http://regexpal.com/
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
