How would have validate an html iframe as of the rules() model?
Only to use the pattern "iframe width=" is enough, because the user has only to copy paste a iframe link from GoogleMaps (to try to validate that user copy a correct link: iframe and not an url or another html code)
Your advice was very useful and the pattern was not correct but aproximate.
This is the solution:
array('link_gmaps', 'match', 'pattern' => '/<iframe[^>]*src="https:\/\/maps.google.es\/[^"]+">\s*<\/iframe>/six', 'message'=>'{attribute} is invalid, must be an iframe.'),