function pattern_checker(v)
	return string.match(v, '^[%d%a_.]+$') ~= nil and -- check if the string only contains digits/letters/_, one or more
	       string.sub(v, 0, 1) ~= '.' and            -- check if the first char is not '.'
	       string.sub(v, -1) ~= '.' and              -- check if the last char is not '.'
	       string.find(v, '%.%.') == nil             -- check if there are 2 consecutive dots in the string
end

-- good examples
print(pattern_checker("hello.com"))
print(pattern_checker("hello"))
print(pattern_checker("hello.com.com.com"))
-- bad examples
print(pattern_checker("hello.com."))
print(pattern_checker(".hello"))
print(pattern_checker("hello..com.com.com"))
print(pattern_checker("%hello.com.com.com"))