[boost] boost regex recursive quesion

This is my first time to using maillist I'm learning to use boost::regex for a month , a few days ago I found a problem( maybe is my mistake ?) I construct a simple recursive regular expression: a(?0)? This regex contains a recursive call (?0). It should match a string with any arbitrary length filled with 'a' characters. regex_match only returns true for "a" and "aa", but failed to match "aaa" and any string longer than three characters. Why? I try it on boost 1.45 and 1.57 , both get same result . I had sent this question on stackoverflow , but had not get a answer now . Is there any flag should be used with regex_match to use recursive call ? Thank you .