$re = '/(?<=vol": \').*(?=\',\s*"tvr)/m';
$str = 'M18.r_16981 = {
"ltt": \'2018/06/21 11:43\',
"np": \'0.050\',
"iep": \'0.000\',
"iev": \'0\',
"ltp": \'0.050\',
"vol": \'940000\',
"tvr": \'49860\',
"dyh": \'0.060\',
"dyl": \'0.049\'
};';
preg_match_all($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