Is it better to use look-behind or capture groups?

Question

I'm not sure if one of these is 'better' then the other, and why it would be, but I've got an original string that looks like this:

$string = '/random_length_user/file.php';

Now, there are two ways to match it, the first, using my new friend, the look-behind, and the 2nd, without:

preg_match("%(?<=^/)([^/]*)%", $string, $capture);
preg_match("%^/([^/]*)%", $string, $capture);

They return, in order:

Array
(
    [0] => random_length_user
)
Array
(
    [0] => /random_length_user
    [1] => random_length_user
)

Essentially I get the result I want in $capture[0] using look-behind, and in $capture[1] without. Now the question is ... is there a reason to prefer one of these methods over the other?

Answer 1

It probably doesn't make a difference with preg_match, but it will matter when using preg_replace, as it affects what will be replaced.

It might also be an issue when you do a global match, because the capturing group will consume characters, while the lookarounds will not

Trivial example:

• /(?<=a)a/g with 'aaaa' gives Array('a', 'a', 'a')
• /(a)a/g with 'aaaa' gives Array('aa', 'aa')

Answer 2

The problem is that the lookbehind approach is not as flexible; it falls down when you start dealing with variable-length matches. For example, suppose you wanted to extract the file name in your example, and you didn't know the name of the directory. The capturing-group technique still works fine:

preg_match("%^/\w+/([^/]*)%", '/random_length_user/file.php');

Array
(
    [0] => /random_length_user/file.php
    [1] => file.php
)

...but the lookbehind approach doesn't, because lookbehind expressions can only match a fixed number of characters. However, there's an even better alternative: \K, the MATCH POINT RESET operator. Wherever you put it, the regex engine pretends the match really started there. So you get the same result as you would with a lookbehind, without the fixed-length limitation:

preg_match('%^/\w+/\K[^/]+$%', '/random_length_user/file.php');

Array
(
    [0] => file.php
)

As far as I know, this feature is only available in Perl 5.10+ and in tools (like PHP's preg_ functions) that are powered by the PCRE library. For the PCRE reference, see the manpage and search (F3) for \K.