regex.ts

export function escapeRegex(string: string) {
  return string.replace(/[.*+?^${}()|[\]\\]/g, '\\$&'); // $& means the whole matched string
}

export function regex(
  strings: TemplateStringsArray,
  ...values: Array<RegExp | string>
): RegExp;

export function regex(
  flags: string,
): (strings: TemplateStringsArray, ...values: Array<RegExp | string>) => RegExp;

/**
 * Usage:
 * regex`${'escaped'} ${regex`nestable`}`
 * regex('gi')`withFlags`
 */
export function regex(...args: any[]) {
  function _regex(
    flags: string,
    strings: TemplateStringsArray,
    ...values: Array<RegExp | string>
  ) {
    const rawStrings = strings.raw; // removes the need for double escapes
    const pattern = values.reduce((pattern, value, index) => {
      value = value instanceof RegExp ? value.source : escapeRegex(value);
      return pattern + value + rawStrings[index + 1];
    }, rawStrings[0]);
    return RegExp(pattern, flags);
  }

  if (Array.isArray(args[0])) {
    // Used as a template tag
    return _regex(
      '',
      ...(args as [TemplateStringsArray, ...Array<RegExp | string>]),
    );
  }

  return (strings: TemplateStringsArray, ...values: Array<RegExp | string>) =>
    _regex(args[0], strings, ...values);
}