$re = '/"Sales_Rep_lookup__c":("?)(?<Salerep>.*?)("?)*?,"Opportunity__c":("?)(?<Opptun>.*?)("?)*?,"Project__c":("?)(?<project>.*?)("?)}]}/m';
$str = '{"searchRecords":[{"attributes":{"type":"Contact","url":"/services/data/v43.0/sobjects/Contact/0031K00002q8BrXQAU"},"Id":"0031K00002q8BrXQAU","Phone":"7032847786","Sales_Rep_lookup__c":"a1J1K00000NLykQUAT","Opportunity__c":"0061K00000eQXdIQAW","Project__c":"a0h1K00000CQzHpQAL"}]}';
preg_match_all($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