Bound the email parts in the autolinker regex

The local part and the domain labels were unbounded, making the
search quadratic in the length of a run of characters preceding
an "@": scanning the text of a single page could take seconds.

test/unit/autolinker_spec.js

@@ -220,4 +220,24 @@ describe("autolinker", function () {
       ["john.doe@uni-cityname.tld", "mailto:john.doe@uni-cityname.tld"],
     ]);
   });
+
+  it("should find emails with the longest parts allowed by the RFCs", function () {
+    const local = "a".repeat(64);
+    const label = "b".repeat(63);
+    testLinks([[`${local}@${label}.com`, `mailto:${local}@${label}.com`]]);
+  });
+
+  it("shouldn't find emails with parts longer than allowed by the RFCs", function () {
+    expect(
+      Autolinker.findLinks(`${"a".repeat(107)}@${"a".repeat(80)}.com`)
+    ).toEqual([]);
+  });
+
+  it("should handle a long run of characters before an @ efficiently", function () {
+    const text = `${"a".repeat(50000)}@`;
+
+    const startTime = performance.now();
+    expect(Autolinker.findLinks(text)).toEqual([]);
+    expect(performance.now() - startTime).toBeLessThan(1000);
+  });
 });

web/autolinker.js

@@ -136,10 +136,13 @@ class Autolinker {
   static #numericTLDRegex;
 
   static findLinks(text) {
-    // Regex can be tested and verified at https://regex101.com/r/rXoLiT/2.
+    // Regex can be tested and verified at https://regex101.com/r/riHjvK/1.
+    // The email parts are bounded to keep the scan linear: a local part can't
+    // exceed 64 characters (RFC 5321, section 4.5.3.1.1) and a domain label
+    // can't exceed 63 (RFC 1035, section 2.3.4).
     this.#regex ??=
       // eslint-disable-next-line regexp/no-super-linear-backtracking
-      /\b(?:https?:\/\/|mailto:|www\.)(?:[\S--[\p{P}<>]]|\/|[\S--[\[\]]]+[\S--[\p{P}<>]])+|(?=\p{L})[\S--[@\p{Ps}\p{Pe}<>]]+@([\S--[[\p{P}--\-]<>]]+(?:\.[\S--[[\p{P}--\-]<>]]+)+)/gv;
+      /\b(?:https?:\/\/|mailto:|www\.)(?:[\S--[\p{P}<>]]|\/|[\S--[\[\]]]+[\S--[\p{P}<>]])+|(?=\p{L})[\S--[@\p{Ps}\p{Pe}<>]]{1,64}@([\S--[[\p{P}--\-]<>]]{1,63}(?:\.[\S--[[\p{P}--\-]<>]]{1,63})+)/gv;
 
     const [normalizedText, diffs] = normalize(text, { ignoreDashEOL: true });
     const matches = normalizedText.matchAll(this.#regex);