Write regular expressions $($REs$)$ for the following languages over the alphabet $\{\text{'}$a$\text{'},\text{'}$b$\text{'}\}.$

$($a$)\{$w : w has no more than two $\text{'}0\text{'}$s$\}$

$($b$)\{$w : length of w is divisible by either two or three$\}$

$($c$)\{$w : w contains exactly one occurrence of substring $01\}$