Perf: Memoize digitsRegex (#1581)

* Memoize digitsRegex

Profiles reveal that a substantial portion of fromFormat time is spent
in digitsRegex. It is only called from unitsForToken and it is always
called with 11 suffixes to match different numbers of digits.

There are 21 numbering systems, so a fully expanded cache would hold
11*21 = 231 regexes.

Benchmarks show about a 1.5x improvement on DateTime.fromFormat

* Allow digitRegex cache to be reset

src/impl/digits.js

@@ -70,6 +70,21 @@ export function parseDigits(str) {
   }
 }
 
+// cache of {numberingSystem: {append: regex}}
+let digitRegexCache = {};
+export function resetDigitRegexCache() {
+  digitRegexCache = {};
+}
+
 export function digitRegex({ numberingSystem }, append = "") {
-  return new RegExp(`${numberingSystems[numberingSystem || "latn"]}${append}`);
+  const ns = numberingSystem || "latn";
+
+  if (!digitRegexCache[ns]) {
+    digitRegexCache[ns] = {};
+  }
+  if (!digitRegexCache[ns][append]) {
+    digitRegexCache[ns][append] = new RegExp(`${numberingSystems[ns]}${append}`);
+  }
+
+  return digitRegexCache[ns][append];
 }

src/settings.js

@@ -4,6 +4,7 @@ import Locale from "./impl/locale.js";
 
 import { normalizeZone } from "./impl/zoneUtil.js";
 import { validateWeekSettings } from "./impl/util.js";
+import { resetDigitRegexCache } from "./impl/digits.js";
 
 let now = () => Date.now(),
   defaultZone = "system",
@@ -171,5 +172,6 @@ export default class Settings {
   static resetCaches() {
     Locale.resetCache();
     IANAZone.resetCache();
+    resetDigitRegexCache();
   }
 }