Why do I need “$$$” (three dollar) if I want “$$” (two dollar) as replace string in Java- / coffescript-

Question

Pre: I have my own library, where I work with namespaced modules and classes in Coffescript. this works like in Ruby "namspace::module::class", since '::' is not allowed as class name I replace Module::ClassName with Module$$ClassName -> replace(/\\$\\$/g,'::') , fine.

Now i struggled doing it reverse: replace(/::/g,'$$') results in Module$ClassName having only one Dollar ($)

So i played a around a bit

a="a:a::b:b::c:c"
a.replace(':','$')         #-> "a$a::b:b::c:c"  clear only first 
a.replace(/:/g,'$')        #-> "a$a$$b$b$$c$c"  better, but wrong we want '::' only 
a.replace(/::/g,'$$')      #-> "a:a$b:b$c:c"    suprise; where is the 2nd Dollar? 
a.replace("::",'$$')       #-> "a:a$b:b::c:c"   try no regexp since dollar has an other meaning? fails only one $
a.replace(/::/g,'\$\$')    #-> "a:a$b:b$c:c"    ESC the guys? nope  
a.replace(/::/g,"\\$\\$")  #-> "a:a\$\$b:b\$\$c:c" ESC ESC to get into the deeper?
                           #   and then replace(/\\\$/g,'$') ? overkill  
a.replace(/::/g,'$$$')     #-> "a:a$$b:b$$c:c"  bingo, but why? 
# trying more
a.replace(/::/g,'$$$$')    #-> "a:a$$b:b$$c:c"  2 get one? one stays alone 
a.replace(/::/g,'$$$$$')   #-> "a:a$$$b:b$$$c:c"  seems so

After all, is logic (and I wonder why I never had the prob before).

So I think (am sure) that '$$' escapes to one '$' because '$n' references to matching groups - but even if there is no regexp in?

Answer 1

Even if you don't have any capture groups, $ can be used in the replacement string. $& refers to everything that was matched by the regexp, $` refers to everything before the match, and $' refers to everything after the match. So $ is always treated specially, and $$ means just a single $ .

Answer 2

The reason is because $ is a special character in the second parameter of replace . See 'Specifying a string as a parameter' in https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/replace

Answer 3

Why do I need “$$$” (three dollar) if I want “$$” (two dollar) as replace string

In fact, you need 4 $ in the replacement string. To replace with a literal $ , you need two $ s, as one $ "escapes" the other.

Thus, you need

 var a = "some::var"; a = a.replace(/::/g,'$$$$'); // this will replace `::` with `$$` document.body.innerHTML = a; 

If you add an odd $ , it will be treated as a literal, or omitted, or whatever a specific browser wants to do with this "wild" escaping symbol. Thus, it is safer to use an even number of $ s in the replacement pattern.