pass tests, but still not complete

src/transforms/tree.js

@@ -169,20 +169,29 @@ function nodeData(field) {
 function normalizer(delimiter = "/") {
   return `${delimiter}` === "/"
     ? (P) => P // paths are already slash-separated
-    : (P) => P.map(slashEscape).map(replaceAll(delimiter, "/")); // TODO string.replaceAll when supported
+    : (P) => P.map(slashDelimiter(delimiter));
 }
 
-function slashEscape(string) {
-  return string.replace(/\//g, "\\/");
+function slashDelimiter(delimiter) {
+  const search = new RegExp(`(\\\\*)(${regexEscape(delimiter)}|/)`, "g");
+  return (value) =>
+    value == null
+      ? null
+      : value.replace(
+          search,
+          (match, a, b) =>
+            b === delimiter
+              ? a.length & 1
+                ? `${a.slice(1)}${delimiter}` // drop one backslash
+                : `${a}/` // replace delimiter with slash
+              : a.length & 1
+              ? `${a}\\\\/` // add two backslashes to escape backslash
+              : `${a}\\/` // add one backslash to escape slash
+        );
 }
 
 function slashUnescape(string) {
-  return string.replace(/\\\//g, "/");
-}
-
-function replaceAll(search, replace) {
-  search = new RegExp(regexEscape(search), "g");
-  return (value) => (value == null ? null : `${value}`.replace(search, replace));
+  return string.replace(/\\\//g, "/").replace(/\\\\/g, "\\"); // TODO count backslashes properly
 }
 
 function regexEscape(string) {

test/plots/tree-delimiter.ts

@@ -3,19 +3,22 @@ import * as Plot from "@observablehq/plot";
 export async function treeDelimiter() {
   return Plot.plot({
     axis: null,
-    height: 120,
+    height: 150,
     margin: 10,
     marginLeft: 40,
     marginRight: 190,
     marks: [
       Plot.tree(
         [
-          "foo;bar;https://example.com",
-          "foo;bar;https://example.com/posts/1",
-          "foo;baz;https://example.com/posts/2",
-          "foo;bar\\;baz;https://example2.com", // “bar;baz” should be a single node
-          "foo;bar/baz;https://example4.com", // "bar/baz" should be a single node, distinct from “bar;baz”
-          "foo;bar\\/baz;https://example3.com" // “bar\/baz” should be a single node
+          "foo;a;//example", // foo → a → //example
+          "foo;a;//example/1", // foo → a → //example/1
+          "foo;b;//example/2", // foo → b → //example/2
+          "foo;c\\;c;//example2", // foo → c;c → //example2
+          "foo;d\\\\;d;//example2", // foo → d\ → d → //example3
+          "foo;d\\\\;\\d;//example2", // foo → d\ → \d → //example3
+          "foo;e\\\\\\;e;//example2", // foo → e\;e → //example3
+          "foo;f/f;//example4", // foo → f/f → //example4
+          "foo;g\\/g;//example3" // foo → g\/g → //example3
         ],
         {delimiter: ";"}
       )
@@ -26,21 +29,22 @@ export async function treeDelimiter() {
 export async function treeDelimiter2() {
   return Plot.plot({
     axis: null,
-    height: 120,
+    height: 150,
     margin: 10,
     marginLeft: 40,
     marginRight: 190,
     marks: [
-      Plot.tree(
-        [
-          "foo/bar/https:\\/\\/example.com",
-          "foo/bar/https:\\/\\/example.com\\/posts\\/1",
-          "foo/baz/https:\\/\\/example.com\\/posts\\/2",
-          "foo/bar;baz/https:\\/\\/example2.com", // “bar;baz” should be a single node
-          "foo/bar\\/baz/https:\\/\\/example4.com", // "bar/baz" should be a single node, distinct from “bar;baz”
-          "foo/bar\\\\\\/baz/https:\\/\\/example3.com" // “bar\/baz” should be a single node
-        ]
-      )
+      Plot.tree([
+        "foo/a/\\/\\/example", // foo → a → //example
+        "foo/a/\\/\\/example\\/1", // foo → a → //example/1
+        "foo/b/\\/\\/example\\/2", // foo → b → //example/2
+        "foo/c;c/\\/\\/example2", // foo → c;c → //example2
+        "foo/d\\\\/d/\\/\\/example2", // foo → d\ → d → //example3
+        "foo/d\\\\/\\d/\\/\\/example2", // foo → d\ → \d → //example3
+        "foo/e\\\\;e/\\/\\/example2", // foo → e\;e → //example3
+        "foo/f\\/f/\\/\\/example4", // foo → f/f → //example4
+        "foo/g\\\\\\/g/\\/\\/example3" // foo → g\/g → //example3
+      ])
     ]
   });
 }