PHP regex backreference not working

Question

I wrote a regex pattern which works perfectly when I test it in Regexr, but when I use it in my PHP code it doesn't always match when it should match.

The regular expression, including some examples that should and shouldn't match.

Example PHP code that should match but doesn't:

preg_match('/^([~]{3,})\s*([\w-]+)?\s*(?:\{([\w-\s]+)\})?\s*(\2[\w-]+)?\s*$/', "~~~ {class} lang", $matches);
echo var_dump($matches);

I believe the problem is caused by the backreference in the last capture group (\2[\w-]+), however, I can't quire figure out how to fix this.

Avinash Raj · Accepted Answer · 2014-09-21 16:10:17Z

3

Because you're referring to a non-existing group(group 2). So remove \2 from the regex.

^([~]{3,})\s*([\w-]+)?\s*(?:\{([-\w\s]+)\})?\s*([\w-]+)?\s*$

DEMO

    ~~~  {class} lang
     |  |   |      |
  Group1| Group3 Group4
        |
Missing group 2

answered Sep 21, 2014 at 16:10

Avinash Raj

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2 Comments

Jonny 5 Over a year ago

I like, how you illustrate it :)

CupOfTea696 Over a year ago

Thanks, you and @hwnd helped me figure it out.

hwnd · Accepted Answer · 2014-09-21 16:43:36Z

2

The problem is caused by capturing group #2, you have made this group optional. So since it may or may not exist, you need to make your backreference optional as well or else it always looks for a required group.

However, since all groups are optional I would just recurse the subpattern of the second group.

^(~{3,})\s*([\w-]+)?\s*(?:{([^}]+)})?\s*((?2))?\s*$

Example:

$str = '~~~ {class} lang';
preg_match('/^(~{3,})\s*([\w-]+)?\s*(?:{([^}]+)})?\s*((?2))?\s*$/', $str, $matches);
var_dump($matches);

Output

array(5) {
  [0]=> string(16) "~~~ {class} lang"
  [1]=> string(3) "~~~"
  [2]=> string(0) ""                   # Returns "" for optional groups that dont exist
  [3]=> string(5) "class"
  [4]=> string(4) "lang"
}

edited Sep 21, 2014 at 16:43

answered Sep 21, 2014 at 16:22

hwnd

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4 Comments

CupOfTea696 Over a year ago

The problem is that this way '~~~ lang {class} lang' gives a match as well, and it shouldn't.

CupOfTea696 Over a year ago

I just solved it my changing the 2nd capturing group to ([\w-]*), making it an empty string when it's not there instead of non-existent.

hwnd Over a year ago

Tell me what it should exactly match and I'll take a look when I get back home

CupOfTea696 Over a year ago

I've already fixed the problem thanks to your help. I've posted an answer myself explaining everything :)

CupOfTea696 · Accepted Answer · 2014-09-21 21:45:35Z

0

The answers below helped me figure out why it wasn't working. However both the answers would give a positive match for $str = '~~~ lang {class} lang'; which I didn't want. I fixed it my changing capturing group 2 to ([\w-]*) so that even if there is no string at that place, the capturing group exists but remains empty. This way all of the following strings match:

$str = '~~~   lang      {no-lines float left}   ';
$str = '~~~     {class}   ';
$str = '~~~ lang';
$str = '~~~ {class } lang ';
$str = '~~~';
$str = '~~~lang{class}';

But this one won't:

$str = '~~~ css {class} php';

Full solution:

$str = '~~~ {class} lang';
preg_match('/^([~]{3,})\s*([\w-]*)?\s*(?:\{([\w-\s]+)\})?\s*(\2[\w-]+)?\s*$/', $str, $matches);
var_dump($matches);

edited Sep 21, 2014 at 21:45

answered Sep 21, 2014 at 21:33

CupOfTea696

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