diff --git a/Sources/_RegexParser/Regex/AST/Atom.swift b/Sources/_RegexParser/Regex/AST/Atom.swift
index 9cc2e9a96..1f6043d72 100644
--- a/Sources/_RegexParser/Regex/AST/Atom.swift
+++ b/Sources/_RegexParser/Regex/AST/Atom.swift
@@ -401,9 +401,6 @@ extension AST.Atom.CharacterProperty {
     /// Some special properties implemented by PCRE and Oniguruma.
     case pcreSpecial(PCRESpecialCategory)
     case onigurumaSpecial(OnigurumaSpecialProperty)
-
-    /// Unhandled properties.
-    case other(key: String?, value: String)
   }
 
   // TODO: erm, separate out or fold into something? splat it in?
diff --git a/Sources/_RegexParser/Regex/Parse/CharacterPropertyClassification.swift b/Sources/_RegexParser/Regex/Parse/CharacterPropertyClassification.swift
index e5b65a46c..911312121 100644
--- a/Sources/_RegexParser/Regex/Parse/CharacterPropertyClassification.swift
+++ b/Sources/_RegexParser/Regex/Parse/CharacterPropertyClassification.swift
@@ -397,8 +397,9 @@ extension Source {
       return .pcreSpecial(pcreSpecial)
     }
 
-    // Otherwise we don't know what this is.
-    return .other(key: nil, value: value)
+    // TODO: This should be versioned, and do we want a more lax behavior for
+    // the runtime?
+    throw ParseError.unknownProperty(key: nil, value: value)
   }
 
   static func classifyCharacterProperty(
@@ -435,6 +436,8 @@ extension Source {
     if let match = match {
       return match
     }
-    return .other(key: key, value: value)
+    // TODO: This should be versioned, and do we want a more lax behavior for
+    // the runtime?
+    throw ParseError.unknownProperty(key: key, value: value)
   }
 }
diff --git a/Sources/_RegexParser/Regex/Parse/Diagnostics.swift b/Sources/_RegexParser/Regex/Parse/Diagnostics.swift
index 621d6ea11..c3d74c30b 100644
--- a/Sources/_RegexParser/Regex/Parse/Diagnostics.swift
+++ b/Sources/_RegexParser/Regex/Parse/Diagnostics.swift
@@ -57,8 +57,8 @@ enum ParseError: Error, Hashable {
   case expectedCustomCharacterClassMembers
   case invalidCharacterClassRangeOperand
 
-  case invalidPOSIXSetName(String)
   case emptyProperty
+  case unknownProperty(key: String?, value: String)
 
   case expectedGroupSpecifier
   case unbalancedEndOfGroup
@@ -142,10 +142,13 @@ extension ParseError: CustomStringConvertible {
       return "expected custom character class members"
     case .invalidCharacterClassRangeOperand:
       return "invalid character class range"
-    case let .invalidPOSIXSetName(n):
-      return "invalid character set name: '\(n)'"
     case .emptyProperty:
       return "empty property"
+    case .unknownProperty(let key, let value):
+      if let key = key {
+        return "unknown character property '\(key)=\(value)'"
+      }
+      return "unknown character property '\(value)'"
     case .expectedGroupSpecifier:
       return "expected group specifier"
     case .unbalancedEndOfGroup:
diff --git a/Sources/_StringProcessing/ConsumerInterface.swift b/Sources/_StringProcessing/ConsumerInterface.swift
index b49804ca1..3c84195aa 100644
--- a/Sources/_StringProcessing/ConsumerInterface.swift
+++ b/Sources/_StringProcessing/ConsumerInterface.swift
@@ -423,10 +423,6 @@ extension AST.Atom.CharacterProperty {
 
       case .onigurumaSpecial(let s):
         throw Unsupported("TODO: map Oniguruma special: \(s)")
-
-      case let .other(key, value):
-        throw Unsupported(
-          "TODO: map other \(key ?? "")=\(value)")
       }
     }()
 
diff --git a/Tests/RegexTests/ParseTests.swift b/Tests/RegexTests/ParseTests.swift
index bdae250ba..4043e4ccb 100644
--- a/Tests/RegexTests/ParseTests.swift
+++ b/Tests/RegexTests/ParseTests.swift
@@ -501,7 +501,6 @@ extension RegexTests {
     parseTest(#"[[:a]]"#, charClass(charClass(":", "a")))
     parseTest(#"[[:}]]"#, charClass(charClass(":", "}")))
     parseTest(#"[[:{]]"#, charClass(charClass(":", "{")))
-    parseTest(#"[[:{:]]"#, charClass(posixProp_m(.other(key: nil, value: "{"))))
     parseTest(#"[[:}:]]"#, charClass(charClass(":", "}", ":")))
 
     parseTest(
@@ -1141,14 +1140,6 @@ extension RegexTests {
       #"\p{C}+"#,
       oneOrMore(of: prop(.generalCategory(.other))))
 
-    // TODO: Start erroring on these?
-    parseTest(#"\p{Lx}"#, prop(.other(key: nil, value: "Lx")))
-    parseTest(#"\p{gcL}"#, prop(.other(key: nil, value: "gcL")))
-    parseTest(#"\p{x=y}"#, prop(.other(key: "x", value: "y")))
-    parseTest(#"\p{aaa(b)}"#, prop(.other(key: nil, value: "aaa(b)")))
-    parseTest("[[:a():]]", charClass(posixProp_m(.other(key: nil, value: "a()"))))
-    parseTest(#"\p{aaa\p{b}}"#, concat(prop(.other(key: nil, value: #"aaa\p{b"#)), "}"))
-
     // UAX44-LM3 means all of the below are equivalent.
     let lowercaseLetter = prop(.generalCategory(.lowercaseLetter))
     parseTest(#"\p{ll}"#, lowercaseLetter)
@@ -2231,12 +2222,12 @@ extension RegexTests {
     diagnosticTest(#"\x{5"#, .expected("}"))
     diagnosticTest(#"\N{A"#, .expected("}"))
     diagnosticTest(#"\N{U+A"#, .expected("}"))
-    diagnosticTest(#"\p{a"#, .expected("}"))
+    diagnosticTest(#"\p{a"#, .unknownProperty(key: nil, value: "a"))
     diagnosticTest(#"\p{a="#, .emptyProperty)
     diagnosticTest(#"\p{a=}"#, .emptyProperty)
-    diagnosticTest(#"\p{a=b"#, .expected("}"))
-    diagnosticTest(#"\p{aaa[b]}"#, .expected("}"))
-    diagnosticTest(#"\p{a=b=c}"#, .expected("}"))
+    diagnosticTest(#"\p{a=b"#, .unknownProperty(key: "a", value: "b"))
+    diagnosticTest(#"\p{aaa[b]}"#, .unknownProperty(key: nil, value: "aaa"))
+    diagnosticTest(#"\p{a=b=c}"#, .unknownProperty(key: "a", value: "b"))
     diagnosticTest(#"(?#"#, .expected(")"))
     diagnosticTest(#"(?x"#, .expected(")"))
 
@@ -2321,6 +2312,16 @@ extension RegexTests {
     diagnosticTest(#"\\#u{E9}"#, .invalidEscape("é"))
     diagnosticTest(#"\˂"#, .invalidEscape("˂"))
 
+    // MARK: Character properties
+
+    diagnosticTest(#"\p{Lx}"#, .unknownProperty(key: nil, value: "Lx"))
+    diagnosticTest(#"\p{gcL}"#, .unknownProperty(key: nil, value: "gcL"))
+    diagnosticTest(#"\p{x=y}"#, .unknownProperty(key: "x", value: "y"))
+    diagnosticTest(#"\p{aaa(b)}"#, .unknownProperty(key: nil, value: "aaa(b)"))
+    diagnosticTest("[[:a():]]", .unknownProperty(key: nil, value: "a()"))
+    diagnosticTest(#"\p{aaa\p{b}}"#, .unknownProperty(key: nil, value: #"aaa\p{b"#))
+    diagnosticTest(#"[[:{:]]"#, .unknownProperty(key: nil, value: "{"))
+
     // MARK: Matching options
 
     diagnosticTest("(?-y{g})", .cannotRemoveTextSegmentOptions)