第一次发现JavaScript中replace() 方法如果直接用str.replace("-","!") 只会替换第一个匹配的字符.
而str.replace(//-/g,"!")则可以全部替换掉匹配的字符(g为全局标志)。

replace()
The replace() method returns the string that results when you replace text matching its first argument
(a regular expression) with the text of the second argument (a string).
If the g (global) flag is not set in the regular expression declaration, this method replaces only the first
occurrence of the pattern. For example,

var s = "Hello. Regexps are fun.";s = s.replace(//./, "!"); // replace first period with an exclamation pointalert(s);

produces the string “Hello! Regexps are fun.” Including the g flag will cause the interpreter to
perform a global replace, finding and replacing every matching substring. For example,

var s = "Hello. Regexps are fun.";s = s.replace(//./g, "!"); // replace all periods with exclamation pointsalert(s);

yields this result: “Hello! Regexps are fun!”

所以可以用以下几种方式.：
string.replace(/reallyDo/g, replaceWith);
string.replace(new RegExp(reallyDo, 'g'), replaceWith);

string：字符串表达式包含要替代的子字符串。
reallyDo：被搜索的子字符串。
replaceWith：用于替换的子字符串。