Allow (?)

This is accepted by PCRE, and forms an empty
option change sequence. We probably ought to warn
on it though as it's a no-op.

Author: hamishknight

Sources/_RegexParser/Regex/Parse/LexicalAnalysis.swift

@@ -802,6 +802,11 @@ extension Source {
   mutating func lexMatchingOptionSequence(
     context: ParsingContext
   ) throws -> AST.MatchingOptionSequence? {
+    // PCRE accepts '(?)'
+    // TODO: This is a no-op, should we warn?
+    if peek() == ")" {
+      return .init(caretLoc: nil, adding: [], minusLoc: nil, removing: [])
+    }
     let ateCaret = recordLoc { $0.tryEat("^") }
 
     // TODO: Warn on duplicate options, and options appearing in both adding

Tests/RegexTests/ParseTests.swift

@@ -1002,6 +1002,9 @@ extension RegexTests {
               concat("a", atomicScriptRun("b"), "c"), throwsError: .unsupported)
 
     // Matching option changing groups.
+    parseTest("(?)", changeMatchingOptions(
+      matchingOptions()
+    ))
     parseTest("(?-)", changeMatchingOptions(
       matchingOptions()
     ))