Regular Expressions (Regex): Complete Guide for Developers

Regular expressions (regex) are one of the most powerful tools in a developer toolkit. Once you understand them, you will use them constantly for validation, parsing, and text manipulation.

Basic Syntax

// Test if string matches pattern
/pattern/.test("string")     // → true/false

// Find matches
"hello world".match(/\w+/g)  // → ["hello", "world"]

// Replace
"hello world".replace(/world/, "developer")  // → "hello developer"

Character Classes

. → Any character except newline
\d → Digit (0-9)
\D → Non-digit
\w → Word character (a-z, A-Z, 0-9, _)
\W → Non-word character
\s → Whitespace (space, tab, newline)
\S → Non-whitespace
[abc] → Character set (a, b, or c)
[^abc] → Negated set (not a, b, or c)
[a-z] → Range (any lowercase letter)
[A-Z0-9] → Multiple ranges

Quantifiers

* → Zero or more
+ → One or more
? → Zero or one (optional)
{3} → Exactly 3
{3,} → 3 or more
{3,6} → Between 3 and 6
*? → Lazy (as few as possible)
+? → Lazy

Real-World Examples

// Email validation
const emailRegex = /^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$/;
emailRegex.test("user@example.com")  // → true

// Phone number (international)
const phoneRegex = /^\+?[\d\s\-\(\)]{10,15}$/;

// URL validation
const urlRegex = /^https?:\/\/(www\.)?[-a-zA-Z0-9@:%._+~#=]{2,256}\.[a-z]{2,6}\b/;

// Password (min 8 chars, letter + number)
const pwdRegex = /^(?=.*[A-Za-z])(?=.*\d)[A-Za-z\d@$!%*#?&]{8,}$/;

// Extract all hashtags from text
const tags = "#flutter #android #kotlin".match(/#\w+/g);
// → ["#flutter", "#android", "#kotlin"]

Kotlin Regex

// Kotlin regex
val emailRegex = Regex("""^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$""")
emailRegex.matches("user@example.com")  // true

// Find all matches
val text = "Call 123-456-7890 or 987-654-3210"
val phoneRegex = Regex("""\d{3}-\d{3}-\d{4}""")
val phones = phoneRegex.findAll(text).map { it.value }.toList()
// → ["123-456-7890", "987-654-3210"]

// Replace with transformation
val result = "hello world".replace(Regex("""\b\w""")) { it.value.uppercase() }
// → "Hello World"