正则表达式匹配以unicode字符开头的单词会返回意外结果

Question

I want to check existence of the word 'açilek' in the context. Running this:

$word = 'açilek';
$article='elma  and  açilek word';
$mat=preg_match('/\b'. $word .'\b/', $article);
var_dump($mat);

Succeeds. This is expected. However, to match the word 'çilek', the code returns False which is not expected:

$word = 'çilek';
$article='elma  and  çilek word';
$mat=preg_match('/\b'. $word .'\b/', $article);
var_dump($mat); //returns false !!!!

Additionally, it will match this word if it is a part of a word, also unexpected:

$word = 'çilek';
$article='elma  and  açilek word';
$mat=preg_match('/\b'. $word .'\b/', $article);
var_dump($mat); //returns true !!!!

Why am I seeing this behavior?

Answer 1

You need to use the /u modifier to make the regex (especially \b) Unicode-aware:

$mat=preg_match('/\b'. $word .'\b/u', $article);

Otherwise, \b only considers positions between ASCII alphanumerics and ASCII non-alnums as word boundaries, therefore matching between a and çilek but not between   and çilek.