第一个“非引用”子表达式只表示“查找类似于http或ftp或其它值的字符串”。
The first of these "non-reference" child subexpressions just says "match something that looks like HTTP or that looks like FTP or...".
表达式的其余部分用来检查值(去掉头尾的空格之后)是否是空字符串。
The rest of the expression is used to see if the value (without leading or trailing blanks) is an empty string.
这是包含被替换值替换了适当正则表达式匹配项的字符串。
This is the string that has the appropriate regular expressions matches replaced with the replacement value.
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