REGEXP empty string inconsistent

DROP TABLE IF EXISTS data_table;

CREATE TABLE `data_table` (

  `id` int(11) NOT NULL AUTO_INCREMENT,

  `where_1` int(11) NOT NULL,

  `where_2` int(11) NOT NULL,

  `where_3` int(11) NOT NULL,

  `pattern` varchar(64) NOT NULL,

  PRIMARY KEY (`id`)

) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_general_ci;

INSERT INTO `data_table` (`id`, `where_1`, `where_2`, `where_3`, `pattern`) VALUES

(9,	11,	40,	1,	''),

(10,	11,	10,	1,	''),

(11,	11,	500,	1,	''),

(12,	12,	500,	1,	''),

(13,	12,	20,	1,	''),

(14,	12,	40,	1,	''),

(15,	11,	920,	1,	''),

(16,	11,	930,	1,	''),

(17,	11,	920,	1,	''),

(18,	11,	930,	1,	''),

(19,	11,	551,	0,	''),

(20,	11,	520,	1,	'/Testwert1/'),

(21,	12,	510,	1,	'/Testwert2*/'),

(22,	11,	60,	0,	''),

(23,	11,	510,	1,	'/Testwert3/'),

(24,	12,	920,	1,	''),

(25,	12,	930,	1,	''),

(26,	12,	920,	1,	''),

(27,	12,	930,	1,	''),

(28,	12,	60,	0,	''),

(29,	12,	510,	1,	'/Testwert1A/'),

(30,	11,	30,	1,	''),

(31,	12,	30,	1,	''),

(32,	12,	30,	1,	'/Testwert4/'),

(33,	12,	30,	1,	'/^(?![\\s|\\S]*Testwert4[\\s|\\S]*)\\b/');

-- Basic Dataset that will be used in REGEXP Query

SELECT *

FROM `data_table`

WHERE `where_1` = '11' AND `where_2` IN (30,510,530,531,532,520);

-- REGEXP against an empty pattern field

SELECT 'This is a test string' REGEXP '' AS 'regexp', LENGTH('') AS 'length';

-- REGEXP with two pattern lines. Empty pattern is valid

SELECT *, 'This is a test string' REGEXP pattern AS 'regexp', LENGTH(pattern) AS 'length'

FROM data_table

WHERE where_1 = 11 AND where_2 IN (30, 510, 530, 531, 532, 520) AND where_3 = 1;

-- REGEPX with one pattern line. Empty pattern is invalid

SELECT *, 'This is a test string' REGEXP pattern AS 'regexp', LENGTH(pattern) AS 'length'

FROM data_table

WHERE where_1 = 11 AND where_2 IN (30, 510, 530, 531, 532) AND where_3 = 1

ORDER BY id ASC;

-- REGEXP with one pattern line in reverse order. Empty pattern is valid

SELECT *, 'This is a test string' REGEXP pattern AS 'regexp', LENGTH(pattern) AS 'length'

FROM data_table

WHERE where_1 = 11 AND where_2 IN (30, 510, 530, 531, 532) AND where_3 = 1

ORDER BY id DESC;

-- REGEXP with one line as the second WHERE condition. Empty pattern is valid

SELECT *, 'This is a test string' REGEXP pattern AS 'regexp', LENGTH(pattern) AS 'length'

FROM data_table

WHERE where_1 = 11

AND 'This is a test string' REGEXP pattern

AND where_2 IN (30, 510, 530, 531, 532)

AND where_3 = 1;

-- REGEXP with one line as the forth WHERE condition. Empty pattern is invalid

SELECT *, 'This is a test string' REGEXP pattern AS 'regexp', LENGTH(pattern) AS 'length'

FROM data_table

WHERE where_1 = 11

AND where_2 IN (30, 510, 530, 531, 532)

AND where_3 = 1 

AND 'This is a test string' REGEXP pattern;

-- Below are the same statment with an IF(pattern = '', '.*', pattern) escape. All results are consitent

SELECT *, 'This is a test string' REGEXP IF(pattern = '', '.*', pattern) AS 'regexp', LENGTH(IF(pattern = '', '.*', pattern)) AS 'length'

FROM data_table

WHERE where_1 = 11 AND where_2 IN (30, 510, 530, 531, 532, 520) AND where_3 = 1;

SELECT *, 'This is a test string' REGEXP IF(pattern = '', '.*', pattern) AS 'regexp', LENGTH(IF(pattern = '', '.*', pattern)) AS 'length'

FROM data_table

WHERE where_1 = 11 AND where_2 IN (30, 510, 530, 531, 532) AND where_3 = 1

ORDER BY id ASC;

SELECT *, 'This is a test string' REGEXP IF(pattern = '', '.*', pattern) AS 'regexp', LENGTH(IF(pattern = '', '.*', pattern)) AS 'length'

FROM data_table

WHERE where_1 = 11 AND where_2 IN (30, 510, 530, 531, 532) AND where_3 = 1

ORDER BY id DESC;

SELECT *, 'This is a test string' REGEXP IF(pattern = '', '.*', pattern) AS 'regexp', LENGTH(IF(pattern = '', '.*', pattern)) AS 'length'

FROM data_table

WHERE where_1 = 11

AND 'This is a test string' REGEXP IF(pattern = '', '.*', pattern)

AND where_2 IN (30, 510, 530, 531, 532)

AND where_3 = 1;

SELECT *, 'This is a test string' REGEXP IF(pattern = '', '.*', pattern) AS 'regexp', LENGTH(IF(pattern = '', '.*', pattern)) AS 'length'

FROM data_table

WHERE where_1 = 11

AND where_2 IN (30, 510, 530, 531, 532)

AND where_3 = 1 

AND 'This is a test string' REGEXP IF(pattern = '', '.*', pattern);