$re = '/^(?:[^{\n]|{(?!{)|{{2}(?:dog|cat)}{2})+$/m';
$str = 'I own a {{dog}} and {{cat}} but not fish
I own a fish
I own a {{cow}}
I own a {{dog}} and a {{cow}}
I own a {{cat}} hah
I {own} a {{dog}}
I {own} a {{cat}} and {cow}
I own a {cow}
';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
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