The problem is when a user enters aaaaaa or xyzzz etc. in the input field, I want to check that the user can't enter 3 similar alphabets repetitively. e.g aabb is valid, but aabbb should be invalid. I want to do it using regular expression. Is there a way to do it..?

The problem is when a user enters aaaaaa or xyzzz etc. in the input field, I want to check that the user can't enter 3 similar alphabets repetitively. e.g aabb is valid, but aabbb should be invalid. I want to do it using regular expression. Is there a way to do it..?

• javascript
• regex
• html

2 Answers 2

You can use a backreference (\1) inside a negative lookahead ((?!…)) like this:

/^(?:(\w)(?!\1\1))+$/

This pattern will match any string consisting of 'word' characters (Latin letters, decimal digits, or underscores) but only if that string doesn't contain three consecutive copies of the same character.

To use the HTML5 pattern attribute, that would be:

<input type="text" pattern="^(?:(\w)(?!\1\1))+$">

Demonstration

You also try this pattern with JavaScript

(\w)\1{2,}

and you can test it on jsfiddle too

The JavaScript code is like this:

jQuery(document).ready(
    function($)
    {
        $('#input').on(
            'keyup',
            function()
            {
                var $regex  = /(\w)\1{2,}/;
                var $string = $(this).val();

                if($regex.test($string))
                {
                    // Do stuff for repeated characters
                }
                else
                {
                    // Do stuff for not repeated characters
                }
            }
        );
    }
);

Where $('#input') selects the text field with ID input. Also with the {2,} in the regex pattern you can control to length if the repeated characters. If you change the 2 to 4 in example, the pattern will match 5 repeated characters or more.