import Foundation
let pattern = #"image(\d)$"#
let regex = try! NSRegularExpression(pattern: pattern, options: .anchorsMatchLines)
let testString = #"""
image1
image2
image3
image4
image5
image6
image7
image8
image9
image10
image11
image12
image13
image14
image15
image100
image101
image102
image103
image104
image105
"""#
let stringRange = NSRange(location: 0, length: testString.utf16.count)
let substitutionString = #"image00$1"#
let result = regex.stringByReplacingMatches(in: testString, range: stringRange, withTemplate: substitutionString)
print(result)
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for Swift 5.2, please visit: https://developer.apple.com/documentation/foundation/nsregularexpression