Can't create NSRegularExpression because of "Couldn't create (Metatype) escape NSRegularExpression"

Question

I'm trying to create the following NSRegularExpression (I'm trying to find all commas ,, left-parens (, right-parens ), and backslashes \):

var error: NSError?
NSRegularExpression(pattern: "[,\\(\\)\\]", options: nil, error: &error)

But it returns .None with the following error:

Couldn't create (Metatype) escape NSRegularExpression

And the following userInfo:

NSInvalidValue: "[,\(\)\]"

What does this mean?

Answer 1

My backslash literal inside the character class wasn't properly escaped. It used two backslashes inside the string literal instead of four.

var error: NSError?
NSRegularExpression(pattern: "[,\\(\\)\\]", options: nil, error: &error)

Should be:

var error: NSError?
NSRegularExpression(pattern: "[,\\(\\)\\\\]", options: nil, error: &error)

I think the Couldn't create (Metatype) escape error refers to the incompletely escaped backslash leading to escaping the final square bracket, which makes the character class definition invalid.