preg_match() returns 0 although regex testers work
<p>I'm trying to validate a string using <code>/[\p{L}\s]{6,}/</code> and trying to match characters only (Unicode ones as well). I used <a href="https://regex101.com" rel="nofollow noreferrer">regex101</a> to test my regex and it works for the string <code>Владимир Алексић</code>. 
However, when I use that regex in <code>preg_match()</code> with the same string, it always returns <code>0</code>. Yet, it returns 1 if I avoid all characters except <code>A-Za-z</code>.</p>

<p>Why is that so?</p>

<p><a href="https://i.stack.imgur.com/hqfGj.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/hqfGj.png" alt="Proof for regex101"></a></p>