--- /dev/null
+/** Used to compose unicode character classes. */
+var rsAstralRange = '\\ud800-\\udfff',
+ rsComboMarksRange = '\\u0300-\\u036f\\ufe20-\\ufe23',
+ rsComboSymbolsRange = '\\u20d0-\\u20f0',
+ rsVarRange = '\\ufe0e\\ufe0f';
+
+/** Used to compose unicode capture groups. */
+var rsAstral = '[' + rsAstralRange + ']',
+ rsCombo = '[' + rsComboMarksRange + rsComboSymbolsRange + ']',
+ rsFitz = '\\ud83c[\\udffb-\\udfff]',
+ rsModifier = '(?:' + rsCombo + '|' + rsFitz + ')',
+ 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(rsFitz + '(?=' + rsFitz + ')|' + 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 + rsComboMarksRange + rsComboSymbolsRange + rsVarRange + ']');
+
+/**
+ * Gets the number of symbols in `string`.
+ *
+ * @private
+ * @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;
+}
+
+module.exports = stringSize;