How do I check if a string includes a certain phrase?

I want to check if a variable named "custom" includes the text "http://www.roblox.com/asset/?id=" How would I go about doing this?

So you could do something like this for an if statement

local Matched = string.match("https://whatsgood.com/", "https?://")

if not Matched then
	return 
end

print("found")

Update:

print(string.match("https://whatsgood.com/", "https?://") ~= nil )

How would I do an if statement with this?