Quote strings with invalid plain scalar sequences involving : and #

Sources/PotentYAML/YAMLWriter.swift

@@ -10,6 +10,8 @@
 
 import Cfyaml
 import Foundation
+import PotentCodables
+import Regex
 
 
 internal struct YAMLWriter {
@@ -36,6 +38,12 @@ internal struct YAMLWriter {
     var explicitDocumentMarkers: Bool = false
   }
 
+  static let disallowedPlainSequencesRegex: Regex = #"(:\s)|(\s#)"#
+
+  public static func hasDisallowedPlainSequences(in string: String) -> Bool {
+    return disallowedPlainSequencesRegex.firstMatch(in: string) != nil
+  }
+
   let emitter: OpaquePointer
   let options: Options
 
@@ -158,7 +166,7 @@ internal struct YAMLWriter {
   private func emit(string: String, style: YAML.StringStyle, anchor: String?, tag: String?) throws {
 
     let stringStyle: YAML.StringStyle
-    if options.schema.requiresQuotes(for: string) {
+    if style == .any && (options.schema.requiresQuotes(for: string) || Self.hasDisallowedPlainSequences(in: string)) {
       stringStyle = (options.preferredStringStyle.isQuoted ? options.preferredStringStyle : .doubleQuoted)
     }
     else {

Tests/YAMLWriterTests.swift

@@ -61,6 +61,58 @@ class YAMLWriterTests: XCTestCase {
 
   }
 
+  func testWriteStringValidSpecialPlain() throws {
+
+    let options = YAMLWriter.Options(pretty: false)
+
+    XCTAssertEqual(
+      try YAMLWriter.write(["simple  :string"], options: options),
+      """
+      simple  :string
+
+      """
+    )
+
+    XCTAssertEqual(
+      try YAMLWriter.write(["simple# string"], options: options),
+      """
+      simple# string
+
+      """
+    )
+
+  }
+
+  func testWriteStringInvalidSpecialPlain() throws {
+
+    let options = YAMLWriter.Options(preferredStringStyle: .plain, pretty: false)
+
+    XCTAssertEqual(
+      try YAMLWriter.write(["simple: string"], options: options),
+      """
+      "simple: string"
+
+      """
+    )
+
+    XCTAssertEqual(
+      try YAMLWriter.write(["simple:\nstring"], options: options),
+      #"""
+      "simple:\nstring"
+
+      """#
+    )
+
+    XCTAssertEqual(
+      try YAMLWriter.write(["simple   #string"], options: options),
+      """
+      "simple   #string"
+
+      """
+    )
+
+  }
+
   func testWriteStringPreferPlain() throws {
 
     let options = YAMLWriter.Options(preferredStringStyle: .plain, pretty: false)