Php preg_match optional group

Question

I wrote a regex:

(^.*)(\[{1}[0-9]+:[0-9]+:[0-9]+:[0-9]+\]{1}) (\"{1}.+\"{1}) ([0-9]+) ([0-9-]+)

to match a string like:

141.243.1.172 [29:23:53:25] "GET /Software.html HTTP/1.0" 200 233

and using the php preg_match.

When I remove from the string for example the first part 141.243.1.172 the preg_match returns me:

array(6
 0  =>  [29:23:53:25] "GET /Software.html HTTP/1.0" 200 233
 1  =>  // correctly empty
 2  =>  [29:23:53:25]
 3  =>  "GET /Software.html HTTP/1.0"
 4  =>  200
 5  =>  233
 )

where the index 1 is correctly empty. But if I remove from the string [29:23:53:25] I get an empty array from preg_match. How can I have the same result as above, getting just the related index empty and not all?

Your regex just doesn't match anymore in the last case, `preg_match()` returns `0` which you don't seem to check for. Optional matches must be marked as such, but before that please explain your usage of `{1}` which is redundant. — AmigoJack, Jul 10 '19 at 14:09
Yes the preg_match does not match anymore, is there a way to make optional groups? — Stefano Maglione, Jul 10 '19 at 14:14
Possible duplicate of [Regex optional group](https://stackoverflow.com/questions/18633334/regex-optional-group) — Stefano Maglione, Jul 10 '19 at 15:50

The fourth bird · Accepted Answer · 2019-07-10T14:30:41.647

For the first part that works due to the .*. If you want to be able to remove the second part as well, you could make both groups optional and the first one non greedy. Move the space into the second group as well.

Note that you don't have to escape the double quote and that the quantifier {1} is superfluous so it can be omitted.

There is only a single double quote following after the first match, but to prevent possible over matching you could make that match also non greedy or use a negated character class ("[^"]+") instead to prevent unnecessary backtracking.

(^.*?)?(\[[0-9]+:[0-9]+:[0-9]+:[0-9]+\] )?(".+?") ([0-9]+) ([0-9-]+)

Regex demo

For example

$strings = [
    '141.243.1.172 [29:23:53:25] "GET /Software.html HTTP/1.0" 200 233',
    '[29:23:53:25] "GET /Software.html HTTP/1.0" 200 233',
    '"GET /Software.html HTTP/1.0" 200 233'
];

$pattern = '/(^.*?)?(\[[0-9]+:[0-9]+:[0-9]+:[0-9]+\] )?(".+?") ([0-9]+) ([0-9-]+)/';

foreach ($strings as $string) {
    preg_match($pattern, $string, $matches);
    print_r($matches);
}

Result

Array
(
    [0] => 141.243.1.172 [29:23:53:25] "GET /Software.html HTTP/1.0" 200 233
    [1] => 141.243.1.172 
    [2] => [29:23:53:25] 
    [3] => "GET /Software.html HTTP/1.0"
    [4] => 200
    [5] => 233
)
Array
(
    [0] => [29:23:53:25] "GET /Software.html HTTP/1.0" 200 233
    [1] => 
    [2] => [29:23:53:25] 
    [3] => "GET /Software.html HTTP/1.0"
    [4] => 200
    [5] => 233
)
Array
(
    [0] => "GET /Software.html HTTP/1.0" 200 233
    [1] => 
    [2] => 
    [3] => "GET /Software.html HTTP/1.0"
    [4] => 200
    [5] => 233
)

Php demo

Note that you really should have a PHP demo setup, as a pure regex is not the same thing as a working PHP script. — Tim Biegeleisen, Jul 10 '19 at 14:07
I tried your solution but it puts together in the same index 141.243.1.172 [29:23:53:25] . While I would like to have them in separated indexes — Stefano Maglione, Jul 10 '19 at 14:12
@TimBiegeleisen Good advice, thank you! I saw the first pattern was not correct. — The fourth bird, Jul 10 '19 at 14:18
I see it works, but if i remove '200' or '233' or 'GET /Software.html HTTP/1.0' I have the same issue with empty array — Stefano Maglione, Jul 10 '19 at 14:18

score 0 · Answer 2 · answered Jul 10 '19 at 15:51

change regex to this

((^.*)(\[{1}[0-9]+:[0-9]+:[0-9]+:[0-9]+\]{1}) )?(\"{1}.+\"{1}) ([0-9]+) ([0-9-]+)

for 141.243.1.172 [29:23:53:25] "GET /Software.html HTTP/1.0" 200 233

result would be

Array
(
    [0] => 141.243.1.172 [29:23:53:25] "GET /Software.html HTTP/1.0" 200 233
    [1] => 141.243.1.172 [29:23:53:25] 
    [2] => 141.243.1.172
    [3] => [29:23:53:25]
    [4] => "GET /Software.html HTTP/1.0"
    [5] => 200
    [6] => 233
)

and for [29:23:53:25] "GET /Software.html HTTP/1.0" 200 233

result would be

Array
(
    [0] => [29:23:53:25] "GET /Software.html HTTP/1.0" 200 233
    [1] => [29:23:53:25] 
    [2] => 
    [3] => [29:23:53:25]
    [4] => "GET /Software.html HTTP/1.0"
    [5] => 200
    [6] => 233
)

Php preg_match optional group

2 Answers2