$re = '/"([^0-9][^"]*)":/m';
$str = 'props: {
"label": "1rem",
"text3": "1rem",
"text2Button": "1rem",
"1": "1rem",
"5spacing": 2
}
';
preg_match_all($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