import Foundation
let pattern = #"\v+(?=To:)"#
let regex = try! NSRegularExpression(pattern: pattern)
let testString = #"""
To: ab@domain.tld
Subject: Der Betreff
From: xp@domain.tld
Reply-To: xp@domain.tld
Hier kommt meine Nachricht
To: ab@domain.tld
Subject: Der Betreff
From: xp@domain.tld
Reply-To: xp@domain.tld
Hier kommt meine Nachricht
To: ab@domain.tld
Subject: Der Betreff
From: xp@domain.tld
Reply-To: xp@domain.tld
Hier kommt meine Nachricht
"""#
let stringRange = NSRange(location: 0, length: testString.utf16.count)
let substitutionString = #"_"#
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