Why are preg_match() Offsets in Bytes, Not Characters, Even with UTF-8 Support?

PREG_OFFSET_CAPTURE and UTF-8 Strings: A Byte-Counting Mismatch

When matching UTF-8 strings using preg_match() with the PREG_OFFSET_CAPTURE parameter, users may encounter an unexpected behavior where offsets are counted in bytes instead of characters. Despite using the u modifier, which enables UTF-8 support for both the pattern and subject, captured offsets remain byte-based.

Resolving the Byte-Counting Issue

To address this discrepancy and obtain character-based offsets, a workaround involving mb_strlen can be employed. This function provides the UTF-8 character count for a specified substring. By utilizing mb_strlen on the substring of the subject string preceding the captured match, we can obtain the accurate character offset.

Here's a modified example:

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

By incorporating mb_strlen, we ensure that offsets represent character positions within the UTF-8 string, providing a precise and expected result.

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