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Capturing groups - J modifier


PCRE in PHP offers J modifier. It can be used either as a flag: /foo/J (since PHP 7.2), or as an in-pattern modifier: /foo(?J)/.

Normally, duplicated pattern names aren't allowed, and such code

pattern('(?<group>)(?<group>)') // MalformedPatternException

would throw MalformedPatternException, with message Two named subpatterns have the same name.

However, J modifier removes that restriction, and it becomes possible to use duplicated group names in one pattern:

pattern('(?<group>)(?<group>)', 'J') // works fine

It doesn't make much sense for two completely separate groups; it rather may have some sense to be used with optional, mutually exclusive groups, like:

pattern('((?<scheme>http)|(?<scheme>ftp))', 'J') // either one or the other

maybe. T-Regx doesn't encourage such patterns, we'd recommend using one enclosing group for that purpose.

pattern('(?<scheme>http|ftp)', 'J')

The complication#

PCRE PHP API returns groups as an array, and PHP arrays can't have duplicate keys. That means, despite multiple groups with the same name being matched, only one will be present in the resulting array. There are some constants, allowing us to handle the duplicate groups in some way, but it's not perfect.

That means, T-Regx isn't able to reliably:

  • assign an index to a named group
  • assign a name to an indexed group
  • determine which of groups are matched or not.

The PHP solution#

The solution is far from perfect, but it's PHP, so what can we do :)

DN - doubly-named

  • We can't reliably assign a duplicated name to an index, and an index to a name:

    • group('group')->index() returns the index of the left-most DN group.
    • group(2)->name() returns the name, only if 2 is the index of the left-most DN group.

    So with PHP we assume the left-most indexed group has the name.

  • We can't reliably handle optional DN groups.

    • So, the whole DN is considered unmatched if, and only if the right-most DN group is not matched.
    • The text() and offset() od the whole DN value, is the text and offset of the right-most DN group.


  • Index/name relation, is taken from the left-most group

    And in consequence: groupNames(), namedGroups(), etc.

  • Text/offset/matched is taken from the right-most group.

    And in consequence: byteOffset(), tail(), etc.

So basically what a group is, what is its name, order and index is determined by the matched subject. Great :|

T-Regx solution#

The solution we came up with offers predictability and reliability.

Previously, group('name') would just read a group by name from the $match returned by PHP. We can't do it anymore, since if J modifier was used, the index and the order of the group would vary based on the matched occurrence (another gotcha).

So first, T-Regx assigns 'name' to an index, and then reads the group. It gives us the advantage of the named group always is in the same place (same order) and has exactly the same index. Unfortunately, to read that, we always read the first group used in pattern (but at least its not so stupidly random, as with PHP).

All methods that handle capturing groups (group(), groups(), inline group(), namedGroups(), etc.) always use that strategy, and basically they ignore J modifier, as if it was never used.

To take advantage of J modifier, we added new method Detail.usingDuplicateName().group('name'). Method usingDuplicateName().group('name') only takes a name as an argument (using it with indexes doesn't make any sense),

It's almost identical to'name') except, it doesn't have index() method. It can't have index() method, since with J modifier it's impossible to reliably assign an index to a named group, since there are many groups that could have this name. We could add method indexes(), to get a list of indexes of the groups that share this name, but it's impossible with PHP API.

use \TRegx\CleanRegex\Match\Details\Detail;
pattern('(?<one>one)? (?<two>two)?', 'J')
->match('one two')
->first(function (Detail $detail) {
// These two are identical
$detail->group('name')->text(); // always 'one'
$detail->group(1)->text(); // always 'one'
// These two rely on subject
$detail->usingDuplicateName()->group('name')->text(); // either 'one' or 'two'
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