import Foundation
let pattern = ##"http(?:s)?\://boards\.4chan\.org/([a-z]*)/thread/([0-9]*)(?:\#[0-9a-z]*)?"##
let regex = try! NSRegularExpression(pattern: pattern)
let testString = ##"""
https://boards.4chan.org/mlp/thread/5
http://boards.4chan.org/b/thread/570277846
https://boards.4chan.org/mlp/thread/19865655#q19865655
https://boards.4chan.org/mlp/thread/19775771
https://boards.4chan.org/co/thread/65737750#q65737750
"""##
let stringRange = NSRange(location: 0, length: testString.utf16.count)
let substitutionString = #"Thread $2 on mlp"#
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