I guess adding type aliases and removing the regex from parser makes it a bit more readable.

typealias Rule = Pair<Int, Int>
typealias PageNumbers = List<Int>

fun part1(input: String): Int {
    val (rules, listOfNumbers) = parse(input)
    return listOfNumbers
        .filter { numbers -> numbers == sort(numbers, rules) }
        .sumOf { numbers -> numbers[numbers.size / 2] }
}

fun part2(input: String): Int {
    val (rules, listOfNumbers) = parse(input)
    return listOfNumbers
        .filterNot { numbers -> numbers == sort(numbers, rules) }
        .map { numbers -> sort(numbers, rules) }
        .sumOf { numbers -> numbers[numbers.size / 2] }
}

private fun sort(numbers: PageNumbers, rules: List<Rule>): PageNumbers {
    return numbers.sortedWith { a, b -> if (rules.contains(a to b)) -1 else 1 }
}

private fun parse(input: String): Pair<List<Rule>, List<PageNumbers>> {
    val (rulesSection, numbersSection) = input.split("\n\n")
    val rules = rulesSection.lines()
        .mapNotNull { line ->
            val parts = line.split('|').map { it.toInt() }
            if (parts.size >= 2) parts[0] to parts[1] else null
        }
    val numbers = numbersSection.lines()
        .map { line -> line.split(',').map { it.toInt() } }
    return rules to numbers
}