preg_replace double replacement

Question

I'm getting some unexpected results from a regular expression, which is meant to be replacing the classname on a namespace. The replacement appears to happen twice, so that the classname getting replaced is duplicated (see example below).

I've actually resolved the problem by changing the reg ex to match 1 or more (+) rather than 0 or more (*) which is actually more accurate for what I want.

However, I'm a little confused as to why I was getting an issue in the first place.

Here is an example of the problem:

$classns  = 'components\groups\GroupsController';
$newclass = 'GroupsAccess';
$classns = preg_replace('/[^\\\\]*$/', $newclass, $classns);
echo $classns;

Result

components\groups\GroupsAccessGroupsAccess

Expected

components\groups\GroupsAccess

Is it possible that the * is matching a word boundary or something of that nature?

The confusing part for me is that a preg_match using the same regex shows only one result, so it would appear to be something specific to how preg_match runs the regex.

e.g.

preg_match('/[^\\\\]*$/', $classns, $m);
var_dump($m);

Result

array(1) { [0]=> string(12) "GroupsAccess" }

Answer 1

The * is not matching a word boundary, it is matching the empty string.

Your expression is at first matching

components\groups\ GroupsController

and the $ is an anchor that is matching a position and that is before the end of the string (or a \n before the end of the string).

So after the first match the position of the regex parser is after the last "r" and before the end of the string, when it tries to match your regex again. And it will find one more match ==> 0 occurrences of the / (the empty string) followed by the end of the string.

Then it moves on, recognize the end of the string and finishes.