How to replace a string with its uppercase with NSRegularExpression in Swift? - regex

Question

I was trying to change hello_world to helloWorld by this snippet of code (Swift 3.0):
import Foundation
let oldLine = "hello_world"
let fullRange = NSRange(location: 0, length: oldLine.characters.count)
let newLine = NSMutableString(string: oldLine)
let regex = try! NSRegularExpression(pattern: "(_)(\\w)", options: [])
regex.replaceMatches(in: newLine, options: [], range: fullRange,
withTemplate: "\\L$2")
The result was newLine = "helloLworld"
I used "\\L$2" as template because I saw this answer: https://stackoverflow.com/a/20742304/5282792 saying \L$2 is the pattern for the second group's uppercase in replacement template. But it didn't work in NSRegularExpression.
So can I replace a string with its uppercase with a replacement template pattern in NSRegularExpression.

Answer 1

One way to work with your case is subclassing NSRegularExpression and override replacementString(for:in:offset:template:) method.
class ToUpperRegex: NSRegularExpression {
override func replacementString(for result: NSTextCheckingResult, in string: String, offset: Int, template templ: String) -> String {
guard result.numberOfRanges > 2 else {
return ""
}
let matchingString = (string as NSString).substring(with: result.rangeAt(2)) as String
return matchingString.uppercased()
}
}
let oldLine = "hello_world"
let fullRange = NSRange(0..<oldLine.utf16.count) //<-
let tuRegex = try! ToUpperRegex(pattern: "(_)(\\w)")
let newLine = tuRegex.stringByReplacingMatches(in: oldLine, range: fullRange, withTemplate: "")
print(newLine) //->helloWorld

Answer 2

This doesn't answer the question pertaining regex, but might be of interest for readers not necessarily needing to use regex to perform this task (rather, using native Swift)
extension String {
func camelCased(givenSeparators separators: [Character]) -> String {
let charChunks = characters.split { separators.contains($0) }
guard let firstChunk = charChunks.first else { return self }
return String(firstChunk).lowercased() + charChunks.dropFirst()
.map { String($0).onlyFirstCharacterUppercased }.joined()
}
// helper (uppercase first char, lowercase rest)
var onlyFirstCharacterUppercased: String {
let chars = characters
guard let firstChar = chars.first else { return self }
return String(firstChar).uppercased() + String(chars.dropFirst()).lowercased()
}
}
/* Example usage */
let oldLine1 = "hello_world"
let oldLine2 = "fOo_baR BAX BaZ_fOX"
print(oldLine1.camelCased(givenSeparators: ["_"])) // helloWorld
print(oldLine2.camelCased(givenSeparators: ["_", " "])) // fooBarBazBazFox