$re = '/(?<=^\#\[\d{2}\])\S*/m';
$str = '?[A7]DA<DA-SG
\'[G7]G%SD\\$DF
#[27]F:./4FFF
?[P9]W3_2SS_F
\'[90]GA\\\\WTER';
$subst = "something_else";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
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