preg_replace('/b^(true|false|TRUE|FALSE)b/', '', 'false'); // Returns an empty string, but i expect 'false'

preg_replace('/b^(true|false|TRUE|FALSE)b/', '', 'test') // Returns 'test', but i expect an empty string

preg_replace('/b^(true|false|TRUE|FALSE)b/', '', 'This regex allow only boolean, such as true, false, TRUE and FALSE');

// This regex allow only boolean, such true, false, TRUE and FALSE

$arr = array('true', 'false', 'TRUE', 'FALSE');

$result = (in_array($str, $arr, true)) ? $str : '';

$re = '/b(?!(?:true|false|TRUE|FALSE))w+b/';
$str = 'I need true to allow only false specific FALSE words in a TRUE string';

$repl = preg_replace($re, "", $str);
//=>   true    false  FALSE    TRUE

if (preg_match('/^(true|false|TRUE|FALSE)$/', $string, $match)) {
    echo "Found : ",$m[1],"n";
} else {
    echo "Not foundn";
}

$regex = '/(?=.)(true|false|TRUE|FALSE|\s{0,})(?!.)/';
$testString = ''; // Fill this in
preg_match($regex, $testString, $matches);
// the $matches variable contains the list of matches