Regex in Swift not working when escaping backslashes

Question

I have a regex that I'm trying to use in Swift to parse out the color values of a string in the form of "rgba(255, 255, 255, 1)"

My regex string is this:

^rgba?$\s*(\d{1,3}(?:\.\d+)?\%?)\s*,\s*(\d{1,3}(?:\.\d+)?\%?)\s*,\s*(\d{1,3}(?:\.\d+)?\%?)\s*(?:\s*,\s*(\d+(?:\.\d+)?)\s*)?$$

You can see it working here:

cl.ly/XxuH

When I escape the backslashes and attempt to use it with NSRegularExpression it no longer matches the given string.

^rgba?\$\\s*(\\d{1,3}(?:\\.\\d+)?\\%?)\\s*,\\s*(\\d{1,3}(?:\\.\\d+)?\\%?)\\s*,\\s*(\\d{1,3}(?:\\.\\d+)?\\%?)\\s*(?:\\s*,\\s*(\\d+(?:\\.\\d+)?)\\s*)?\$$

Is there something I am doing wrong?

Code Example:

import Foundation

var pattern = "^rgba?\\(\\s*(\\d{1,3}(?:\\.\\d+)?\\%?)\\s*,\\s*(\\d{1,3}(?:\\.\\d+)?\\%?)\\s*,\\s*(\\d{1,3}(?:\\.\\d+)?\\%?)\\s*(?:\\s*,\\s*(\\d+(?:\\.\\d+)?)\\s*)?\\)$"

var string = "rgba(255, 234, 243, 1)"

var regex = NSRegularExpression(pattern: pattern, options: NSRegularExpressionOptions(0), error: nil)

regex.numberOfMatchesInString(string, options: nil, range: NSMakeRange(0, countElements(string))) // Yields 1. Should yield 4

Why is question is tagged with Objective-C when the question is about Swift? – A small self-contained code example would be extremely helpful. — Martin R, Oct 09 '14 at 21:02
@MartinR because it's using NSRegularExpression and regexes in Swift vs. Obj-c are one to one. — erickreutz, Oct 09 '14 at 21:03
The common factor is the Cocoa (Touch) framework; the ObjC _language_ has nothing to do with this. — jscs, Oct 09 '14 at 21:05

Martin R · Accepted Answer · 2023-03-30T12:03:14.253

numberOfMatchesInString returns 1 because the entire pattern matches once. Use rangeAtIndex() to get the ranges matched by the capture groups:

let pattern = "^rgba?\\(\\s*(\\d{1,3}(?:\\.\\d+)?\\%?)\\s*,\\s*(\\d{1,3}(?:\\.\\d+)?\\%?)\\s*,\\s*(\\d{1,3}(?:\\.\\d+)?\\%?)\\s*(?:\\s*,\\s*(\\d+(?:\\.\\d+)?)\\s*)?\\)$"
let regex = NSRegularExpression(pattern: pattern, options: nil, error: nil)

let rgba : NSString = "rgba(255, 234, 243, 1)"
if let match = regex!.firstMatchInString(rgba, options: nil, range: NSMakeRange(0, rgba.length)) {
    println(rgba.substringWithRange(match.rangeAtIndex(1)))
    println(rgba.substringWithRange(match.rangeAtIndex(2)))
    println(rgba.substringWithRange(match.rangeAtIndex(3)))
    println(rgba.substringWithRange(match.rangeAtIndex(4)))
}

Output:

Update: Using the Regex type that was introduced with Swift 5.7, requires macOS 13+ or iOS 16+):

let rgba = "rgba(255, 234, 243, 1)"

let pattern = /rgba?\(\s*(\d{1,3}(?:\.\d+)?\%?)\s*,\s*(\d{1,3}(?:\.\d+)?\%?)\s*,\s*(\d{1,3}(?:\.\d+)?\%?)\s*(?:\s*,\s*(\d+(?:\.\d+)?)\s*)?\)/

if let match = try? pattern.wholeMatch(in: rgba) {
    print(match.1)
    print(match.2)
    print(match.3)
    print(match.4)
}

Regex in Swift not working when escaping backslashes

1 Answers1