An optional directive that can be used to check values for input: Input control follows HTML5 input types and polyfills the HTML5 validation behavior for older browsers.
Sets pattern validation error key if the ngModel value does not match a RegExp found by evaluating the Angular expression given in the attribute value. If the expression evaluates to a RegExp object, then this is used directly. If the expression evaluates to a string, then it will be converted to a RegExp after wrapping it in ^ and $ characters. For instance, "abc" will be converted to new RegExp('^abc$').
Note: Avoid using the g flag on the RegExp, as it will cause each successive search to start at the index of the last search's match, thus not taking the whole input value into account.
