I've had a quick poke about in the code and think I can give you the fixes for this issue.
In Register.aspx.cs...
RegularExpressionValidator passwordRegex
= (RegularExpressionValidator)CreateUserWizardStep1.ContentTemplateContainer.FindControl("PasswordRegex");
// CF bug fix part 1 https://www.mojoportal.com/Forums/Thread.aspx?pageid=5&t=13155~1
if (siteSettings.PasswordRegexWarning.Length > 0)
{
passwordRegex.ErrorMessage = siteSettings.PasswordRegexWarning;
}
else
{
passwordRegex.ErrorMessage = Resource.RegisterPasswordRegexWarning;
}
//end fix
AND (further down)
if (Membership.Provider.PasswordStrengthRegularExpression.Length == 0)
{
passwordRegex.Visible = false;
}
//CF bug fix part 2 https://www.mojoportal.com/Forums/Thread.aspx?pageid=5&t=13155~1
else
{
passwordRegex.ValidationExpression = Membership.Provider.PasswordStrengthRegularExpression;
}
//end fix