🔍 Regular Expressions

📖 What are Regular Expressions?

Regular Expressions (Regex) are a way to express string patterns. They are used for searching, validation, extraction, and replacement!

💡 Basic Usage

Creating Regex

fun main() {
    // Create Regex object
    val regex1 = Regex("hello")
    val regex2 = "hello".toRegex()

    // Check matching
    println(regex1.matches("hello"))  // true
    println(regex1.matches("Hello"))  // false (case sensitive)
}

Simple Matching

fun main() {
    val text = "My email is hong@example.com"

    // contains
    val hasEmail = Regex("@").containsMatchIn(text)
    println(hasEmail)  // true

    // find
    val match = Regex("\\w+@\\w+\\.\\w+").find(text)
    println(match?.value)  // hong@example.com
}

🎯 Basic Patterns

Character Classes

fun main() {
    // \d - digits
    println("123".matches(Regex("\\d+")))  // true

    // \w - word characters (letters, digits, _)
    println("hello_123".matches(Regex("\\w+")))  // true

    // \s - whitespace
    println(" ".matches(Regex("\\s")))  // true

    // . - any character
    println("abc".matches(Regex("...")))  // true (3 characters)
}

Quantifiers

fun main() {
    // + : 1 or more
    println("aaa".matches(Regex("a+")))  // true

    // * : 0 or more
    println("".matches(Regex("a*")))  // true

    // ? : 0 or 1
    println("a".matches(Regex("a?")))  // true

    // {n} : exactly n
    println("aaa".matches(Regex("a{3}")))  // true

    // {n,m} : between n and m
    println("aaa".matches(Regex("a{2,4}")))  // true
}

🎨 Practical Examples

Email Validation

fun isValidEmail(email: String): Boolean {
    val pattern = "[a-zA-Z0-9._-]+@[a-z]+\\.+[a-z]+"
    return email.matches(pattern.toRegex())
}

fun main() {
    println(isValidEmail("hong@example.com"))  // true
    println(isValidEmail("invalid.email"))     // false
    println(isValidEmail("test@domain.co.kr")) // true
}

Phone Number Validation

fun isValidPhone(phone: String): Boolean {
    // 010-1234-5678 or 01012345678
    val pattern = "010-?\\d{4}-?\\d{4}"
    return phone.matches(pattern.toRegex())
}

fun main() {
    println(isValidPhone("010-1234-5678"))  // true
    println(isValidPhone("01012345678"))    // true
    println(isValidPhone("02-1234-5678"))   // false
}

Password Strength

fun checkPasswordStrength(password: String): String {
    val hasLength = password.length >= 8
    val hasDigit = Regex("\\d").containsMatchIn(password)
    val hasLower = Regex("[a-z]").containsMatchIn(password)
    val hasUpper = Regex("[A-Z]").containsMatchIn(password)
    val hasSpecial = Regex("[!@#$%^&*]").containsMatchIn(password)

    return when {
        hasLength && hasDigit && hasLower && hasUpper && hasSpecial -> "Very Strong"
        hasLength && hasDigit && (hasLower || hasUpper) -> "Strong"
        hasLength -> "Medium"
        else -> "Weak"
    }
}

fun main() {
    println(checkPasswordStrength("Pass123!"))      // Very Strong
    println(checkPasswordStrength("password123"))   // Strong
    println(checkPasswordStrength("password"))      // Medium
    println(checkPasswordStrength("pass"))          // Weak
}

URL Extraction

fun extractUrls(text: String): List<String> {
    val pattern = "https?://[\\w./]+"
    return Regex(pattern).findAll(text).map { it.value }.toList()
}

fun main() {
    val text = """
        Visit https://kotlin.org for info.
        Check http://example.com too!
    """.trimIndent()

    val urls = extractUrls(text)
    urls.forEach { println(it) }
    // https://kotlin.org
    // http://example.com
}

Hashtag Extraction

fun extractHashtags(text: String): List<String> {
    val pattern = "#\\w+"
    return Regex(pattern).findAll(text).map { it.value }.toList()
}

fun main() {
    val tweet = "Beautiful weather today #weather #happiness #kotlin"
    val hashtags = extractHashtags(tweet)

    println("Hashtags:")
    hashtags.forEach { println(it) }
    // #weather
    // #happiness
    // #kotlin
}

🔧 Advanced Features

Group Capture

fun parseDate(text: String): Triple<String, String, String>? {
    val pattern = "(\\d{4})-(\\d{2})-(\\d{2})"
    val match = Regex(pattern).find(text) ?: return null

    val (year, month, day) = match.destructured
    return Triple(year, month, day)
}

fun main() {
    val date = parseDate("Today is 2024-12-25")
    if (date != null) {
        val (year, month, day) = date
        println("Year: $year, Month: $month, Day: $day")
    }
}

Replacement

fun main() {
    val text = "My phone is 010-1234-5678"

    // Mask phone number
    val masked = text.replace(Regex("\\d{4}$"), "****")
    println(masked)  // My phone is 010-1234-****

    // Multiple spaces to one
    val multiSpace = "Hello    World    !"
    val cleaned = multiSpace.replace(Regex("\\s+"), " ")
    println(cleaned)  // Hello World !
}

Splitting

fun main() {
    // Split by comma or space
    val text = "apple, banana orange,grape"
    val fruits = text.split(Regex("[,\\s]+"))
    println(fruits)  // [apple, banana, orange, grape]

    // Split by digits
    val text2 = "item1,item2,item3"
    val items = text2.split(Regex("\\d+"))
    println(items)  // [item, ,item, ,item, ]
}

🎯 Practical Patterns

Remove HTML Tags

fun removeHtmlTags(html: String): String {
    return html.replace(Regex("<[^>]*>"), "")
}

fun main() {
    val html = "<p>Hello <b>World</b>!</p>"
    val text = removeHtmlTags(html)
    println(text)  // Hello World!
}

Extract Numbers

fun extractNumbers(text: String): List<Int> {
    return Regex("\\d+").findAll(text)
        .map { it.value.toInt() }
        .toList()
}

fun main() {
    val text = "I have 3 apples and 5 oranges"
    val numbers = extractNumbers(text)
    println(numbers)  // [3, 5]
}

Currency Formatting

fun formatCurrency(amount: Int): String {
    return amount.toString().replace(Regex("(\\d)(?=(\\d{3})+$)"), "$1,")
}

fun main() {
    println(formatCurrency(1000))      // 1,000
    println(formatCurrency(1234567))   // 1,234,567
}

🤔 Frequently Asked Questions

Q1. How to ignore case?

A: Use RegexOption!

fun main() {
    val regex = Regex("hello", RegexOption.IGNORE_CASE)

    println(regex.matches("Hello"))  // true
    println(regex.matches("HELLO"))  // true
    println(regex.matches("hello"))  // true
}

Q2. How to escape characters?

A: Use double backslash!

fun main() {
    // Match . literally
    val dotRegex = Regex("\\.")
    println("3.14".contains(dotRegex))  // true

    // Match ( ) literally
    val parens = Regex("\\(.*\\)")
    println("(test)".matches(parens))  // true
}

Q3. What about performance?

A: Reuse compiled Regex!

// ❌ Compile every time
fun bad(text: String): Boolean {
    return text.matches(Regex("\\d+"))  // slow
}

// ✅ Compile once
val NUMBER_REGEX = Regex("\\d+")
fun good(text: String): Boolean {
    return text.matches(NUMBER_REGEX)  // fast
}

📚 Pattern Reference

Characters:
^       - start
$       - end
.       - any character
\d      - digit [0-9]
\D      - not digit
\w      - word character [a-zA-Z0-9_]
\W      - not word character
\s      - whitespace
\S      - not whitespace

Quantifiers:
*       - 0 or more
+       - 1 or more
?       - 0 or 1
{n}     - exactly n
{n,}    - n or more
{n,m}   - between n and m

Groups:
(...)   - group
|       - OR
[abc]   - a, b, or c
[^abc]  - not a, b, or c
[a-z]   - a through z

🎬 Conclusion

Powerful string processing with regular expressions!

Key Takeaways:
✅ Create patterns with Regex()
✅ Full matching with matches()
✅ Partial matching with find()
✅ Replacement with replace()
✅ Improve performance by reusing patterns

Next Step: Learn to create Kotlin-style APIs in DSL!