主要使用的有下面三种：

1.  (...)

Grouping. Group items into a single unit that can be used with *, +, ?, |, and so on. Also remember the characters that match this group for use with later references.

2. (?:...)

Grouping only. Group items into a single unit, but do not remember the characters that match this group.

3.

(?=p)  A positive lookahead assertion. 或

(?!p)  A negative lookahead assertion.

       eg: (\/)匹配url(skins/default/images/index/default.png)

           (\/)default\1匹配url(skins/default/images/index/default.png)

第二种，用于分组，不记录匹配该分组的字符串

       eg: (?:\/)default匹配url(skins/default/images/index/default.png)

第一、二两种方式匹配的结果里都包含有该分组匹配到的结果，在例子中是"/default"中的"/"

第三种，只用于确定位置，最终结果里并不包含该括号匹配到的结果

       eg:  (?!\/)default匹配url(skins/default/images/index/default.png)

       eg:  default(?=\/)匹配url(skins/default/images/index/default.png)