import Foundation
let pattern = #"((?:(?!<[/]mark\s*>).){0,5})<mark\s*>([^<]*)<[/]mark\s*>(?=((?:(?!<mark\s*>).){0,5}))"#
let regex = try! NSRegularExpression(pattern: pattern, options: .caseInsensitive)
let testString = #"""
Lorem ipsum mark> <MARK >dolor</MARK > < mark sitamet. <mark>Lorem</mark> ipsum again and <mark>dolor</mark>.
um <mark>dolor</mark> sit amet. <mark>Lorem</mark> ipsum again and <mark>dolor</mark>.
um <mark>dolor</mark> sit <test amet. <mark>Lorem</mark> ipsum again and <mark>dolor</mark>.
Lorem ipsum <MARK >dolor</MARK > sit <mark>amet.</mark><mark>Lorem</mark> ipsum again and <mark>dolor</mark>.
"""#
let stringRange = NSRange(location: 0, length: testString.utf16.count)
let substitutionString = #"\n$1 $2 $3\n"#
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