verboseRegExp.js

function verboseRegExp(input) {
  if (input.raw.length !== 1) {
    throw Error('verboseRegExp: interpolation is not supported');
  }

  let source = input.raw[0];
  let regexp = /(?<!\\)\s|[/][/].*|[/][*][\s\S]*[*][/]/g;
  let result = source.replace(regexp, '');

  return new RegExp(result);
}

let example1 = new RegExp(verboseRegExp`
    (?<!\\) \s             // Ignore whitespace, but not
                           // when escaped with backslash.
  | [/][/] .*              // Single-line comment.
  | [/][*] [\s\S]* [*][/]  // Multi-line comment.
                           // Note: /[\s\S]/ is same as /./s
                           // with dot-all flag (s).
`, 'g');

console.log(example1);

let example2 = verboseRegExp`
  (0 | [1-9] [0-9]*)   // Integer part with no leading zeroes
  \.                   // Dot
  [0-9]*               // Fractional part (optional)
  ([eE] [+-]? [0-9]+)? // Exponent part (optional)
`;

console.log(example2);

function assertRegExpEqual(regexp1, regexp2) {
  if (regexp1.toString() !== regexp2.toString()) {
    console.error(`Assertion error: ${regexp1} !== ${regexp2}`);
  } else {
    console.log('.');
  }
}

assertRegExpEqual(verboseRegExp`hai`, /hai/);
assertRegExpEqual(verboseRegExp`[]`, /[]/);

// Comments and whitespace are ignored;
assertRegExpEqual(verboseRegExp`x y`, /xy/);
assertRegExpEqual(/xy/, verboseRegExp`
  x // ignored
  y
`);
assertRegExpEqual(verboseRegExp`x /* ignored */ y`, /xy/);

// Also inside character classes:
assertRegExpEqual(verboseRegExp`[x y]`, /[xy]/);
assertRegExpEqual(verboseRegExp`[x//comment
  y]`, /[xy]/); // XXX
assertRegExpEqual(verboseRegExp`[x/*z][x*/z]`, /[xz]/);
assertRegExpEqual(verboseRegExp`[x/*z*/y]`, /[xy]/);

// Escaped whitespace is not ignored:
assertRegExpEqual(verboseRegExp`\ `, /\ /);
assertRegExpEqual(verboseRegExp`\ x `, /\ x/);
assertRegExpEqual(verboseRegExp`\       `, /\   /);

// Also not in brackets:
assertRegExpEqual(verboseRegExp`[\ ]`, /[\ ]/);

// Other escapes are passed through
assertRegExpEqual(verboseRegExp`\s`, /\s/);
assertRegExpEqual(verboseRegExp`\n`, /\n/);

// Nested brackets are allowed
assertRegExpEqual(verboseRegExp` [x [y ]] `, /[x[y]]/);

Related work: http://xregexp.com