This means "match a subpattern, but don't include the match in the back-references." So let's examine the subexpression
这表示“查找子模式的匹配字符串，但不包括反向引用中的匹配结果”。
2
The POSIX standard does not support back-references for "extended" regular expressions, this is a compatible extension to that standard.
POSIX标准并不支持“扩展”正则表达式的后向引用，这是对标准的兼容扩展。
3
This Web service application will enable users to upload any number of files to the server and get referencesback to those files, as well as download the files they want using these references.