Fix HexDigit definition in RegexSyntax.md

Author: hamishknight

Documentation/Evolution/RegexSyntax.md

@@ -263,7 +263,7 @@ UnicodeScalar -> '\u{' HexDigit{1...} '}'
                | '\o{' OctalDigit{1...} '}'
                | '\0' OctalDigit{0...3}
 
-HexDigit   -> [0-9a-zA-Z]
+HexDigit   -> [0-9a-fA-F]
 OctalDigit -> [0-7]
 
 NamedScalar -> '\N{' ScalarName '}'

Tests/RegexTests/ParseTests.swift

@@ -2313,6 +2313,10 @@ extension RegexTests {
     diagnosticTest("{5}", .quantifierRequiresOperand("{5}"))
     diagnosticTest("{1,3}", .quantifierRequiresOperand("{1,3}"))
 
+    // MARK: Unicode scalars
+
+    diagnosticTest(#"\u{G}"#, .expectedNumber("G", kind: .hex))
+
     // MARK: Matching options
 
     diagnosticTest(#"(?^-"#, .cannotRemoveMatchingOptionsAfterCaret)