Handle missing group specifier

Avoid crashing on '(?' by emitting a new
diagnostic.

Author: hamishknight

Sources/_MatchingEngine/Regex/Parse/Diagnostics.swift

@@ -24,6 +24,8 @@ enum ParseError: Error, Hashable {
 
   case invalidPOSIXSetName(String)
   case emptyProperty
+
+  case expectedGroupSpecifier
 }
 
 extension ParseError: CustomStringConvertible {
@@ -57,6 +59,8 @@ extension ParseError: CustomStringConvertible {
       return "invalid character set name: '\(n)'"
     case .emptyProperty:
       return "empty property"
+    case .expectedGroupSpecifier:
+      return "expected group specifier"
     }
   }
 }

Sources/_MatchingEngine/Regex/Parse/LexicalAnalysis.swift

@@ -502,8 +502,10 @@ extension Source {
           return .namedCapture(name)
         }
 
-        throw ParseError.misc(
-          "Unknown group kind '(?\(src.peek()!)'")
+        guard let next = src.peek() else {
+          throw ParseError.expectedGroupSpecifier
+        }
+        throw ParseError.misc("Unknown group kind '(?\(next)'")
       }
 
       // Explicitly spelled out PRCE2 syntax for some groups.

Tests/RegexTests/LexTests.swift

@@ -107,6 +107,10 @@ extension RegexTests {
     diagnose(#"\p{a="#, expecting: .expected("}")) { try $0.lexBasicAtom() }
     diagnose(#"(?#"#, expecting: .expected(")")) { _ = try $0.lexComment() }
 
+    diagnose(#"(?"#, expecting: .expectedGroupSpecifier) {
+      _ = try $0.lexGroupStart()
+    }
+
     // TODO: want to dummy print out source ranges, etc, test that.
   }