Regular Expression

There's an input string like "{str1,str2,...,strN}". I want to capture all these strings and due to complexity of them I use a preg_match_all() instead of simple split. A pattern for the matching strings is ready but I cannot imagine how to specify that strings are separated by commas and the last one is not followed by comma. For example, I'm afraid that this pattern "/^{(?:(pattern),)*|(pattern)?}$/" can match and capture properly constructed input string, but, in addition, this matches if the string end is ",}". Any ideas?