小结：
?与/U有类似功能，但同时出现彼此抵消
如下：
$a="asdf/*asdfaldsfasdf*/asfdasldf;kfldsj*/asfddsaf";
$pattern="/\/\*.*?\*\//";
//$pattern="/\/\*.*\*\//U";
//$pattern="/\/\*.*?\*\//U";
preg_match($pattern,$a,$match);
print_r($match);
?>
2.Assertions
\w+(?=;)
matchesawordfollowedbyasemicolon,butdoesnotinclude
thesemicoloninthematch,and
foo(?!bar)
matchesanyoccurrenceof"foo"thatisnotfollowedby
"bar".Notethattheapparentlysimilarpattern
小结：
(?!)只前向判断匹配，如bar(?!foo)，而(?!foo)bar没有意义
(?
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