$re = '/(?<=>)[a-zA-Z]*:/m';
$str = '0,<|endoftext|>ERRDLLRFKH:GAGCGCCGCGACCTGTTACGATTTAAACAC<|endoftext|>
1,<|endoftext|>RRDLLRFKHG:CGCCGCGACCTGTTACGATTTAAACACGGC<|endoftext|>
2,<|endoftext|>RDLLRFKHGD:CGCGACCTGTTACGATTTAAACACGGCGAC<|endoftext|>
3,<|endoftext|>DLLRFKHGDS:GACCTGTTACGATTTAAACACGGCGACAGT<|endoftext|>';
preg_match_all($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