Multibyte trim in PHP?

Apparently there’s no mb_trim in the mb_* family, so I’m trying to implement one for my own.

I recently found this regex in a comment in php.net:

/(^s+)|(s+$)/u

So, I’d implement it in the following way:

function multibyte_trim($str)
{
    if (!function_exists("mb_trim") || !extension_loaded("mbstring")) {
        return preg_replace("/(^s+)|(s+$)/u", "", $str);
    } else {
        return mb_trim($str);
    }
}

The regex seems correct to me, but I’m extremely noob with regular expressions. Will this effectively remove any Unicode space in the beginning/end of a string?

Answer

The standard trim function trims a handful of space and space-like characters. These are defined as ASCII characters, which means certain specific bytes from 0 to 0100 0000.

Proper UTF-8 input will never contain multi-byte characters that is made up of bytes 0xxx xxxx. All the bytes in proper UTF-8 multibyte characters start with 1xxx xxxx.

This means that in a proper UTF-8 sequence, the bytes 0xxx xxxx can only refer to single-byte characters. PHP’s trim function will therefore never trim away “half a character” assuming you have a proper UTF-8 sequence. (Be very very careful about improper UTF-8 sequences.)

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The s on ASCII regular expressions will mostly match the same characters as trim.

The preg functions with the /u modifier only works on UTF-8 encoded regular expressions, and /s/u match also the UTF8’s nbsp. This behaviour with non-breaking spaces is the only advantage to using it.

If you want to replace space characters in other, non ASCII-compatible encodings, neither method will work.

In other words, if you’re trying to trim usual spaces an ASCII-compatible string, just use trim. When using /s/u be careful with the meaning of nbsp for your text.

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Take care:

  $s1 = html_entity_decode(" Hello   "); // the NBSP
  $s2 = " ???? exotic test ホ ???? ";

  echo "nCORRECT trim: [". trim($s1) ."], [".  trim($s2) ."]";
  echo "nSAME: [". trim($s1) ."] == [". preg_replace('/^s+|s+$/','',$s1) ."]";
  echo "nBUT: [". trim($s1) ."] != [". preg_replace('/^s+|s+$/u','',$s1) ."]";

  echo "n!INCORRECT trim: [". trim($s2,'???? ') ."]"; // DANGER! not UTF8 safe!
  echo "nSAFE ONLY WITH preg: [". 
       preg_replace('/^[????s]+|[????s]+$/u', '', $s2) ."]";