import Foundation
let pattern = ##"^YOUR_ANSWER\s+.*?\s+#'END_ANSWER$"##
let regex = try! NSRegularExpression(pattern: pattern, options: [.anchorsMatchLines, .dotMatchesLineSeparators])
let testString = ##"""
random text!!!!
YOUR_ANSWER
As_samp2 = 36
As_samp3 = 38
#'END_ANSWER
not your answer
"""##
let stringRange = NSRange(location: 0, length: testString.utf16.count)
let substitutionString = #"space for answer"#
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