$re = '/public_key=#STARTKEY#(.*)#ENDKEY#/s';
$str = 'public_key=#STARTKEY#c3NoLXJzYSBBQUFBQjNOemFDMXljMkVBQUFBREFRQUJBQUFCZ1FEWVZVZDY5eVBDS2hvejdjMUI5
dWx0TStNZmpRR3BnZG9xZGhFTDRKVStsRkt5OFpwazVTdy9Ic3RlVnZvZm5qbGlLajRjNWNXbzdr
NEZUSy81dzZCcXlmSHpkcnFSRHNWbHNWOERLSUpJejBrTFJoMUFHdlQ3c1lWcXpwNUhFcnN4ZnZI
K0pEZlNVa0pUOElTWE4xSkxkcllxUjJzQVBmMjhsbVl1eXpkaGprQTNzdWZlZ2dQQWYzam1ndFFP
Q1JHWi9ZdmZKcjFLcVZIZDdvYUdOOE1Ndm9BMy9reE9iTWVCSk5ldkl5VHNwdWF2bWxGU29qMkVU
Z12RGeUphQ0FGamQ3VFF2ZXJGM0JSeUVxNDhDMUR4OTE3cWJvRVJaQXZ6elQxU0lIZHk2MG5QVHpa
OEM5akRxbzZlVmJYNk8xT05ZckViM0eYhRlmk0WkpjMWcvRi83WVA3SHVhSy91V0llb2JkN2JNRmtj
NFNhSlJxRE1JNE9ERFdzSHhKL29ZalZrL1VrU1dqSXJZTVBaUDA4VW1XaVljcHh4cFF6YmZKb3pH
bmNqMEFodUlqZzQ3blJwMTJTUWY5cDRnQ1UzK2NxdVNPQmZjTXExd0ZHcXVxMEk0ZEVIdCtiNmZi
OFpmYll1Z0lpa2xRZUlYMzQ4cnl2aFpzaVQvZ3AvRllSaFZEZTA9IG5pY2NoYXVkQGNpc2NvLmNv
bUBOSUNDSEFVRC1NLUZCVE4K#ENDKEY#';
preg_match($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