JavaScript RegExp : Why causes a double backslash ( \\ ) an error? [duplicate]

Question

Have found this out by accident and have no idea what's the reason.

// Results in "Syntax error in regular Expression".
var re = RegExp('\\');

I know that the constructor-function expects a string as parameter. And that the backslash is used within strings to escape characters with special meaning. I know that I have to escape characters like \d to \\d .

So therefore: The right backslash should the interpreted as some normal character.

Instead it throws an error. Why?

Can anyone explain this to me?

Answer 1

\ is used to escape \ in strings, so to get \d as you wrote you need to do \\d.

Also in regexp you need to escape \ with \\.

So you have two escape syntaxes that need to take place in regexps, using a single \\ will mean \ in regexp which is not correct, because it needs to be escaped.

So to workaround this you need double escape: \\\\ - this will be a regex looking for \.