String Patterns Deep Dive

Lua doesn't have full regular expressions, instead it has a lighter-weight pattern system built into string.find, string.match, string.gmatch, and string.gsub. Patterns cover the vast majority of real-world text processing without needing a heavyweight regex engine.

Character classes

Uppercase versions (%A, %D, ...) match the complement. Quantifiers *, +, -, and ? work similarly to regex, though - is a lazy (shortest) repeat, unlike *'s greedy repeat.

Capturing groups

local date = "2026-07-06"
local year, month, day = date:match("(%d+)-(%d+)-(%d+)")
print(year, month, day) -- 2026  07  06

Parentheses in a pattern create a capture, string.match returns one value per capture (or the whole match if there are none).

Iterating all matches with gmatch

local text = "cat, dog, bird"
for word in text:gmatch("%a+") do
  print(word)
end
-- cat
-- dog
-- bird

Search-and-replace with gsub

local sentence = "Lua is great, Lua is fast"
local replaced, count = sentence:gsub("Lua", "Kodstigen")
print(replaced) -- Kodstigen is great, Kodstigen is fast
print(count)    -- 2

-- gsub can also take a function to compute each replacement
local upper = sentence:gsub("%a+", function(word) return word:upper() end)
print(upper) -- LUA IS GREAT, LUA IS FAST

gsub returns two values: the new string, and how many replacements were made, don't forget the second one if you only want the string.