import Foundation
let pattern = #"(?<!^\s*)(?=<GTOL-[A-Z]+>)"#
let regex = try! NSRegularExpression(pattern: pattern, options: .anchorsMatchLines)
let testString = #"""
<GTOL-POSI>.010<MOD-MMC>B-C<GTOL-POSI>.002<MOD-FMC>
<GTOL-POSI><MOD-DIAM>.004<MOD-MMC>HC<MOD-MMC><GTOL-POSI>.030<MOD-MMC>D-E<MOD-FMC>
<GTOL-POSI><MOD-DIAM>.005<MOD-MMC>AD-E<MOD-MMC><GTOL-PERP>.001A
"""#
let stringRange = NSRange(location: 0, length: testString.utf16.count)
let substitutionString = #"\n\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