 
FROM table1 LEFT JOIN table2 on (table1_id = tabl2_id)
Where table1_id NOT IN (SELECT DISTINCT table1_id FROM table3)
and  b ~* 'pattern'
-- the result is 5 rows

Query2
--this select return 3 rows
SELECT a, b 
FROM table1 LEFT JOIN table2 on (table1_id = tabl2_id)
Where table1_id NOT IN (SELECT DISTINCT table1_id FROM table3)
and  b !~* 'pattern'

Why query1 and query2  return different set. note that query two return a 
subset 
of query1