function prefix(prop) {
	let match = prop.match(/^(-\w+-)/);

	if (match) {
		return match[0];
	}

	return '';
}
Returns the vendor prefix extracted from an input string.

@param {string} prop String with or without vendor prefix.

@return {string} vendor prefix or empty string

@example
vendor.prefix('-moz-tab-size') //=> '-moz-'
vendor.prefix('tab-size')      //=> ''