Function domainRegex

domainRegex(options: RegexPatternOptions, flags?: string): RegExp
Builds a regular expression for matching domains based on route options.
Parameters
- options: RegexPatternOptions
  The route options to build the regex from.
- flags: string = 'i'
  Regular expression flags, defaults to 'i' (case insensitive).
Returns RegExp
A regular expression for matching domains or undefined if no domain is specified.
Example
```
const regex = domainRegex({ domain: '{subdomain}.example.com' });
console.log(regex?.test('api.example.com')); // true
```