Allow numbered \k backreferences

Allow e.g `\k<1>`, which is supported by Oniguruma
and .NET.

Author: hamishknight

Sources/_MatchingEngine/Regex/Parse/LexicalAnalysis.swift

@@ -1176,11 +1176,22 @@ extension Source {
         }
 
         if src.tryEat("k") {
-          // Perl/.NET-style backreferences.
-          if let openChar = src.tryEat(anyOf: "<", "'", "{") {
+          // Perl/.NET/Oniguruma-style backreferences.
+          if let openChar = src.tryEat(anyOf: "<", "'") {
             let closing = String(Source.getClosingDelimiter(for: openChar))
+
+            // Perl only accept named references here, but Oniguruma and .NET
+            // also accepts numbered references. This shouldn't be an ambiguity
+            // as named references may not begin with a digit, '-', or '+'.
+            let ref = try src.expectNamedOrNumberedReference(
+              endingWith: closing)
+
+            return .backreference(ref)
+          }
+          // Perl/.NET also allow a named references with the '{' delimiter.
+          if src.tryEat("{") {
             return .backreference(
-              try src.expectNamedReference(endingWith: closing))
+              try src.expectNamedReference(endingWith: "}"))
           }
           return nil
         }
@@ -1225,7 +1236,6 @@ extension Source {
     try recordLoc { src in
       try src.tryEating { src in
         guard src.tryEat(sequence: "(?") else { return nil }
-        let _start = src.currentPosition
 
         // Note the below should be covered by canLexGroupLikeReference.
 
@@ -1243,8 +1253,7 @@ extension Source {
         }
 
         // Whole-pattern recursion, which is equivalent to (?0).
-        if src.tryEat("R") {
-          let loc = Location(_start ..< src.currentPosition)
+        if let loc = src.tryEatWithLoc("R") {
           try src.expect(")")
           return .subpattern(.init(.recurseWholePattern, innerLoc: loc))
         }

Tests/RegexTests/ParseTests.swift

@@ -846,6 +846,10 @@ extension RegexTests {
     parseTest(#"\g{52}"#, backreference(.absolute(52)))
     parseTest(#"\g{-01}"#, backreference(.relative(-1)))
     parseTest(#"\g{+30}"#, backreference(.relative(30)))
+    parseTest(#"\k<+4>"#, backreference(.relative(4)))
+    parseTest(#"\k<2>"#, backreference(.absolute(2)))
+    parseTest(#"\k'-3'"#, backreference(.relative(-3)))
+    parseTest(#"\k'1'"#, backreference(.absolute(1)))
 
     parseTest(#"\k{a0}"#, backreference(.named("a0")))
     parseTest(#"\k<bc>"#, backreference(.named("bc")))
@@ -1335,8 +1339,8 @@ extension RegexTests {
     diagnosticTest(#"\k'#'"#, .groupNameMustBeAlphaNumeric)
     diagnosticTest(#"(?&#)"#, .groupNameMustBeAlphaNumeric)
 
-    diagnosticTest(#"\k'1'"#, .groupNameCannotStartWithNumber)
     diagnosticTest(#"(?P>1)"#, .groupNameCannotStartWithNumber)
+    diagnosticTest(#"\k{1}"#, .groupNameCannotStartWithNumber)
 
     // MARK: Conditionals