preg_match and UTF-8 in PHP

Question

I'm trying to search a UTF8-encoded string using preg_match.

preg_match('/H/u', "\xC2\xA1Hola!", $a_matches, PREG_OFFSET_CAPTURE);
echo $a_matches[0][1];

This should print 1, since "H" is at index 1 in the string "¡Hola!". But it prints 2. So it seems like it's not treating the subject as a UTF8-encoded string, even though I'm passing the "u" modifier in the regular expression.

I have the following settings in my php.ini, and other UTF8 functions are working:

mbstring.func_overload = 7
mbstring.language = Neutral
mbstring.internal_encoding = UTF-8
mbstring.http_input = pass
mbstring.http_output = pass
mbstring.encoding_translation = Off

Any ideas?

Answer 1

Although the u modifier makes both the pattern and subject be interpreted as UTF-8, the captured offsets are still counted in bytes.

You can use mb_strlen to get the length in UTF-8 characters rather than bytes:

$str = "\xC2\xA1Hola!";
preg_match('/H/u', $str, $a_matches, PREG_OFFSET_CAPTURE);
echo mb_strlen(substr($str, 0, $a_matches[0][1]));

Answer 2

Try adding this (*UTF8) before the regex:

preg_match('(*UTF8)/H/u', "\xC2\xA1Hola!", $a_matches, PREG_OFFSET_CAPTURE);

Magic, thanks to a comment in https://www.php.net/manual/function.preg-match.php#95828

Answer 3

Looks like this is a "feature", see http://bugs.php.net/bug.php?id=37391

'u' switch only makes sense for pcre, PHP itself is unaware of it.

From PHP's point of view, strings are byte sequences and returning byte offset seems logical (i don't say "correct").

Answer 4

Excuse me for necroposting, but may be somebody will find it useful: code below can work both as replacement for preg_match and preg_match_all functions and returns correct matches with correct offset for UTF8-encoded strings.

     mb_internal_encoding('UTF-8');

     /**
     * Returns array of matches in same format as preg_match or preg_match_all
     * @param bool   $matchAll If true, execute preg_match_all, otherwise preg_match
     * @param string $pattern  The pattern to search for, as a string.
     * @param string $subject  The input string.
     * @param int    $offset   The place from which to start the search (in bytes).
     * @return array
     */
    function pregMatchCapture($matchAll, $pattern, $subject, $offset = 0)
    {
        $matchInfo = array();
        $method    = 'preg_match';
        $flag      = PREG_OFFSET_CAPTURE;
        if ($matchAll) {
            $method .= '_all';
        }
        $n = $method($pattern, $subject, $matchInfo, $flag, $offset);
        $result = array();
        if ($n !== 0 && !empty($matchInfo)) {
            if (!$matchAll) {
                $matchInfo = array($matchInfo);
            }
            foreach ($matchInfo as $matches) {
                $positions = array();
                foreach ($matches as $match) {
                    $matchedText   = $match[0];
                    $matchedLength = $match[1];
                    $positions[]   = array(
                        $matchedText,
                        mb_strlen(mb_strcut($subject, 0, $matchedLength))
                    );
                }
                $result[] = $positions;
            }
            if (!$matchAll) {
                $result = $result[0];
            }
        }
        return $result;
    }

    $s1 = 'Попробуем русскую строку для теста';
    $s2 = 'Try english string for test';

    var_dump(pregMatchCapture(true, '/обу/', $s1));
    var_dump(pregMatchCapture(false, '/обу/', $s1));

    var_dump(pregMatchCapture(true, '/lish/', $s2));
    var_dump(pregMatchCapture(false, '/lish/', $s2));

Output of my example:

    array(1) {
      [0]=>
      array(1) {
        [0]=>
        array(2) {
          [0]=>
          string(6) "обу"
          [1]=>
          int(4)
        }
      }
    }
    array(1) {
      [0]=>
      array(2) {
        [0]=>
        string(6) "обу"
        [1]=>
        int(4)
      }
    }
    array(1) {
      [0]=>
      array(1) {
        [0]=>
        array(2) {
          [0]=>
          string(4) "lish"
          [1]=>
          int(7)
        }
      }
    }
    array(1) {
      [0]=>
      array(2) {
        [0]=>
        string(4) "lish"
        [1]=>
        int(7)
      }
    }

Answer 5

You can calculate the real UTF-8 offset by cutting the string to the offset returned by the preg_mach with the byte-counting substr and then measuring this prefix with the correct-counting mb_strlen.

$utf8Offset = mb_strlen(substr($text, 0, $offsetFromPregMatch), 'UTF-8');

Answer 6

I wrote small class to convert offsets returned by preg_match to proper utf offsets:

final class NonUtfToUtfOffset
{
    /** @var int[] */
    private $utfMap = [];

    public function __construct(string $content)
    {
        $contentLength = mb_strlen($content);

        for ($offset = 0; $offset < $contentLength; $offset ++) {
            $char = mb_substr($content, $offset, 1);
            $nonUtfLength = strlen($char);

            for ($charOffset = 0; $charOffset < $nonUtfLength; $charOffset ++) {
                $this->utfMap[] = $offset;
            }
        }
    }

    public function convertOffset(int $nonUtfOffset): int
    {
        return $this->utfMap[$nonUtfOffset];
    }
}

You can use it like that:

$content = 'aą bać d';
$offsetConverter = new NonUtfToUtfOffset($content);

preg_match_all('#(bać)#ui', $content, $m, PREG_OFFSET_CAPTURE);

foreach ($m[1] as [$word, $offset]) {
    echo "bad: " . mb_substr($content, $offset, mb_strlen($word))."\n";
    echo "good: " . mb_substr($content, $offsetConverter->convertOffset($offset), mb_strlen($word))."\n";
}

https://3v4l.org/8Y32J

Answer 7

You might want to look at T-Regx library.

pattern('/Hola/u')->match('\xC2\xA1Hola!')->first(function (Match $match) 
{
    echo $match->offset();     // characters
    echo $match->byteOffset(); // bytes
});

This $match->offset() is UTF-8 safe offset.

Answer 8

If all you want to do is find the multi-byte safe position of H try mb_strpos()

mb_internal_encoding('UTF-8');
$str = "\xC2\xA1Hola!";
$pos = mb_strpos($str, 'H');
echo $str."\n";
echo $pos."\n";
echo mb_substr($str,$pos,1)."\n";

Output:

¡Hola!
1
H

Answer 9

The problem was solved to me just by using casual substr instead of expected mb_substr (PHP 7.4).

The mb_substr together with preg_match_all / PREG_OFFSET_CAPTURE (despite using or not using /u modifier)resulted in incorrect position when text contained euro sign symbol (€).

Also iconv and utf8_encode did not help, and I was not able to use htmlentities.

Just reverting to simple substr helped, and it worked with € and other characters correctly.