Regex to match first character and no more than 2 identical consecutives

Question

I'm using jQuery to add a pattern attribute to a text field based on a letter I select from an array. I'm trying to restrict the values that text field can accept with a regex, but it doesn't work properly.

What I want is that the first char of the value must be the letter I choose of the array, and then don't accept more than 2 identical consecutive caracters.

My regex is this:

^["+letter+"](?!(.)\1).{2}.*

And it seems to work when I'm testing it in regexr.com, but when I test it in my page, just the part of match the 1st char works, and the rest don't. When I type something like "Aaaaron", the message of "invalid entry" doesn't show.

Thanks in advance.

Answer 1

Description

^(.)(?!\1{2})

This regex will do the following:

• capture the first character
• validate the first character is then not repeated 2 more times. If two more of the same character are present after the first occurrence, then you have 3 of the same characters in a row.

Note to make this expression view upper and lower case versions of a letter as the same character you'll need to use the case insensitive flag.

Live Example

https://regex101.com/r/xG9mE9/2

Explanation

NODE                     EXPLANATION
----------------------------------------------------------------------
  ^                        the beginning of a "line"
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    .                        any character except \n
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
  (?!                      look ahead to see if there is not:
----------------------------------------------------------------------
    \1{2}                    what was matched by capture \1 (2 times)
----------------------------------------------------------------------
  )                        end of look-ahead
----------------------------------------------------------------------

Answer 2

try this regex: (with letter being 'a')

$("form").find("input[type=text]").attr("pattern", "(?=[Aa])(?!.*(.)\\1\\1).*");

it validates the starting letter, and that no character appears more than 2 times consecutively:

jsfiddle

notes:

• you can't do case insensitive matching with HTML5 pattern attribute, so 'a' and 'A' are not the same thing ('Aaaron' isn't 3 a's in a row)
• if adding pattern via a string (not a regex literal) in jquery/javascript, remember there's string interpolation first and then regex interpolation second (the backslash means something to String as well to Regex, you might need to double escape them: (\\1 for a backreference in this case)
• you don't need ^ or $ to make the input value match the entire pattern only, the regex is wrapped in ^(?:regex)$ for you. This means that if your pattern does not consume the entire string it will not work: (?=[Aa])(?!.*(.)\\1\\1), which are just a couple of lookarounds, and would normally validate the input just fine, is a zero-width pattern, and without the .* at the end, does not work.