Strings in Lua

Beginner ~25 min read

Strings are sequences of characters used to represent text. Lua provides powerful string manipulation capabilities through its built-in string library. Whether you're processing user input, formatting output, or parsing data, understanding strings is essential. Let's explore everything you need to know about working with strings in Lua!

Creating Strings

Lua offers several ways to create strings:

1. Double Quotes

local message = "Hello, World!"
local name = "Alice"

2. Single Quotes

local greeting = 'Hello, Lua!'
local text = 'It\'s a beautiful day'  -- Escape single quote

3. Long Strings (Multi-line)

local multiline = [[
This is a
multi-line
string
]]

local code = [[
function hello()
    print("Hello!")
end
]]

Tip: Use long strings [[...]] for multi-line text or when you need to include quotes without escaping. They're perfect for embedding code or SQL queries!

String Operations

Concatenation

Use .. to join strings together:

Output

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String Length

Use the # operator to get string length:

local text = "Hello"
print(#text)  -- 5

local empty = ""
print(#empty)  -- 0

Important: Strings in Lua are immutable. When you concatenate or modify a string, Lua creates a new string. The original string remains unchanged.

String Library Functions

Lua's string library provides powerful functions for string manipulation:

Function	Description	Example
`string.upper(s)`	Convert to uppercase	`string.upper("hello")` → "HELLO"
`string.lower(s)`	Convert to lowercase	`string.lower("HELLO")` → "hello"
`string.reverse(s)`	Reverse string	`string.reverse("abc")` → "cba"
`string.len(s)`	Get length	`string.len("hello")` → 5
`string.sub(s, i, j)`	Extract substring	`string.sub("hello", 1, 3)` → "hel"
`string.rep(s, n)`	Repeat string n times	`string.rep("", 5)` → "****"

Try String Functions

-- String Library Functions

local text = "Hello, Lua Programming!"

-- string.upper() and string.lower()
print("=== Case Conversion ===")
print("Upper:", string.upper(text))
print("Lower:", string.lower(text))

-- string.len() - length
print("\n=== Length ===")
print("Length:", string.len(text))
print("Using #:", #text)

-- string.sub() - substring
print("\n=== Substring ===")
print("First 5 chars:", string.sub(text, 1, 5))
print("From 8 onwards:", string.sub(text, 8))
print("Last 12 chars:", string.sub(text, -12))

-- string.find() - search
print("\n=== Find ===")
local start, finish = string.find(text, "Lua")
print("'Lua' found at:", start, finish)

-- string.gsub() - replace
print("\n=== Replace ===")
local replaced = string.gsub(text, "Lua", "Python")
print("Replaced:", replaced)

-- string.rep() - repeat
print("\n=== Repeat ===")
print(string.rep("Ha", 5))
print(string.rep("-", 20))

-- string.reverse() - reverse
print("\n=== Reverse ===")
print("Reversed:", string.reverse("Lua"))

-- string.format() - formatting
print("\n=== Format ===")
local name = "Alice"
local age = 25
print(string.format("Name: %s, Age: %d", name, age))
print(string.format("Pi: %.2f", 3.14159))

-- Method syntax (syntactic sugar)
print("\n=== Method Syntax ===")
print("Hello":upper())
print("WORLD":lower())
print("Lua":rep(3))

Output

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Best Practice: You can use method syntax for string functions: str:upper() instead of string.upper(str). Both work the same!

String Formatting

Use string.format() for formatted output (like C's printf):

-- Format numbers
local price = 19.99
print(string.format("Price: $%.2f", price))  -- Price: $19.99

-- Format strings
local name = "Alice"
local age = 25
print(string.format("Name: %s, Age: %d", name, age))

-- Padding and alignment
print(string.format("%10s", "hello"))     -- Right-aligned
print(string.format("%-10s", "hello"))    -- Left-aligned
print(string.format("%05d", 42))          -- 00042

Format	Type	Example
`%s`	String	`string.format("%s", "hello")`
`%d`	Integer	`string.format("%d", 42)`
`%f`	Float	`string.format("%.2f", 3.14159)`
`%x`	Hexadecimal	`string.format("%x", 255)` → "ff"

Pattern Matching

Lua uses patterns (similar to regular expressions) for searching and replacing:

Common Pattern Functions

string.find(s, pattern) - Find pattern in string
string.match(s, pattern) - Extract matching part
string.gmatch(s, pattern) - Iterate over matches
string.gsub(s, pattern, replacement) - Replace pattern

-- Lua Pattern Matching (simpler than regex)

local text = "The price is $25.99 and the code is ABC123"

-- Basic pattern matching
print("=== Basic Patterns ===")
print("Find number:", string.match(text, "%d+"))
print("Find letters:", string.match(text, "%a+"))
print("Find word:", string.match(text, "%w+"))

-- Pattern classes
-- %a = letters, %d = digits, %w = alphanumeric
-- %s = space, %p = punctuation, %x = hexadecimal

-- Find all numbers
print("\n=== Find All Numbers ===")
for num in string.gmatch(text, "%d+") do
    print("Found:", num)
end

-- Find all words
print("\n=== Find All Words ===")
for word in string.gmatch(text, "%a+") do
    print("Word:", word)
end

-- Replace patterns
print("\n=== Replace Patterns ===")
local censored = string.gsub(text, "%d", "X")
print("Numbers censored:", censored)

-- Capture groups
print("\n=== Capture Groups ===")
local email = "user@example.com"
local username, domain = string.match(email, "(%w+)@(%w+%.%w+)")
print("Username:", username)
print("Domain:", domain)

-- Split string
print("\n=== Split String ===")
local csv = "apple,banana,cherry"
for fruit in string.gmatch(csv, "([^,]+)") do
    print("Fruit:", fruit)
end

-- Validate patterns
print("\n=== Validation ===")
local phone = "123-456-7890"
if string.match(phone, "%d%d%d%-%d%d%d%-%d%d%d%d") then
    print("Valid phone number!")
end

Output

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Pattern Classes

Pattern	Matches
`%a`	Letters (a-z, A-Z)
`%d`	Digits (0-9)
`%w`	Alphanumeric (letters and digits)
`%s`	Whitespace
`.`	Any character
`+`	One or more
`*`	Zero or more
`-`	Zero or more (non-greedy)

Tip: Lua patterns are NOT regular expressions! They're simpler but still very powerful. Use % to escape special characters.

Practice Exercise

Try these string manipulation challenges:

-- Exercise: String Manipulation
-- TODO: Complete the following exercises

-- 1. Create a full name from first and last name
local firstName = "John"
local lastName = "Doe"
local fullName = -- your code here
print("Full name:", fullName)

-- 2. Convert a string to uppercase
local message = "hello world"
local upperMessage = -- your code here
print("Uppercase:", upperMessage)

-- 3. Get the first 3 characters of a string
local word = "Lua Programming"
local first3 = -- your code here (hint: use string.sub)
print("First 3 chars:", first3)

-- 4. Count the length of your name
local myName = "YourName"  -- Replace with your name
local nameLength = -- your code here
print("Name length:", nameLength)

-- 5. Replace "bad" with "good" in a sentence
local sentence = "This is a bad example"
local fixed = -- your code here (hint: use string.gsub)
print("Fixed:", fixed)

-- 6. Check if a string contains "Lua"
local text = "I love Lua programming"
local hasLua = -- your code here (hint: use string.find)
print("Contains 'Lua':", hasLua ~= nil)

-- 7. Format a greeting with name and age
local name = "Alice"
local age = 25
local greeting = -- your code here (hint: use string.format)
print(greeting)

Output

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Summary

In this lesson, you learned:

Creating strings with quotes and long strings [[...]]
String concatenation with ..
String library functions: upper, lower, sub, find, etc.
String formatting with string.format()
Pattern matching for searching and replacing
Common pattern classes: %a, %d, %w, %s

What's Next?

Now that you can work with strings, it's time to learn about control flow. In the next lesson, we'll explore conditional statements (if, elseif, else) to make decisions in your code. Let's continue! 🚀

Previous Operators

Next Conditionals

Creating Strings

1. Double Quotes

2. Single Quotes

3. Long Strings (Multi-line)

String Operations

Concatenation

String Length

String Library Functions

Try String Functions

String Formatting

Pattern Matching

Common Pattern Functions

Pattern Classes

Practice Exercise

Summary

What's Next?

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