Can preg_match() capture unknown number of occurrences? - php

Question

Let's say I'm having the following string:
$string = 'cats[Garfield,Tom,Azrael]';
I need to capture the following strings:
cats
Garfield
Tom
Azrael
That string can be any word-like text, followed by brackets with the list of comma-separated word-like entries. I tried the following:
preg_match('#^(\w+)\[(\w+)(?:,(\w+))*\]$#', $string, $matches);
The problem is that $matches ignores Tom, matching only the first and the last cat.
Now, I know how to do that with more calls, perhaps combining preg_match() and explode(), so the question is not how to do it in general.
The question is: can that be done in single preg_match(), so I could validate and match on one go?

Answer 1

The underlying question seems to be: is it possible to extract each occurrence of a repeated capture group?
The answer is no.
However, several workarounds exists:
The most understandable uses two steps: you capture the full list and then you split it. Something like:
$str = 'cats[Garfield,Tom,Azrael,Supermatou]';
if ( preg_match('~(?<item>\w+)\[(?<list>\w+(?:,\w+)*)]~', $str, $m) )
$result = [ $m['item'], explode(',', $m['list']) ];
(or any structure you want)
An other workaround uses preg_match_all in conjunction with the \G anchor that matches either the start of the string or the position after a successful match:
$pattern = '~(?:\G(?!\A),|(?<item>\w+)\[(?=[\w,]+]))(?<elt>\w+)~';
if ( preg_match_all($pattern, $str, $matches) )
print_r($matches);
This design ensures that all elements are between the brackets.
To obtain a more flat result, you can also write it like this:
$pattern = '~\G(?!\A)[[,]\K\w+|\w+(?=\[[\w,]+])~';
details of this last pattern:
~
# first alternative (can't be the first match)
\G (?!\A) # position after the last successful match
# (the negative lookahead discards the start of the string)
[[,] # an opening bracket or a comma
\K # return the whole match from this position
\w+ # an element
| # OR
# second alternative (the first match)
\w+ # the item
(?= # lookahead to check forward if the format is correct
\[ # opening bracket
[\w,]+ # word characters and comma (feel free to be more descriptive
# like \w+(?:,\w+)* or anything you want)
] # closing bracket
)
~

Answer 2

Why not a simple preg_match_all:
$string = 'cats[Garfield,Tom,Azrael], entity1[child11,child12,child13], entity2:child21&child22&child23';
preg_match_all('#\w+#', $string, $matches);
print_r($matches);
Output:
Array
(
[0] => Array
(
[0] => cats
[1] => Garfield
[2] => Tom
[3] => Azrael
[4] => entity1
[5] => child11
[6] => child12
[7] => child13
[8] => entity2
[9] => child21
[10] => child22
[11] => child23
)
)