how to use dotall flag for regex.exec()

In 2018, with the ECMA2018 standard implemented in some browsers for the time being, JS regex now supports s DOTALL modifier:

Browser support

console.log("foo\r\nbar".match(/.+/s)) // => "foo\r\nbar"

Actually, JS native match-all-characters regex construct is

[^]

It means match any character that is not nothing. Other regex flavors would produce a warning or an exception due to an incomplete character class (demo), though it will be totally valid for JavaScript (demo).

The truth is, the [^] is not portable, and thus is not recommendable unless you want your code to run on JS only.

regex = /--Head([^]*)--\/Head/

To have the same pattern matching any characters in JS and, say, Java, you need to use a workaround illustrated in the other answers: use a character class with two opposite shorthand character classes when portability is key: [\w\W], [\d\D], [\s\S] (most commonly used).

NOTE that [^] is shorter.

javascript doesn't support s (dotall) modifier. The only workaround is to use a "catch all" class, like [\s\S] instead of a dot:

regex = new RegExp("\-{2}Head([\\s\\S]*)-{2}\/\Head")

Also note that your expression can be written more concisely using a literal:

regex = /--Head([\s\S]*)--\/Head/

Use catch all character class [\s\S] which means space or non space

var regex = new RegExp("\-{2}Head([\s\S]*)-{2}\/\Head","m");      
var content = "--Head \n any Code \n and String --/Head";
var match = regex.exec(content);