Updates the regex for the joind.in validation to better match possible URLs
Updates the regex for the joind.in validation to better match possible URLs

localheinz/opencfp,GrUSP/opencfp,PHPBenelux/opencfp,localheinz/opencfp,PHPBenelux/opencfp,localheinz/opencfp,PHPBenelux/opencfp,GrUSP/opencfp,opencfp/opencfp,localheinz/opencfp,opencfp/opencfp,PHPBenelux/opencfp,GrUSP/opencfp,opencfp/opencfp,GrUSP/opencfp