word boundary on non latin characters in php

This example works fine:

echo preg_replace("/\bI\b/u", 'we', "I can"); // we can

This one were russian letters are used does not work even though I use "u" modifier:

echo preg_rep开发者_如何学Pythonlace("/\bЯ\b/u", 'мы', 'Я могу'); // still "Я могу"

So the question is what should I do to fix this?

Thanks.

In PCRE (the library used by preg_replace), \b refers only to word boundaries in an ASCII sense, i.e., only [a-zA-Z0-9_] are word characters.

If you want to match a Я character that has no letters, digits or _ immediately before or after, you can use:

(?<![\p{L}0-9_])Я(?![\p{L}0-9_])

You still have to use the u modifier.

Word boundaries are often counter-intuitive.