REGEXP predicate unexpectedly results in an error "No valid regexp pattern found"

[mrigger]

Consider the following test case:

CREATE TABLE t0(c0 INT);
CREATE TABLE t1(c0 INT);
INSERT INTO t0 VALUES (NULL), (1);
INSERT INTO t1 VALUES (0);
SELECT t0.c0 FROM t0 JOIN t1 ON (t0.c0 REGEXP 1) | t1.c0  WHERE BINARY STRCMP(t1.c0, t0.c0); --  No valid regexp pattern found

Unexpectedly, the SELECT results in an error ERROR 1105 (HY000): No valid regexp pattern found.

Environment:

mysql> select tidb_version();
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tidb_version()                                                                                                                                                                                                                                                                                          |
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Release Version: v4.0.0-beta.2-167-gaa7ef9c46
Git Commit Hash: aa7ef9c46435bb20d42fa0badcf09fc16f9609a9
Git Branch: master
UTC Build Time: 2020-04-02 09:15:47
GoVersion: go1.13.4
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false |
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)