These should use validate_re from stdlib - https://github.com/puppetlabs/puppetlabs-stdlib/tree/3.2.0#validate_re

You can use rubular - http://rubular.com/ - to help with your regex

downcase can be used from stdlib as well - https://github.com/puppetlabs/puppetlabs-stdlib/tree/3.2.0#downcase

ie:
<pre>
$sshd_config_passwordauth_real = downcase($sshd_config_passwordauth)
validate_re($sshd_config_passwordauth_real, '^yes|no$', "sshd_config_passwordauth may be either 'yes' or 'no' and is set to ${sshd_config_passwordauth}")
</pre>