import Foundation
var str = "###Word1 Word2\nWord3\nWord4 Word5 Word6\n##Word7\n#Word8 Word9\n"
// Define the regex pattern
let pattern = "#+(.*)\n"
// Create the regular expression
if let regex = try? NSRegularExpression(pattern: pattern, options: []) {
// Perform the replacement
let range = NSRange(location: 0, length: str.utf16.count)
str = regex.stringByReplacingMatches(in: str, options: [], range: range, withTemplate: "_$1_\n")
}
print(str)
/*
run:
_Word1 Word2_
Word3
Word4 Word5 Word6
_Word7_
_Word8 Word9_
*/