07 Strings

In this tutorial, you will learn about Julia Strings Guide. We cover key concepts, practical examples, and best practices to help you master this topic.

Julia strings are UTF-8 encoded immutable byte sequences supporting interpolation with $, concatenation with * or string(), substring extraction, and regex pattern matching with r"..." literals and match/sub/replace functions.

Creating Strings

# Literal strings
s1 = "Hello, World!"
s2 = """Multi-line
string"""

# Interpolation
name = "Alice"
age = 30
greeting = "Hello, $name! You are $age years old."
calc = "2 + 2 = $(2 + 2)"

# Concatenation
s = "Hello" * " " * "World"
s = string("Hello", " ", "World")

String Operations

s = "Hello, Julia!"

# Length (characters, not bytes)
length(s)          # 14

# Indexing (byte positions, 1-based)
s[1]               # 'H' (Char, not string)
s[8]               # 'J'

# Substring
s[1:5]             # "Hello"
s[8:12]            # "Julia"

# Search
findfirst("Julia", s)  # 8:12 (UnitRange)
occursin("Julia", s)   # true
contains(s, "Julia")   # true

# Split and join
split("a,b,c", ",")    # ["a", "b", "c"]
join(["a", "b", "c"], "-")  # "a-b-c"

Regular Expressions

# Regex literal
email_pattern = r"[\w.]+@[\w.]+\.\w+"
phone_pattern = r"\d{3}-\d{3}-\d{4}"

# Matching
m = match(email_pattern, "Contact: user@example.com")
m.match          # "user@example.com"
m.captures       # [] (no capture groups)

# With capture groups
m = match(r"(\w+)@(\w+)\.(\w+)", "user@example.com")
m.captures       # ["user", "example", "com"]

# Replace
replace("Hello World", r"\w+" => uppercase)
# "HELLO WORLD"

# Test
occursin(r"\d+", "abc123")  # true

Useful Functions

# Case conversion
uppercase("hello")    # "HELLO"
lowercase("HELLO")    # "hello"
titlecase("hello world")  # "Hello World"

# Trimming
strip("  hello  ")    # "hello"
lstrip("  hello")     # "hello  "
rstrip("hello  ")     # "  hello"

# Padding
lpad("5", 3, "0")     # "005"
rpad("5", 3, "0")     # "500"

# Repeat
repeat("ha", 3)       # "hahaha"

Common Mistakes

1. Using + for concatenation

Julia uses * for string concatenation, not +. "Hello" + " " + "World" errors.

2. 1-based string indexing

s[0] errors. Strings use 1-based indexing like arrays. s[1] is the first character.

3. Byte vs character indexing

length(s) counts characters. sizeof(s) counts bytes. Non-ASCII characters use multiple bytes.

Practice Questions

1. How do you interpolate a variable into a string? Use $ prefix: "Hello, $name". For expressions: "Result: $(a + b)".

2. How do you concatenate strings? Use * operator: "Hello" * " " * "World", or string() function.

3. How do you test if a string matches a pattern? occursin(pattern, string) or contains(string, pattern) or match(pattern, string) !== nothing.

FAQ

{{< faq question="Are Julia strings mutable?" >}} No. Strings are immutable. Operations like replace return new strings. Use SubString for zero-copy substrings. {{< /faq >}}

{{< faq question="How do I convert a number to a string?" >}} Use string(42), repr(42), or sprintf("%.2f", 3.14) from Printf module. {{< /faq >}}

{{< faq question="How do I handle Unicode in strings?" >}} Julia strings are UTF-8. length counts characters (codepoints). Use eachindex(s) for valid byte positions. {{< /faq >}}

What's Next

Now learn about control flow in Julia.