Merge pull request #69 from rxwei/regex-string-initializer

Add 'init(_regexString:)' for compiler integration.

Author: rxwei

Sources/_StringProcessing/RegexDSL/Core.swift

@@ -12,9 +12,11 @@ public protocol RegexProtocol {
 }
 
 /// A regular expression.
+@frozen
 public struct Regex<Capture>: RegexProtocol {
   /// A program representation that caches any lowered representation for
   /// execution.
+  @usableFromInline
   internal class Program {
     /// The underlying AST.
     let ast: AST
@@ -34,13 +36,20 @@ public struct Regex<Capture>: RegexProtocol {
     }
   }
 
+  @usableFromInline
   let program: Program
   var ast: AST { program.ast }
 
   init(ast: AST) {
     self.program = Program(ast: ast)
   }
 
+  // Compiler interface. Do not change independently.
+  @usableFromInline
+  init(_regexString pattern: String) {
+    self.init(ast: try! parse(pattern, .traditional))
+  }
+
   public init<Content: RegexProtocol>(
     _ content: Content
   ) where Content.Capture == Capture {