Change lookahead function to Lookahead type

Author: rxwei

Sources/RegexBuilder/Anchor.swift

@@ -107,16 +107,26 @@ extension Anchor {
   }
 }
 
-public func lookahead<R: RegexComponent>(
-  negative: Bool = false,
-  @RegexComponentBuilder _ content: () -> R
-) -> Regex<R.Output> {
-  Regex(node: .nonCapturingGroup(negative ? .negativeLookahead : .lookahead, content().regex.root))
-}
-  
-public func lookahead<R: RegexComponent>(
-  _ component: R,
-  negative: Bool = false
-) -> Regex<R.Output> {
-  Regex(node: .nonCapturingGroup(negative ? .negativeLookahead : .lookahead, component.regex.root))
+public struct Lookahead<Output>: _BuiltinRegexComponent {
+  public var regex: Regex<Output>
+
+  init(_ regex: Regex<Output>) {
+    self.regex = regex
+  }
+
+  public init<R: RegexComponent>(
+    _ component: R,
+    negative: Bool = false
+  ) where R.Output == Output {
+    self.init(node: .nonCapturingGroup(
+      negative ? .negativeLookahead : .lookahead, component.regex.root))
+  }
+
+  public init<R: RegexComponent>(
+    negative: Bool = false,
+    @RegexComponentBuilder _ component: () -> R
+  ) where R.Output == Output {
+    self.init(node: .nonCapturingGroup(
+      negative ? .negativeLookahead : .lookahead, component().regex.root))
+  }
 }

Tests/RegexBuilderTests/RegexDSLTests.swift

@@ -306,8 +306,8 @@ class RegexDSLTests: XCTestCase {
       matchType: Substring.self, ==)
     {
       OneOrMore("a")
-      lookahead(CharacterClass.digit)
-      lookahead("2", negative: true)
+      Lookahead(CharacterClass.digit)
+      Lookahead("2", negative: true)
       CharacterClass.word
     }
   }