Sometimes, you may have a large string (such as the contents of a file) and need to find out if it contains a smaller substring. This can be done with the string.substring function. This function takes two variables: the large string to search and a smaller string to look for. On success, it returns a number, which is the index at which the substring first appears. On failure, the function returns nil:
local sentence = "The quick brown fox"
local word = "quick"
local index = string.find(sentence, word)
print ("substring found at index: " .. index)
The second argument is interesting; it doesn't just have to be a string. The second argument to string.substring can be a pattern, which results in regex like searching. For more info, check out http://lua-users.org/wiki/PatternsTutorial