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"https://www.jb51.net/"开头，(?=\/)表示以"https://www.jb51.net/"结尾