regex does not work in swift

Question

I am using a regular expression to find all \n occurrences in a string.

The regular expression itself is working:

The expression finds \n but not \\n. Which is, what I want.

However, when I want to implement this in Swift for an iOS-application I get the error: invalid regex: The value „(?<!\)\n“ is invalid.

My code looks like this (after implementing the \ approach from the comments:

import UIKit

var str = "Hello, \n \\n playground"

let regex = try? NSRegularExpression(pattern: "(?<!\\\\)\\n", options: .caseInsensitive)

let matches = regex?.matches(in: str, options: .anchored, range: NSMakeRange(0, str.count))

print(matches)

matches is nil. It should find \n.

Answer 1

The regex is compiled with the .anchored option that requires the pattern to only match at the start of the string:

Specifies that matches are limited to those at the start of the search range.

You need to remove this option, e.g.

let matches = regex?.matches(in: str, options: [], range: NSMakeRange(0, str.count))

Note the "(?<!\\\\)\\n" string literal defines a (?<!\\)\n regex pattern that matches

• (?<!\\) - a location in string that is not immediately preceded with a \ char
• \n - a newline character, LF.

See the regex demo online.