invalid escape sequence: `\\u`' from regexp

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)
```sql
set sql_mode = "";
select regexp_like('莼潔婕', '^[\\u4e00-\\u9fa5]{0,}$');
set sql_mode = "NO_BACKSLASH_ESCAPES";
select regexp_like('莼潔婕', '^[\u4e00-\u9fa5]{0,}$');

```

### 2. What did you expect to see? (Required)
```sql
MySQL root@172.16.4.90:hchwang> set sql_mode = "NO_BACKSLASH_ESCAPES";
Query OK, 0 rows affected
Time: 0.043s
MySQL root@172.16.4.90:hchwang> select regexp_like('莼潔婕', '^[\u4e00-\u9fa5]{0,}$');
+------------------------------------------------+
| regexp_like('莼潔婕', '^[\u4e00-\u9fa5]{0,}$') |
+------------------------------------------------+
| 1 |
+------------------------------------------------+

1 row in set
Time: 0.050s
MySQL root@172.16.4.90:hchwang> set sql_mode = "";
Query OK, 0 rows affected
Time: 0.052s
MySQL root@172.16.4.90:hchwang> select regexp_like('莼潔婕', '^[\\u4e00-\\u9fa5]{0,}$');
+--------------------------------------------------+
| regexp_like('莼潔婕', '^[\\u4e00-\\u9fa5]{0,}$') |
+--------------------------------------------------+
| 1 |
+--------------------------------------------------+
```
### 3. What did you see instead (Required)
```sql
MySQL root@127.0.0.1:reg> set sql_mode = "";
Query OK, 0 rows affected
Time: 0.001s
MySQL root@127.0.0.1:reg> select regexp_like('莼潔婕', '^[\\u4e00-\\u9fa5]{0,}$');
(1139, "Got error 'error parsing regexp: invalid escape sequence: `\\u`' from regexp")
MySQL root@127.0.0.1:reg> set sql_mode = "NO_BACKSLASH_ESCAPES";
Query OK, 0 rows affected
Time: 0.001s
MySQL root@127.0.0.1:reg> select regexp_like('莼潔婕', '^[\u4e00-\u9fa5]{0,}$');
(1139, "Got error 'error parsing regexp: invalid escape sequence: `\\u`' from regexp")
MySQL root@127.0.0.1:reg>
```
### 4. What is your TiDB version? (Required)
Git Commit Hash: 95f4b119c03f11580e0cdd9c0875f52e13c14611