上面这个网站可以用于在线检测JS的正则表达式语法

除了熟知的几个固定字符表示位置：

^ : Match the beginning of the string and, in multiline searches, the beginning of a line.

$ : Match the end of the string and, in multiline searches, the end of a line.

b: 

Match a word boundary. That is, match the position between a w character and a W character or between a w character and the beginning or end of a string. (Note, however, that [b] matches backspace.)

B: Match a position that is not a word boundary.

还有的就是使用正则表达式来确定要匹配的位置，也叫做Zero-Width Test（零宽断言）

(?=p) :

A positive lookahead assertion. Require that the following characters match the pattern p, but do not include those characters in the match.

(?!p) ：

A negative lookahead assertion. Require that the following characters do not match the pattern p.

对于(?=p)和(?!p)的使用举一个例子：

要在url(skins/default/images/index/default.png)中匹配"/default/"中的"default"，而不匹配"/default.png"中的"default"?

正则表达式: (?!/)default(?=/)

其中(?!/)表示以"/"开头，(?=/)表示以"/"结尾