Add a simple command-line regex tester

Package.swift

@@ -100,6 +100,13 @@ let package = Package(
                 "_RegexParser",
                 "_StringProcessing"
             ]),
+        .executableTarget(
+            name: "RegexTester",
+            dependencies: [
+                .product(name: "ArgumentParser", package: "swift-argument-parser"),
+                "_RegexParser",
+                "_StringProcessing"
+            ]),
 
         // MARK: Exercises
         .target(

Sources/RegexTester/RegexTester.swift

@@ -0,0 +1,50 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2021-2022 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+//
+//===----------------------------------------------------------------------===//
+
+import ArgumentParser
+import _RegexParser
+import _StringProcessing
+
+@main
+@available(macOS 9999, *)
+struct RegexTester: ParsableCommand {
+  typealias MatchFunctionType = (String) throws -> Regex<AnyRegexOutput>.Match?
+
+  @Argument(help: "The regex pattern to test.")
+  var pattern: String
+
+  @Argument(help: "One or more input strings to test against <pattern>.")
+  var inputs: [String]
+
+  @Flag(
+    name: [.customShort("p"), .customLong("partial")],
+    help: "Allow partial matches.")
+  var allowPartialMatch: Bool = false
+
+  mutating func run() throws {
+    print("Using pattern \(pattern.halfWidthCornerQuoted)")
+    let regex = try Regex(pattern)
+
+    for input in inputs {
+      print("Input \(input.halfWidthCornerQuoted)")
+
+      let matchFunc: MatchFunctionType = allowPartialMatch
+        ? regex.firstMatch(in:)
+        : regex.wholeMatch(in:)
+
+      if let result = try matchFunc(input) {
+        print("   matched: \(result.0.halfWidthCornerQuoted)")
+      } else {
+        print("   no match")
+      }
+    }
+  }
+}