import Foundation
let pattern = #"(?<=.\n)(.+)\n(?=.)"#
let regex = try! NSRegularExpression(pattern: pattern, options: .anchorsMatchLines)
let testString = #"""
ChapterName
text text text
text text text
text text text
AnotherChapterName
text text text
text text text
text text text
AnotherChapterName
text text text
text text text
text text text
"""#
let stringRange = NSRange(location: 0, length: testString.utf16.count)
let substitutionString = #"$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