BREAKING: Resolves #19, throw an error when the permalink resolves wi…

…th invalid filepath chars

src/index.js

@@ -256,6 +256,14 @@ function permalinks(options) {
               }
         let ppath = replace(linkset.pattern, data, opts) || resolve(file, normalizedOptions.directoryIndex)
 
+        // invalid on Windows, but best practice not to use them anyway
+        const invalidFilepathChars = /\||:|<|>|\*|\?|"/
+        if (invalidFilepathChars.test(ppath)) {
+          const msg = `Filepath "${file}" contains invalid filepath characters (one of :|<>"*?) after resolving as linkset pattern "${linkset.pattern}"`
+          debug.error(msg)
+          done(new Error(msg))
+        }
+
         // Override the path with `permalink`  option
         if (Object.prototype.hasOwnProperty.call(data, 'permalink') && data.permalink !== false) {
           ppath = data.permalink

test/fixtures/invalid-filename-chars/src/post.html

@@ -0,0 +1,3 @@
+---
+title: ":::|||"
+---

test/index.cjs

@@ -258,6 +258,7 @@ describe('@metalsmith/permalinks', () => {
       })
   })
 
+  // this test was probably added to address an oddity in regexparam library path handling related to having a leading slash
   it('should use the resolve path for false values (not root)', (done) => {
     Metalsmith(path.join(fixturesBase, 'falsy'))
       .use(permalinks(':falsy/:title'))
@@ -272,6 +273,21 @@ describe('@metalsmith/permalinks', () => {
       })
   })
 
+  // better to error the build and make users aware of an anti-pattern,
+  // than silently strip/replace the invalid chars and let them follow bad practice
+  it('should error when encountering invalid filepath characters after permalink pattern resolution', done => {
+    Metalsmith(path.join(fixturesBase, 'invalid-filename-chars'))
+      .use(permalinks(':title'))
+      .build((err) => {
+          try {
+            assert.strictEqual(err.message, 'Filepath "post.html" contains invalid filepath characters (one of :|<>"*?) after resolving as linkset pattern ":title"')
+            done()
+          } catch (err) {
+            done(err)
+          }
+      })
+  })
+
   it('should allow an alternative directoryIndex', (done) => {
     const basepath = path.join(fixturesBase, 'custom-indexfile')
     Metalsmith(basepath)