Reject regex literals involving unbalanced close parentheses

Author: DougGregor

Sources/SwiftParser/Lexer.swift

@@ -2007,6 +2007,8 @@ extension Lexer.Cursor {
   ) -> RawTokenKind? {
     var Tmp = TokStart
     var poundCount = 0
+    var parenCount = 0
+
     while Tmp.advance(matching: UInt8(ascii: "#")) != nil {
       poundCount += 1
     }
@@ -2072,6 +2074,15 @@ extension Lexer.Cursor {
 //        // delimiter.
 //        throw DelimiterLexError(.unprintableASCII, resumeAt: cursor.successor())
 
+      case UInt8(ascii: "("):
+        parenCount += 1
+
+      case UInt8(ascii: ")"):
+        if parenCount == 0 {
+          return nil
+        }
+        parenCount -= 1
+
       default:
         continue DELIMITLOOP
       }

Tests/SwiftParserTest/LexerTests.swift

@@ -347,6 +347,13 @@ public class LexerTests: XCTestCase {
         lexeme(.pound, "#"),
         lexeme(.eof, ""),
       ]),
+      ("/a)/", [
+        lexeme(.prefixOperator, "/"),
+        lexeme(.identifier, "a"),
+        lexeme(.rightParen, ")"),
+        lexeme(.postfixOperator, "/"),
+        lexeme(.eof, ""),
+      ]),
     ]
     for (fixture, expectation) in fixtures {
       var fixture = fixture