Parse kebab-case sentence with predictable components

Question

Example strings:

accuracy-is-5

accuracy-is-5-or-15

accuracy-is-5-or-15-or-20

package-is-dip-8-or-dip-4-or-dip-16

My current regexp:

/^([a-z0-9\-]+)\-is\-([a-z0-9\.\-]*[a-z0-9])(?:\-or\-([a-z0-9\.\-]*[a-z0-9]))*$/U

No fixed length, part:

\-or\-[a-z0-9\.\-]

can be repeated.

But now from string accuracy-is-5-or-15-or-20, I get:

Array (
    [0] => accuracy-is-5-or-15-or-20
    [1] => accuracy
    [2] => 5
    [3] => 20
)

Where is 15?

Answer 1

When a capture group is repeated in a pattern the previous values are overwritten with the last. So it is not possible to design your pattern like this with preg_match.

A possible workaround consists to use preg_match_all that searches all occurrences of a pattern and the \G anchor that is the position after the previous match. The pattern must be written to find one value at a time.

The \G ensures that all matches are contiguous. To be sure that the end of the string has been reached (in other word that the string is correctly formatted from start to the end), a convenient way is to create an empty capture group at the end. So if this capture group appears in the last match, that means that the format is correct.

define('PARSE_SENTENCE_PATTERN', '~
(?:                                       # two possible beginings:
    \G(?!\A)                              # - immediatly after a previous match 
  |                                       # OR
    \A                                    # - at the start of the string
    (?<subject> \w+ (?>[-.]\w+)*? ) -is-  #  (in this case the subject is captured)
)
(?<value> \w+ (?>[-.]\w+)*? )  # capture the value
(?: -or- | \z (?<check>) )     # must be followed by "-or-" OR the end of the string \z
                               # (then the empty capture group "check" is created)
~x');

function parseSentence ($sentence) {

    if (preg_match_all(PARSE_SENTENCE_PATTERN, $sentence, $matches, PREG_SET_ORDER) &&
        isset(end($matches)['check']) ) 
        return [ 'subject' => $matches[0]['subject'],
                 'values'  => array_reduce ($matches, function($c, $v) {
                                  $c[] = $v['value']; return $c; }, $c = []) ];

    return false; // wrong format

}

// tests
$test_strings = ['accuracy-is-5', 'accuracy-is-5-or-15', 'accuracy-is-5-or-15-or-20',
                 'package-is-dip-8-or-dip-4-or-dip-16',
                 'bad-format', 'bad-format-is-', 'bad-format-is-5-or-'];

foreach ($test_strings as $test_string) {
    var_dump(parseSentence($test_string));
}