Bump to v4.0.0.

internal/stringSize.js

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+/** Used to compose unicode character classes. */
+var rsAstralRange = '\\ud800-\\udfff',
+    rsComboRange = '\\u0300-\\u036f\\ufe20-\\ufe23',
+    rsVarRange = '\\ufe0e\\ufe0f';
+
+/** Used to compose unicode capture groups. */
+var rsAstral = '[' + rsAstralRange + ']',
+    rsCombo = '[' + rsComboRange + ']',
+    rsModifier = '(?:\\ud83c[\\udffb-\\udfff])',
+    rsNonAstral = '[^' + rsAstralRange + ']',
+    rsRegional = '(?:\\ud83c[\\udde6-\\uddff]){2}',
+    rsSurrPair = '[\\ud800-\\udbff][\\udc00-\\udfff]',
+    rsZWJ = '\\u200d';
+
+/** Used to compose unicode regexes. */
+var reOptMod = rsModifier + '?',
+    rsOptVar = '[' + rsVarRange + ']?',
+    rsOptJoin = '(?:' + rsZWJ + '(?:' + [rsNonAstral, rsRegional, rsSurrPair].join('|') + ')' + rsOptVar + reOptMod + ')*',
+    rsSeq = rsOptVar + reOptMod + rsOptJoin,
+    rsSymbol = '(?:' + [rsNonAstral + rsCombo + '?', rsCombo, rsRegional, rsSurrPair, rsAstral].join('|') + ')';
+
+/** Used to match [string symbols](https://mathiasbynens.be/notes/javascript-unicode). */
+var reComplexSymbol = RegExp(rsSymbol + rsSeq, 'g');
+
+/** Used to detect strings with [zero-width joiners or code points from the astral planes](http://eev.ee/blog/2015/09/12/dark-corners-of-unicode/). */
+var reHasComplexSymbol = RegExp('[' + rsZWJ + rsAstralRange + rsComboRange + rsVarRange + ']');
+
+/**
+ * Gets the number of symbols in `string`.
+ *
+ * @param {string} string The string to inspect.
+ * @returns {number} Returns the string size.
+ */
+function stringSize(string) {
+  if (!(string && reHasComplexSymbol.test(string))) {
+    return string.length;
+  }
+  var result = reComplexSymbol.lastIndex = 0;
+  while (reComplexSymbol.test(string)) {
+    result++;
+  }
+  return result;
+}
+
+export default stringSize;