A regular expression to match lone surrogates only

regex-lone-surrogates.js

var assert = require('assert');

// The goal is to match lone surrogates, i.e. any high surrogates
// (`[\uD800-\uDBFF]`) that are not directly followed by a low surrogate 
// (`[\uDC00-\uDFFF]`), and any low surrogates (`[\uDC00-\uDFFF]`) that are not
// directly preceded by a high surrogate (`[\uD800-\uDBFF]`).

var regex = /[\uD800-\uDBFF](?![\uDC00-\uDFFF])|(?:[^\uD800-\uDBFF]|^)[\uDC00-\uDFFF]/;

assert.equal(regex.test('foo\uDC00bar'), true);
assert.equal(regex.test('\uDC00bar'), true);
assert.equal(regex.test('foo\uDC00'), true);
assert.equal(regex.test('foo\uDBFFbar'), true);
assert.equal(regex.test('\uDBFFbar'), true);
assert.equal(regex.test('foo\uDBFF'), true);
assert.equal(regex.test('foo\uD800bar'), true);
assert.equal(regex.test('\uD800bar'), true);
assert.equal(regex.test('foo\uD800'), true);
assert.equal(regex.test('foo\uDFFFbar'), true);
assert.equal(regex.test('\uDFFFbar'), true);
assert.equal(regex.test('foo\uDFFF'), true);
assert.equal(regex.test('foo\uD834\uDF06bar'), false); // astral symbol
assert.equal(regex.test('\uD834\uDF06bar'), false); // astral symbol
assert.equal(regex.test('foo\uD834\uDF06'), false); // astral symbol

console.log('If you can see this, the implementation is correct.');

As used in regenerate.

Incredibly helpful, thank you.

Thank you!