$re = '/\b(?:APPLES|BANANAS|PEARS)\b(?:\s*(?:,\s*)?(?:(?:or|and)\s+)?(?:APPLES|BANANAS|PEARS))*\b/mi';
$str = 'apples, pears, bananas are fruit
pears, apples, and bananas are in the fruit basket
you can use bananas, pears, and apples in a fruit salad
he also likes pears, bananas, and apples for a snack
she likes cheese for a snack but also apples or pears';
$subst = "[\g<0>]";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php