diff --git a/Sources/_RegexParser/Regex/Parse/Sema.swift b/Sources/_RegexParser/Regex/Parse/Sema.swift
index 83c014d2a..c66ab9246 100644
--- a/Sources/_RegexParser/Regex/Parse/Sema.swift
+++ b/Sources/_RegexParser/Regex/Parse/Sema.swift
@@ -106,8 +106,13 @@ extension RegexValidator {
     case .byteSemantics:
       throw error(.unsupported("byte semantic mode"), at: loc)
 
+    case .unicodeScalarSemantics:
+      throw error(.unsupported("unicode scalar semantic mode"), at: loc)
+      
+    case .graphemeClusterSemantics:
+      throw error(.unsupported("grapheme semantic mode"), at: loc)
+      
     case .caseInsensitive, .possessiveByDefault, .reluctantByDefault,
-        .unicodeScalarSemantics, .graphemeClusterSemantics,
         .singleLine, .multiline, .namedCapturesOnly, .extended, .extraExtended,
         .asciiOnlyDigit, .asciiOnlyWord, .asciiOnlySpace, .asciiOnlyPOSIXProps:
       break
diff --git a/Tests/RegexTests/CompileTests.swift b/Tests/RegexTests/CompileTests.swift
index 9e94a886a..3f6532d82 100644
--- a/Tests/RegexTests/CompileTests.swift
+++ b/Tests/RegexTests/CompileTests.swift
@@ -124,9 +124,13 @@ extension RegexTests {
       "(?im)(?s).",
       matchingOptions(adding: [.caseInsensitive, .multiline, .singleLine]))
     try expectInitialOptions(".", matchingOptions())
-    try expectInitialOptions(
-      "(?im)(?s).(?u)",
-      matchingOptions(adding: [.caseInsensitive, .multiline, .singleLine]))
+    
+    // FIXME: Figure out (?X) and (?u) semantics
+    try XCTExpectFailure("Figure out (?X) and (?u) semantics") {
+      try expectInitialOptions(
+        "(?im)(?s).(?u)",
+        matchingOptions(adding: [.caseInsensitive, .multiline, .singleLine]))
+    }
     
     try expectInitialOptions(
       "(?i:.)",
diff --git a/Tests/RegexTests/MatchTests.swift b/Tests/RegexTests/MatchTests.swift
index 8cecd26c8..98aa23ca8 100644
--- a/Tests/RegexTests/MatchTests.swift
+++ b/Tests/RegexTests/MatchTests.swift
@@ -1020,10 +1020,12 @@ extension RegexTests {
       #"\u{65}\y"#,           // Grapheme boundary assertion
       ("Cafe\u{301}", nil),
       ("Sol Cafe", "e"))
+    
+    // FIXME: Figure out (?X) and (?u) semantics
     firstMatchTests(
       #"(?u)\u{65}\Y"#,       // Grapheme non-boundary assertion
       ("Cafe\u{301}", "e"),
-      ("Sol Cafe", nil))
+      ("Sol Cafe", nil), xfail: true)
   }
 
   func testMatchGroups() {
@@ -1586,7 +1588,8 @@ extension RegexTests {
     // a single Unicode scalar value, leaving any other grapheme scalar
     // components to be matched.
     
-    firstMatchTest(#"(?u:.)"#, input: eDecomposed, match: "e")
+    // FIXME: Figure out (?X) and (?u) semantics
+    firstMatchTest(#"(?u:.)"#, input: eDecomposed, match: "e", xfail: true)
 
     matchTest(
       #".\u{301}"#,
@@ -1597,18 +1600,30 @@ extension RegexTests {
       (eComposed, false),
       (eDecomposed, false))
     
+    // FIXME: Figure out (?X) and (?u) semantics
     // FIXME: \O is unsupported
-    firstMatchTest(#"(?u)\O\u{301}"#, input: eDecomposed, match: eDecomposed)
-    firstMatchTest(#"(?u)e\O"#, input: eDecomposed, match: eDecomposed)
+    firstMatchTest(
+      #"(?u)\O\u{301}"#,
+      input: eDecomposed,
+      match: eDecomposed,
+      xfail: true
+    )
+    firstMatchTest(
+      #"(?u)e\O"#,
+      input: eDecomposed,
+      match: eDecomposed,
+      xfail: true
+    )
     firstMatchTest(#"\O"#, input: eComposed, match: eComposed)
     firstMatchTest(#"\O"#, input: eDecomposed, match: nil,
               xfail: true)
 
+    // FIXME: Figure out (?X) and (?u) semantics
     matchTest(
       #"(?u).\u{301}"#,
       (eComposed, false),
-      (eDecomposed, true))
-    firstMatchTest(#"(?u).$"#, input: eComposed, match: eComposed)
+      (eDecomposed, true), xfail: true)
+    firstMatchTest(#"(?u).$"#, input: eComposed, match: eComposed, xfail: true)
     
     // Option permutations for 'u' and 's'
     matchTest(
@@ -1621,14 +1636,16 @@ extension RegexTests {
       ("e\u{301}ab", false),
       ("e\u{301}abc", true),
       ("e\u{301}\nab", true))
+    
+    // FIXME: Figure out (?X) and (?u) semantics
     matchTest(
       #"(?u)...."#,
       ("e\u{301}ab", true),
-      ("e\u{301}\na", false))
+      ("e\u{301}\na", false), xfail: true)
     matchTest(
       #"(?us)...."#,
       ("e\u{301}ab", true),
-      ("e\u{301}\na", true))
+      ("e\u{301}\na", true), xfail: true)
   }
   
   // TODO: Add test for implied grapheme cluster requirement at group boundaries
diff --git a/Tests/RegexTests/ParseTests.swift b/Tests/RegexTests/ParseTests.swift
index 5beb67448..97f9ba908 100644
--- a/Tests/RegexTests/ParseTests.swift
+++ b/Tests/RegexTests/ParseTests.swift
@@ -1074,9 +1074,11 @@ extension RegexTests {
       .singleLine, .reluctantByDefault, .extraExtended, .extended,
       .unicodeWordBoundaries, .asciiOnlyDigit, .asciiOnlyPOSIXProps,
       .asciiOnlySpace, .asciiOnlyWord, .textSegmentGraphemeMode,
-      .textSegmentWordMode, .graphemeClusterSemantics, .unicodeScalarSemantics,
+      .textSegmentWordMode,
+      .graphemeClusterSemantics, .unicodeScalarSemantics,
       .byteSemantics
     ]
+    
     parseTest("(?iJmnsUxxxwDPSWy{g}y{w}Xub-iJmnsUxxxwDPSW)", changeMatchingOptions(
       matchingOptions(adding: allOptions, removing: allOptions.dropLast(5))
     ), throwsError: .unsupported)
@@ -2787,8 +2789,9 @@ extension RegexTests {
     diagnosticTest("(?-y{g})", .cannotRemoveTextSegmentOptions)
     diagnosticTest("(?-y{w})", .cannotRemoveTextSegmentOptions)
 
-    diagnosticTest("(?-X)", .cannotRemoveSemanticsOptions)
-    diagnosticTest("(?-u)", .cannotRemoveSemanticsOptions)
+    // FIXME: Reenable once we figure out (?X) and (?u) semantics
+    //diagnosticTest("(?-X)", .cannotRemoveSemanticsOptions)
+    //diagnosticTest("(?-u)", .cannotRemoveSemanticsOptions)
     diagnosticTest("(?-b)", .cannotRemoveSemanticsOptions)
 
     diagnosticTest("(?a)", .unknownGroupKind("?a"))
diff --git a/Tests/RegexTests/UTS18Tests.swift b/Tests/RegexTests/UTS18Tests.swift
index 7306632da..be01fecb3 100644
--- a/Tests/RegexTests/UTS18Tests.swift
+++ b/Tests/RegexTests/UTS18Tests.swift
@@ -62,7 +62,7 @@ fileprivate func expectFirstMatch<Output: Equatable>(
 }
 
 #if os(Linux)
-func XCTExpectFailure(_ message: String? = nil, body: () -> Void) {}
+func XCTExpectFailure(_ message: String? = nil, body: () throws -> Void) rethrows {}
 #endif
 
 // MARK: - Basic Unicode Support: Level 1
@@ -466,7 +466,12 @@ extension UTS18Tests {
 
     // Matching semantic level
     XCTAssertFalse("👩‍👩‍👧‍👦".contains(regex(#".\N{ZERO WIDTH JOINER}"#)))
-    XCTAssertTrue("👩‍👩‍👧‍👦".contains(regex(#"(?u).\N{ZERO WIDTH JOINER}"#)))
+    
+    // FIXME: Figure out (?X) and (?u) semantics
+    XCTExpectFailure("Figure out (?X) and (?u) semantics") {
+      XCTFail(#"(?u).\N{ZERO WIDTH JOINER}"#)
+      //XCTAssertTrue("👩‍👩‍👧‍👦".contains(regex(#"(?u).\N{ZERO WIDTH JOINER}"#)))
+    }
   }
 
   func testIndividuallyNamedCharacters_XFail() {