Select characters that appear only once in a string

Question

Is it possible to select characters who appear only once?

I am familiar with negative look-behind, and tried the following

/(.)(?<!\1.*)/

but could not get it to work.

examples:

given AXXDBD it should output ADBD
       ^^ - this is unacceptable
given 123558 it should output 1238
         ^^ - this is unacceptable

thanks in advance for the help

Either I misunderstand what you are asking or the output for your first example - `AXXDBD` - should be `ADBD` not `ABDD`? — Bananaapple, Aug 01 '18 at 10:48
See https://stackoverflow.com/questions/1660694/regular-expression-to-match-any-character-being-repeated-more-than-10-times, https://stackoverflow.com/questions/12258622/regular-expression-to-check-for-repeating-characters, https://stackoverflow.com/questions/664194/how-can-i-find-repeated-characters-with-a-regex-in-java, https://stackoverflow.com/questions/6306098/regexp-match-repeated-characters, [**and more**](https://www.google.pl/search?q=regex+repeated+chars+site:stackoverflow.com&rlz=1C1GGRV_enPL782PL782&sa=X&ved=2ahUKEwi5oKTC2MvcAhUDaVAKHQOcB30QrQIoBDAAegQIABAN&biw=1745&bih=861). — Wiktor Stribiżew, Aug 01 '18 at 10:55
@WiktorStribiżew, thank you very much for the answer, useful topics — wnull, Aug 01 '18 at 10:59
The question is confusing, you can't select any nonconsecutive substring with a regex so iainn's replacing (throwing away) sequences of identical characters option is probably your best bet with regex — Veltzer Doron, Aug 01 '18 at 11:37

score 7 · Accepted Answer · answered Aug 01 '18 at 10:53

7

There are probably a lot of approaches to this, but I think you're looking for something like

(.)\1{1,}

That is, any character followed by the same character at least once.

Your question is tagged with both PHP and JS, so:

PHP:

$str = preg_replace('/(.)\1{1,}/', '', $str);

JS:

str = str.replace(/(.)\1{1,}/g, '');

answered Aug 01 '18 at 10:53

iainn

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4

you can use `+` instead of `{1,}` – bobble bubble Aug 01 '18 at 11:42

AymDev · Answer 2 · 2018-08-01T11:14:32.537

Without using a regular expression:

function not_twice ($str) {
    $str = (string)$str;
    $new_str = '';
    $prev = false;

    for ($i=0; $i < strlen($str); $i++) {
        if ($str[$i] !== $prev) {
            $new_str .= $str[$i];
        }
        $prev = $str[$i];
    }
    return $new_str;
}

Removes consecutives characters (1+) and casts numbers to string in case you need that too.

Testing:

$string = [
    'AXXDBD',
    '123558',
    12333
];
$string = array_map('not_twice', $string);
echo '<pre>' . print_r($string, true) . '</pre>';

Outputs:

Array
(
    [0] => AXDBD
    [1] => 12358
    [2] => 123
)

Select characters that appear only once in a string

2 Answers2