<?php
 
    $str = "23456789ABCDEGHJKMNPQRSTUVXYZabcdeghjkmnpqrstuvxyz";
    $excludes = ['cp','cb','ck','c6','c9','rn','rm','mm','co','do','cl','db','qp','qb','dp','ww'];
 
    $indexedExcludes = array_combine($excludes, $excludes);
    $dictionaryArray = str_split($str);
 
    function fastCaptcha(array $dictionary, array $excludedSyllables, int $length): string
    {
        $captcha = '';
        $generatedLength = 0;
 
        while ($generatedLength < $length) {
            $randomCharacter = $dictionary[array_rand($dictionary)];
            $captcha .= $randomCharacter;
            $generatedLength++;
        }
 
        return $captcha;
    }
 
    $excludesRegex = '/' . implode('|', $excludes) . '/';
 
    $shortCaptchas  = [];
    $mediumCaptchas = [];
    $longCaptchas   = [];
    for ($i = 0; $i < 100000; $i++)
        $shortCaptchas[]  = fastCaptcha($dictionaryArray, $indexedExcludes, 2);
        $mediumCaptchas[] = fastCaptcha($dictionaryArray, $indexedExcludes, 5);
        $longCaptchas[]   = fastCaptcha($dictionaryArray, $indexedExcludes, 10);
 
 
    $t0 = microtime(true);
 
    foreach($shortCaptchas as $captcha) {
    	preg_match($excludesRegex, $captcha);
    }
 
    $t1 = microtime(true);
 
    foreach($longCaptchas as $captcha) {
    	preg_match($excludesRegex, $captcha);
    }
 
    $t2 = microtime(true);
 
    foreach($mediumCaptchas as $captcha) {
    	preg_match($excludesRegex, $captcha);
    } 
 
    $t3 = microtime(true);
 
    echo "short: " . ($t1 - $t0) . "\n";
    echo "long:  " . ($t2 - $t1) . "\n";
    echo "medium:" . ($t3 - $t2) . "\n";

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