$re = '/(?<=\bRate: )(?:[5-9]\d{3}|[1-9]\d{4,})\b/';
$str = ' - Item: Oldman\'s_Romance
Rate: 50
- Item: Grasshopper\'s_Leg
Rate: 8000
- Item: Azure_Jewel
Rate: 9000
- Item: Grasshopper_Doll
Rate: 1500
- Item: Angel\'s_Arrival
Rate: 1000
- Item: Center_Potion
Rate: 700
- Item: Angel\'s_Departure
Rate: 10600
- Item: Left_Potion
Rate: 23471
- Item: Right_Potion
aRate: 6000';
$subst = "5000";
$result = preg_replace($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