Allow POSIX character properties outside of custom character classes

This matches the ICU behavior, and appears to be
suggested by UTS#18.

Author: hamishknight

Sources/_RegexParser/Regex/Parse/LexicalAnalysis.swift

@@ -1064,14 +1064,13 @@ extension Source {
   }
 
   mutating func lexCustomCCStart(
-    context: ParsingContext
   ) throws -> Located<CustomCC.Start>? {
     recordLoc { src in
       // Make sure we don't have a POSIX character property. This may require
       // walking to its ending to make sure we have a closing ':]', as otherwise
       // we have a custom character class.
       // TODO: This behavior seems subtle, could we warn?
-      guard !src.canLexPOSIXCharacterProperty(context: context) else {
+      guard !src.canLexPOSIXCharacterProperty() else {
         return nil
       }
       if src.tryEat("[") {
@@ -1104,11 +1103,8 @@ extension Source {
   }
 
   private mutating func lexPOSIXCharacterProperty(
-    context: ParsingContext
   ) throws -> Located<AST.Atom.CharacterProperty>? {
-    // Only allowed in a custom character class.
-    guard context.isInCustomCharacterClass else { return nil }
-    return try recordLoc { src in
+    try recordLoc { src in
       try src.tryEating { src in
         guard src.tryEat(sequence: "[:") else { return nil }
         let inverted = src.tryEat("^")
@@ -1127,10 +1123,10 @@ extension Source {
     }
   }
 
-  private func canLexPOSIXCharacterProperty(context: ParsingContext) -> Bool {
+  private func canLexPOSIXCharacterProperty() -> Bool {
     do {
       var src = self
-      return try src.lexPOSIXCharacterProperty(context: context) != nil
+      return try src.lexPOSIXCharacterProperty() != nil
     } catch {
       // We want to tend on the side of lexing a POSIX character property, so
       // even if it is invalid in some way (e.g invalid property names), still
@@ -1818,8 +1814,9 @@ extension Source {
       if !customCC && (src.peek() == ")" || src.peek() == "|") { return nil }
       // TODO: Store customCC in the atom, if that's useful
 
-      // POSIX character property.
-      if let prop = try src.lexPOSIXCharacterProperty(context: context)?.value {
+      // POSIX character property. Like \p{...} this is also allowed outside of
+      // a custom character class.
+      if let prop = try src.lexPOSIXCharacterProperty()?.value {
         return .property(prop)
       }
 

Sources/_RegexParser/Regex/Parse/Parse.swift

@@ -403,7 +403,7 @@ extension Parser {
     }
 
     // Check if we have the start of a custom character class '['.
-    if let cccStart = try source.lexCustomCCStart(context: context) {
+    if let cccStart = try source.lexCustomCCStart() {
       return .customCharacterClass(
         try parseCustomCharacterClass(cccStart))
     }
@@ -487,7 +487,7 @@ extension Parser {
     while source.peek() != "]" && source.peekCCBinOp() == nil {
 
       // Nested custom character class.
-      if let cccStart = try source.lexCustomCCStart(context: context) {
+      if let cccStart = try source.lexCustomCCStart() {
         members.append(.custom(try parseCustomCharacterClass(cccStart)))
         continue
       }

Sources/_StringProcessing/Utility/ASTBuilder.swift

@@ -354,6 +354,11 @@ func prop(
 ) -> AST.Node {
   atom(.property(.init(kind, isInverted: inverted, isPOSIX: false)))
 }
+func posixProp(
+  _ kind: AST.Atom.CharacterProperty.Kind, inverted: Bool = false
+) -> AST.Node {
+  atom(.property(.init(kind, isInverted: inverted, isPOSIX: true)))
+}
 
 // Raw atom constructing variant
 func atom_a(

Tests/RegexTests/ParseTests.swift

@@ -477,12 +477,9 @@ extension RegexTests {
     // These are custom character classes, not invalid POSIX character classes.
     // TODO: This behavior is subtle, we ought to warn.
     parseTest("[[:space]]", charClass(charClass(":", "s", "p", "a", "c", "e")))
-    parseTest("[:space:]", charClass(":", "s", "p", "a", "c", "e", ":"))
     parseTest("[:a]", charClass(":", "a"))
     parseTest("[a:]", charClass("a", ":"))
     parseTest("[:]", charClass(":"))
-    parseTest("[::]", charClass(":", ":"))
-    parseTest("[:=:]", charClass(":", "=", ":"))
     parseTest("[[:]]", charClass(charClass(":")))
     parseTest("[[:a=b=c:]]", charClass(charClass(":", "a", "=", "b", "=", "c", ":")))
 
@@ -522,6 +519,12 @@ extension RegexTests {
                         posixProp_m(.binary(.uppercase), inverted: true),
                         "c", "d"))
 
+    // Like ICU, we allow POSIX character properties outside of custom character
+    // classes. This also appears to be suggested by UTS#18.
+    // TODO: We should likely emit a warning.
+    parseTest("[:space:]", posixProp(.binary(.whitespace)))
+    parseTest("[:script=Greek:]", posixProp(.script(.greek)))
+
     parseTest("[[[:space:]]]", charClass(charClass(
       posixProp_m(.binary(.whitespace))
     )))
@@ -2252,6 +2255,8 @@ extension RegexTests {
     diagnosticTest("[[:a:", .expected("]"))
     diagnosticTest("[[:a[:]", .expected("]"))
 
+    diagnosticTest("[::]", .emptyProperty)
+    diagnosticTest("[:=:]", .emptyProperty)
     diagnosticTest("[[::]]", .emptyProperty)
     diagnosticTest("[[:=:]]", .emptyProperty)