First working version.

Plugins/TailwindCSS/TailwindCSSBuildPlugin.swift

@@ -0,0 +1,117 @@
+import Foundation
+import PackagePlugin
+import RegexBuilder
+
+@main
+struct TailwindCSSBuildPlugin: BuildToolPlugin {
+  let inputCSSFilename = "Tailwind.css"
+  let outputBundleName = "TailwindCSS.bundle"
+  let outputCSSFilename = "tw.css"
+
+  let importStatementRegex = Regex {
+    Anchor.startOfLine
+    ZeroOrMore(.whitespace)
+    "@import"
+    ZeroOrMore(.whitespace)
+    "\"tailwindcss\""
+    ZeroOrMore(.whitespace)
+    "source("
+    ZeroOrMore(.whitespace)
+    "none"
+    ZeroOrMore(.whitespace)
+    ")"
+    ZeroOrMore(.whitespace)
+    ";"
+  }.anchorsMatchLineEndings()
+
+  let sourceDeclarationRegex = Regex {
+    Anchor.startOfLine
+    ZeroOrMore(.whitespace)
+    "@source"
+    ZeroOrMore(.whitespace)
+    "\""
+    Capture(ZeroOrMore(.word))
+    "\""
+    ZeroOrMore(.whitespace)
+    ";"
+  }
+
+  let sourceNotDeclarationRegex = Regex {
+    Anchor.startOfLine
+    ZeroOrMore(.whitespace)
+    "@source"
+    ZeroOrMore(.whitespace)
+    "not"
+  }.anchorsMatchLineEndings()
+
+  func createBuildCommands(
+    context: PluginContext,
+    target: Target
+  ) throws -> [Command] {
+    let tailwindCSSURL: URL = target.directoryURL.appending(component: "Tailwind.css")
+    guard let cssContent = try? String(contentsOf: tailwindCSSURL) else {
+      throw BuildError.missingTailwindCSSFile
+    }
+    let matches = cssContent.matches(of: importStatementRegex)
+    guard !matches.isEmpty else {
+      throw BuildError.missingImportStatement
+    }
+    if (try? sourceNotDeclarationRegex.firstMatch(in: cssContent)) != nil {
+      throw BuildError.sourceNotDeclarationUnsupported
+    }
+
+    let sourcePaths =
+      cssContent
+      .matches(of: sourceDeclarationRegex)
+      .compactMap { String($0.output.1) }
+    let sourceURLs: [URL] = sourcePaths.map { path in
+      target.directoryURL
+        .appending(component: path, directoryHint: .inferFromPath)
+        .resolvingSymlinksInPath()
+    }
+
+    let tailwindCLIURL: URL = try context.tool(named: "tailwindcss").url
+    let outputBundleURL = context.pluginWorkDirectoryURL
+      .appending(component: outputBundleName, directoryHint: .isDirectory)
+    let outputURL = outputBundleURL.appending(
+      component: outputCSSFilename, directoryHint: .notDirectory)
+
+    print("Tailwind CSS Build Plugin: \(tailwindCSSURL.path)")
+    print("Tailwind CSS File: \(tailwindCSSURL.path)")
+    print("Source files: \n -\(sourceURLs.map(\.path).joined(separator: "\n -"))")
+    print("Output: \(outputURL.path)")
+
+    return [
+      .buildCommand(
+        displayName: "Building Tailwind CSS",
+        executable: tailwindCLIURL,
+        arguments: [
+          "--input", tailwindCSSURL.path,
+          "--output", outputURL.path,
+          "--minify",
+        ],
+        inputFiles: [tailwindCSSURL] + sourceURLs,
+        outputFiles: [outputBundleURL]
+      )
+    ]
+  }
+}
+
+extension TailwindCSSBuildPlugin {
+  enum BuildError: Error {
+    case missingTailwindCSSFile
+    case missingImportStatement
+    case sourceNotDeclarationUnsupported
+
+    var localizedDescription: String {
+      switch self {
+      case .missingTailwindCSSFile:
+        "Tailwind.css file not found in the target directory."
+      case .missingImportStatement:
+        "No `@import \"tailwind\"` statement found, or `source(none)` is missing."
+      case .sourceNotDeclarationUnsupported:
+        "`@source not` declarations are not supported. Please explicitly declare sources with `@source \"<path>\";`."
+      }
+    }
+  }
+}