{input
 {@ type="text"
    placeholder="Please enter a string and dblclick"
    ondblclick="alert( 'You wrote « ' +
                       this.value +
                       ' » and it is ' +
                       ((isNaN(this.value)) ? 'not' : '') +
                       ' a number.' )"
}}

Please enter a string and dblclick

Input: Hello World
Output: You wrote « Hello World » and it is not a number.

Input: 123
Output: You wrote « 123 » and it is a number.