Replace occurance(s) of ' with \'

I'm trying to find an easy way to search a string and if a single quote is found, replace it with \'

So:

var a = "It's doing my head in!"

OR

var a = "I can't do it - it's too hard."


How can I turn either of those into:

It\'s doing my head in!

OR

I can\'t do it - it\'s too hard.


Cheers.


Edit: I cant just use:

var a = "I can\'t do it - it\'s too hard."

'a' will get its value from a form when I'm done testing so I won't know if or where in the string an apostrophe will be used.

string.replace(/'/g,'\\\'');

Excellent, thanks.