PHP mb_strpos fails for Greek strings

Question

I have a file browser and I'm trying to find which file names contain a given query.The code goes like this:

$query = (isset($_POST['s']))? mb_strtolower($_POST['s'],'UTF-8') : ''; 
$res = opendir($dir); 
    while(false!== ($file = readdir($res))) { 
if(mb_strpos(mb_strtolower($file,'UTF-8'),mb_strtolower($query,'UTF-8'),0,'UTF-8')!== false) {
    echo $file;
}}

For English words this works fine,but when the text is in Greek,the results are not as expected,meaning that it works for some but not all of Greek words.Could anyone help me solve this?

Answer 1

The graphemes may render the same or similar but they are not represented the same way. For example:

• ά is represented here as Unicode Character 'GREEK SMALL LETTER ALPHA WITH TONOS' (U+03AC)
• ά is represented here as Unicode Character 'GREEK SMALL LETTER ALPHA' (U+03B1) followed by Unicode Character 'COMBINING ACUTE ACCENT' (U+0301)

These were copied directly from your comment.

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In order to compare them you should first use normalizer_normalize() on both strings to obtain them in their normalized forms. Which type of normalization form to use is ultimately up to you. There are four:

1. NFD (Canonical Decomposition)
2. NFC (Canonical Decomposition, followed by Canonical Composition)
3. NFKD (Compatibility Decomposition)
4. NFKC (Compatibility Decomposition, followed by Canonical Composition)

Because this normalization is being used completely internally just ignore NFC and NFKC, there's no need to recompose. This leaves you with the option of either NFD or NFKD - canonical or compatible. The names give you a bit of a clue on how strict they are regarding equivalence.

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1.1 Canonical and Compatibility Equivalence:

Canonical equivalence is a fundamental equivalency between characters or sequences of characters that represent the same abstract character, and when correctly displayed should always have the same visual appearance and behavior.

Compatibility equivalence is a weaker equivalence between characters or sequences of characters that represent the same abstract character, but may have a different visual appearance or behavior.

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For searching I would go with the latter.

Example:

$foo = "παράρτημα";
$bar = "παράρτημα";
var_dump($foo === $bar);
var_dump(
    normalizer_normalize($foo, Normalizer::FORM_KD) ===
    normalizer_normalize($bar, Normalizer::FORM_KD)
);

Output:

bool(false)
bool(true)