Add parser support for \p{L&}

This is a PCRE spelling for a cased letter.

Author: hamishknight

Sources/_RegexParser/Regex/Parse/CharacterPropertyClassification.swift

@@ -32,8 +32,8 @@ extension Source {
   static private func classifyGeneralCategory(
     _ str: String
   ) -> Unicode.ExtendedGeneralCategory? {
-    // This uses the aliases defined in
-    // https://www.unicode.org/Public/UCD/latest/ucd/PropertyValueAliases.txt.
+    // This uses the aliases defined in https://www.unicode.org/Public/UCD/latest/ucd/PropertyValueAliases.txt.
+    // Additionally, uses the `L& = Lc` alias defined by PCRE.
     withNormalizedForms(str) { str in
       switch str {
       case "c", "other":                   return .other
@@ -43,7 +43,7 @@ extension Source {
       case "co", "privateuse":             return .privateUse
       case "cs", "surrogate":              return .surrogate
       case "l", "letter":                  return .letter
-      case "lc", "casedletter":            return .casedLetter
+      case "lc", "l&", "casedletter":      return .casedLetter
       case "ll", "lowercaseletter":        return .lowercaseLetter
       case "lm", "modifierletter":         return .modifierLetter
       case "lo", "otherletter":            return .otherLetter

Tests/RegexTests/MatchTests.swift

@@ -698,6 +698,8 @@ extension RegexTests {
     firstMatchTest(#"\p{L}"#, input: "\u{0374}\u{00AA}123abcXYZ", match: "\u{0374}")
     firstMatchTest(#"\p{Lc}"#, input: "\u{0374}\u{00AA}123abcXYZ", match: "a")
     firstMatchTest(#"\p{Lc}"#, input: "\u{0374}\u{00AA}123XYZ", match: "X")
+    firstMatchTest(#"\p{L&}"#, input: "\u{0374}\u{00AA}123abcXYZ", match: "a")
+    firstMatchTest(#"\p{L&}"#, input: "\u{0374}\u{00AA}123XYZ", match: "X")
 
     firstMatchTest(
       #"\P{Cc}"#, input: "\n\n\nXYZ", match: "X")

Tests/RegexTests/ParseTests.swift

@@ -1156,6 +1156,9 @@ extension RegexTests {
       #"\p{C}+"#,
       oneOrMore(of: prop(.generalCategory(.other))))
 
+    // L& defined by PCRE.
+    parseTest(#"\p{L&}"#, prop(.generalCategory(.casedLetter)))
+
     // UAX44-LM3 means all of the below are equivalent.
     let lowercaseLetter = prop(.generalCategory(.lowercaseLetter))
     parseTest(#"\p{ll}"#, lowercaseLetter)