JS 正则表达式的位置匹配

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