Question

The below code should replace all X’s not followed by a 1 with K,
but the it doesn’t work as intended. Can anyone shed some light on what the issue could be? Thanks!

<?php
    $test = "XXXX X1 X2 XXX X1";
    $test = preg_replace("/X([^1])/", 'K$1', $test);

    echo $test;
?>

Input: XXXX X1 X2 XXX X1
Expected output: KKKK X1 K2 KKK X1

Actual output: KXKX X1 K2 KXK X1

Answer 1

Matches cannot overlap. So after finding XX the regex engine will continue its search at the third character.

Use a negative lookahead instead (which will not be part of the match itself, but only assure that your X is not followed by a 1):

    $test = preg_replace("/X(?!1)/", 'K', $test);

This will also make replacing an X at the very end of your string work (which would not have worked before, because [^1] requires that there is actually a character (just not a 1).