上面這個網站可以用于在線檢測JS的正則表達式語法

除了熟知的幾個固定字符表示位置：

^ : 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.

還有的就是使用正則表達式來確定要匹配的位置，也叫做Zero-Width Test（零寬斷言）

(?=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.

對于(?=p)和(?!p)的使用舉一個例子：

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"?

正則表達式: (?!//)default(?=//)

其中(?!//)表示以"/"開頭，(?=//)表示以"/"結尾