How clear duplicate consecutive non-alphabetic characters in a string?

Matching the string only for: ,.:- how I can remove duplicate values from a string? For example having:

"ab::::c ---------d,,,e ..........f ::a-b,,,c..d"

Expected output:

"ab:c -d,e .f :a-b,c.d" 

Here we are using preg_replace to achieve desired output.

Regex: ([,.:-])\1+ Regex demo

Or

Regex: (,|\.|:|-)\1+Regex demo

1. This will match a character and add that in captured group

2. using that captured group for \1 more than one occurence.

Replacement: $1

Try this code snippet here

<?php
ini_set('display_errors', 1);

$string="ab::::c ---------d,,,e ..........f ::a-b,,,c..d";
echo preg_replace('/([,.:-])\1+/', '$1', $string);

Solution 2: using foreach loop

Try this code snippet here

$string="aab::::css ---------ddd,,,esddsff ..........f ::a-b,,,c..d";
$chars=  str_split($string);
$result=array();
foreach($chars as $character)
{
    if($character!=end($result) ||  !in_array($character, array(":",",",".","-")))
    {
        $result[]=$character;
    }
}
print_r(implode("",$result));