正则表达式，尽量匹配短的部分

*、+等后面加上?，就表示尽量匹配短的部分，比如：

%(<span class="normal"> </span><span class="keyword">def</span>).sub!(/(<span class="normal">)(.*)(<\/span>)/) {|s| puts s}
=> <span class="normal"> </span><span class="keyword">def</span>

%(<span class="normal"> </span><span class="keyword">def</span>).sub!(/(<span class="normal">)(.*?)(<\/span>)/) {|s| puts s}
=> <span class="normal"> </span>