Regex - PHP - PCRE English Pluralization

<?php
 function add_s($word) {
    $rg_strt="(.{2,})"; // the word is greater then 2 characters
    $rg_es="((t|c|s)h|s|x|i|o)"; // the word needs only an 'e'
    $rg_yo="(([^aeiou])y)"; // 'y' transform, but not 'ay', etc
    $rg_v="([^(f|r|o|a)])(fe?)"; // 'f' needs to transform to 've'
    $rp_strt="'\\2'!=''"; // Replacement should be done
    $rp_1="'\\1'"; // Keep match made by $rg_start
    $rp_y="'\\6'!=''"; // $rg_yo was matched with a 'y'
    $rp_xi="'\\6i'"; // replace 'y' with an 'i' (keep non-vowel match)
    $rp_v="'\\8'!=''"; // $rg_v was matched
    $rp_xv="'\\7v'"; // transform 'f' to 'v' (keep leading char)
    $rp_xe="'\\2'"; // $rg_es was matched, keep match
    $reg="^$rg_strt($rg_es|$rg_yo|$rg_v)$";
    $rep="$rp_strt?($rp_1.($rp_y?$rp_xi:($rp_v?$rp_xv:$rp_xe)).'e'):''";
    return preg_replace("/$reg/e",$rep,$word)."s";
 }
?>

<?php
 header("Content-type: text/plain");
 $types=array();
 $types["s"]=array("boy","girl","chair","cat","lap","clock","day","monkey");
 $types["es"]=array("dish","glass","judge","phase","witch");
 $types["oes"]=array("hero","potato","volcano");
 $types["ies"]=array("cherry","lady");
 $output=array();
 foreach ($types as $type => $words) {
    $output[$type]=array();
    foreach ($words as $word) {
       $output[$type][$word]=add_s($word);
    }
 }
 print_r($output);
?>

Array
(
    [s] => Array
        (
            [boy] => boys
            [girl] => girls
            [chair] => chairs
            [cat] => cats
            [lap] => laps
            [clock] => clocks
            [day] => days
            [monkey] => monkeys
        )
    [es] => Array
        (
            [dish] => dishes
            [glass] => glasses
            [judge] => judges
            [phase] => phases
            [witch] => witches
        )
    [oes] => Array
        (
            [hero] => heroes
            [potato] => potatoes
            [volcano] => volcanoes
        )
    [ies] => Array
        (
            [cherry] => cherries
            [lady] => ladies
        )
)