Fix crash on lone backslash

Author: hamishknight

Sources/_MatchingEngine/Regex/Parse/LexicalAnalysis.swift

@@ -1472,7 +1472,9 @@ extension Source {
         return ref
       }
 
-      let char = src.eat()
+      guard let char = src.tryEat() else {
+        throw ParseError.expectedEscape
+      }
 
       // Single-character builtins.
       if let builtin = AST.Atom.EscapedBuiltin(

Sources/_MatchingEngine/Regex/Parse/Source.swift

@@ -86,6 +86,12 @@ extension Source {
     tryEat(anyOf: set)
   }
 
+  /// Try to eat any character, returning `nil` if the input has been exhausted.
+  mutating func tryEat() -> Char? {
+    guard !isEmpty else { return nil }
+    return eat()
+  }
+
   mutating func eat(asserting c: Char) {
     assert(peek() == c)
     advance()

Tests/RegexTests/ParseTests.swift

@@ -1753,6 +1753,10 @@ extension RegexTests {
     diagnosticTest("(?<a-b", .expected(">"))
     diagnosticTest("(?<a-b>", .expected(")"))
 
+    // MARK: Bad escapes
+
+    diagnosticTest("\\", .expectedEscape)
+
     // MARK: Text Segment options
 
     diagnosticTest("(?-y{g})", .cannotRemoveTextSegmentOptions)