ฟังก์ชัน String สำคัญ: trim, split, replace, contains

val s = "   hello   "
println(s.trim())           // "hello"
println(s.trimStart())      // "hello   "
println(s.trimEnd())        // "   hello"
println("##hi##".trim('#')) // "hi"

val csv = "a,b,c,d"
val parts = csv.split(",")
println(parts)       // [a, b, c, d]
println(parts.size)  // 4
val words = "one  two   three".split(Regex("\\s+"))

val s = "a:b:c:d"
println(s.split(":", limit = 2))  // [a, b:c:d]
println(s.split(":", limit = 3))  // [a, b, c:d]

val s = "hello hello hello"
println(s.replace("hello", "hi"))           // hi hi hi
println(s.replaceFirst("hello", "hey"))     // hey hello hello

val text = "I have 3 cats and 12 dogs"
val noNumbers = text.replace(Regex("\\d+"), "N")
println(noNumbers)  // I have N cats and N dogs

val s = "Hello, Kotlin!"
println(s.contains("Kotlin"))      // true
println(s.startsWith("Hello"))     // true
println(s.endsWith("!"))           // true
println("kotlin" in s)             // false (case-sensitive)

val s = "Hello World"
println(s.contains("hello", ignoreCase = true))   // true
println(s.startsWith("HELLO", ignoreCase = true)) // true
println(s.equals("hello world", ignoreCase = true)) // true

val text = "line1\nline2\nline3"
val lines = text.lines()
println(lines.size)  // 3
println(lines.joinToString(" | "))

println("42".padStart(6, '0'))  // 000042
println("hi".padEnd(5))         // "hi   "
println("-".repeat(20))          // --------------------

println("42".toIntOrNull())       // 42
println("abc".toIntOrNull())      // null
println("3.14".toDoubleOrNull())  // 3.14
val n = "abc".toIntOrNull() ?: 0  // safe default