ACZoom  ·  Archive-2016

Fun with perl vs php regular expression handling.

PHP has a regular expression search and replace function, preg_replace. This is supposed to be the standard greedy algorithm for patterns, for example, A* means zero or more A characters, and it matches the longest string of A's at that point.

So, if you want to match zero or more slash (/) characters at end of a string the pattern to use is: /*$

And, if we want to replace zero or more slash characters at end of a string with a single slash character, the perl code is:   s!/*$!/!

Full example in perl:

foreach ('path/to', 'path/to//', 'path/to///', 'path/to////') {
    my $s = $_;
    $s =~ s!/*$!/!;
    print "string: $_ changed to $s\n";
}

Output is: 
string: path/to changed to path/to/
string: path/to// changed to path/to/
string: path/to/// changed to path/to/
string: path/to//// changed to path/to/

And the above output is all correct.

Equivalent PHP code does not work, using PHP version 5.1.6.

Following PHP code:  $t = preg_replace('!/*$!', '/', $s); fails, it will end up with one / if the input has 0 or 1 / characters, but it will end up with two / characters if the input has 2 or more / characters, instead of a single / character. So, the match was not greedy, but for reason, was split into two matches, and then each matched group was replaced with a single / character.
The workaround for this is to specify a limit count of 1 to preg_replace, which makes the code work fine. preg_match seems to work fine, only preg_replace has this problem.

Here's php code, and its output, showing the failure:

<?php
foreach (array('path/to', 'path/to//', 'path/to///', 'path/to////') as $s) {
     # preg_match('!/*$!', $s, $matches); // works fine, is greedy - $matches[0] is zero or all slashes
     # $t = preg_replace('!/*$!', '/', $s, 1); // works, limit of 1 helps
     $t = preg_replace('!/*$!', '/', $s); // does not work: not greedy
     print "string: $s changed to $t\n";
}
?>

Output is:
string: path/to changed to path/to/
string: path/to// changed to path/to//
string: path/to/// changed to path/to//
string: path/to//// changed to path/to//

Did have a discussion on this in the php newsgroup and posted to php.net, no other update yet, for now, sticking with the workaround of forcing a limit count of 1 to suppress getting 2 matches for a * regular expression.