$re = '/(?<="signature":")[^"]+/';
$str = '{"payload":"eyJjaGFsbGVuZ2UiOiIwMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAiLCJtaW5pbXVtTW9iaWxlVmVyc2lvbiI6MSwibm9uY2UiOiJPQ1dwdjkzSzRMSCtQM0FOQmJDM0NCcHRQMmtES1M5TFZPVGFpWU9wUXFFPSJ9","signature":"hlMKmwJt8f24jv6MluM0W+0Kzxo96XlfM1GDop3CHPgLnLFpLmJ9imHFeCCU5bedA4jREuuQ5Y9pY+AqU5qt5QHAjPrtFyT69PktV8B7e+i9tqXae8haIB5iQUD9ln2yjMPE3XfG8+4jHEnHv7x7dePKr4zRgRTRQj6ZbMBaFGt1nhrDexfwHTiGfwuMGuneHUsBeb/EHjaKyGcOoLNq1xjHijlYVb3C3RF+y04VraN3Pt1mfjG9O+3Jvt3cF1xL4nhQO93ZvZ6UU2a0x74PutXxhfoWaeMiZSG0nCBGVP8WNDil1V00cX1Darpp/Xti6RlJJO+CKESInoBMxIWWYw=="}';
preg_match($re, $str, $matches, PREG_OFFSET_CAPTURE, 0);
// Print the entire match result
var_dump($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