Steps to reproduce.

1. Turn on Admin -> Security -> Site policies -> Strict validation of required fields
2. Set your editor to be TinyMCE
3. Go and create a Multiple choice question (or anything else with a required HTML field.)
4. Enter a question name.
5. Type b into the Question text.
6. Try to submit the form.

Actual result:

The form does not submit. Client-side JS marks the Question text as "You must supply a value here."

Expected result:

The form submits. b is not blank. (Of course the value of the field is actually '<p>b</p>'.

The problem is the regex in getValidationScript in lib/formslib.php. It is:

return array('', "

.replace(/(<[^img|hr|canvas]>)| |\s/ig, '') == ''");

That regex, added by MDL-28987, does not do what was intended. I will fix it.