This is to suggest complexity for Enrolment key for "Self enrolment (Student)", similar to Enrolment Key used by Groups.

Steps to replicate:

1. Create a course.
2. Click Participants tab.
3. Select Enrolment Methods in the dropdown control.
4. Enable Self enrolment (Student) by clicking the "eye" icon.
5. Click Cog icon to configure.
6. Set "Enrolment key" = 1
7. Use group Enrolment Keys = Yes
8. Save it. Notice there are no warnings, it just saves successfully.
9. Click Groups in the dropdown.
10. Click Create group.
11. Set "Group name" = g1
12. Set "Enrolment key" = 1
13. Save it.
14. Notice you will have the following message:

Passwords must be at least 8 characters long.
Passwords must have at least 1 lower case letter(s).
Passwords must have at least 1 upper case letter(s).
The password must have at least 1 special character(s) such as *, -, or #.????

Edited to add: As noted in the comments below, it's easy to miss the setting 'Use password policy' (usepasswordpolicy) in Site administration > Plugins > Enrolments > Self enrolment because it's disabled by default.

Instead, it should be enabled by default - for security, and also for consistency with the setting 'Group enrolment key policy' (which is enabled by default).