gistfile1.kt

fun String.pluralize() : String{
    val rules = listOf(
    "$" to "s",
    "s$"to "s",
    "(ax|test)is$" to "$1es",
    "us$" to "i",
    "(octop|vir)us$" to "$1i",
    "(octop|vir)i$" to "$1i",
    "(alias|status)$" to "$1es",
    "(bu)s$" to "$1ses",
    "(buffal|tomat)o$" to "$1oes",
    "([ti])um$" to "$1a",
    "([ti])a$" to "$1a",
    "sis$" to "ses",
    "(,:([^f])fe|([lr])f)$" to "$1$2ves",
    "(hive)$" to "$1s",
    "([^aeiouy]|qu)y$" to "$1ies",
    "(x|ch|ss|sh)$" to "$1es",
    "(matr|vert|ind)ix|ex$" to "$1ices",
    "([m|l])ouse$" to "$1ice",
    "([m|l])ice$" to "$1ice",
    "^(ox)$" to "$1en",
    "(quiz)$" to "$1zes",
    "f$" to "ves",
    "fe$" to "ves")
    val unCountable = listOf("equipment", "information", "rice", "money",
            "species", "series", "fish", "sheep", "aircraft","bison", "flounder", "pliers", "bream",
            "gallows", "proceedings", "breeches", "graffiti", "rabies",
            "britches", "headquarters", "salmon", "carp", "herpes",
            "scissors", "chassis", "high-jinks", "sea-bass", "clippers",
            "homework", "cod", "innings", "shears",
            "contretemps", "jackanapes", "corps", "mackerel",
            "swine", "debris", "measles", "trout", "diabetes", "mews",
            "tuna", "djinn", "mumps", "whiting", "eland", "news",
            "wildebeest", "elk", "pincers", "sugar")
    val exceptions = listOf("person" to "people",
    "man" to "men",
    "goose" to "geese",
    "child" to "children",
    "sex" to "sexes",
    "move" to "moves",
    "stadium" to "stadiums",
    "deer" to "deer")
    if(unCountable.contains(this)) return this
    val rule = rules.last { Pattern.compile(it.component1(), Pattern.CASE_INSENSITIVE).matcher(this).find()}
    var found = Pattern.compile(rule.component1(), Pattern.CASE_INSENSITIVE).matcher(this).replaceAll(rule.component2())
    val excep = exceptions.firstOrNull() { this.equals(it.component1())}
    if(excep !=null) found = excep.component2()
    val endswith =    exceptions.firstOrNull { this.endsWith(it.component1())}
    if(endswith != null) found = this.replace(endswith.component1(),endswith.component2())
    return found
}

pluralizetests.kt

package org.humanizer.jvm.tests

import org.spek.Spek
import org.spek.givenData
import org.humanizer.jvm.titleize
import org.spek.shouldEqual
import org.humanizer.jvm.pluralize
import org.humanizer.jvm.singularize

public class PluralizeTests() : Spek() {
    {
        var data = listOf(
                "search" to  "searches",
                "switch" to  "switches",
                "fix" to  "fixes",
                "box" to  "boxes",
                "process" to  "processes",
                "address" to  "addresses",
                "case" to  "cases",
                "stack" to  "stacks",
                "wish" to  "wishes",
                "fish" to  "fish",

                "category" to  "categories",
                "query" to  "queries",
                "ability" to  "abilities",
                "agency" to  "agencies",
                "movie" to  "movies",

                "archive" to  "archives",

                "index" to  "indices",

                "wife" to  "wives",
                "safe" to  "saves",
                "half" to  "halves",

                "move" to  "moves",

                "salesperson" to  "salespeople",
                "person" to  "people",

                "spokesman" to  "spokesmen",
                "man" to  "men",
                "woman" to  "women",

                "basis" to  "bases",
                "diagnosis" to  "diagnoses",

                "datum" to  "data",
                "medium" to  "media",
                "analysis" to  "analyses",

                "node_child" to  "node_children",
                "child" to  "children",

                "experience" to  "experiences",
                "day" to  "days",

                "comment" to  "comments",
                "foobar" to  "foobars",
                "newsletter" to  "newsletters",

                "old_news" to  "old_news",
                "news" to  "news",

                "series" to  "series",
                "species" to  "species",

                "quiz" to  "quizzes",

                "perspective" to  "perspectives",

                "ox" to  "oxen",
                "photo" to  "photos",
                "buffalo" to  "buffaloes",
                "tomato" to  "tomatoes",
                "dwarf" to  "dwarves",
                "elf" to  "elves",
                "information" to  "information",
                "equipment" to  "equipment",
                "bus" to  "buses",
                "status" to  "statuses",
                "status_code" to  "status_codes",
                "mouse" to  "mice",

                "louse" to  "lice",
                "house" to  "houses",
                "octopus" to  "octopi",
                "virus" to  "viri",
                "alias" to  "aliases",
                "portfolio" to  "portfolios",

                "vertex" to  "vertices",
                "matrix" to  "matrices",

                "axis" to  "axes",
                "testis" to  "testes",
                "crisis" to  "crises",

                "rice" to  "rice",
                "shoe" to  "shoes",

                "horse" to  "horses",
                "prize" to  "prizes",
                "edge" to  "edges",

                "deer" to  "deer",
                "sheep" to  "sheep",
                "wolf" to  "wolves"
        )

        givenData(data) {
            val (value, expected) = it
            on("calling pluralize on String", {
                val actual = value.pluralize()
                it("should be ${expected}", {
                    shouldEqual(expected, actual)
                })
            })
        }

        /*givenData(data) {
            val (expected, value) = it
            on("calling singularize on String", {
                val actual = value.singularize()
                it("should be ${expected}", {
                    shouldEqual(expected, actual)
                })
            })
        }*/
    }
}

Your problem is that first is not finding a matching element. That's what the exception is throwing.

the compiler hints are wrong because when I run the tests I get an error on that line. Added tests.

So maybe I should use some other method?

Compiler hints aren't wrong. You're assigning a value to found, not using it. Then assigning a new value, not using it, then assigning another value, not using it.

you are saying I'm wrong. The nerve.

Ok I was wrong, the last version is now there.