replace be safely used on a UTF

 

PHP's str_replace() was intended only for ANSI strings and as such can mangle UTF-8 strings. However, given that it's binary-safe would it work properly if it was only given valid UTF-8 strings as arguments?

Edit: I'm not looking for a replacement function, I would just like to know if this hypothesis is correct.

Yes. UTF-8 is deliberately designed to allow this and other similar non-Unicode-aware processing.

In UTF-8, any non-ASCII byte sequence representing a valid character always begins with a byte in the range xC0-xFF . This byte may not appear anywhere else in the sequence, so you can't make a valid UTF-8 sequence that matches part of a character.

This is not the case for older multibyte encodings, where different parts of a byte sequence are indistinguishable. This caused a lot of problems, for example trying to replace an ASCII backslash in a Shift-JIS string (where byte x5C might be the second byte of a character sequence representing something else).

It's correct because UTF-8 multibyte characters are exclusively non-ASCII (128+ byte value) characters beginning with a byte that defines how many bytes follow, so you can't accidentally end up matching a part of one UTF-8 multibyte character with another.

To visualise (abstractly):

  • a for an ASCII character
  • 2x for a 2-byte character
  • 3xx for a 3-byte character
  • 4xxx for a 4-byte character

    • If you're matching, say, a2x3xx ( a bytes in ASCII range), since a < x , and 2x cannot be a subset of 3xx or 4xxx , et cetera, you can be safe that your UTF-8 will match correctly, given the prerequisite that all strings are definitely valid UTF-8.

    Edit: See bobince's answer for a less abstract explanation.

    Well, I do have a counter example: I have a UTF8 encoded settings ".ini' file specifying appliation settings like email sender name. it says something like: 

    email_from = Märta

    and I read it from there to variable $sender . Now that I replace the message body (UTF8 again)

    regards {sender} 

    $message = str_replace("{sender}",$sender_name,$message);

    The email is absolutely correct in every respect but the sender is totally broken. There are other cases (like explode() ) when something goes wrong with a UTF string. It is healthy before the conversion but not after it. Sorry to say there seems to be no way of correcting this behaviour.

    Edit : Actually, explode() is involved in parsing the .ini file so the problem may well lie in that very function so the str_replace() may well be innocent.

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