Multibyte trim in PHP?

Asked 2012-04-08T21:23:38.033

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?

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3 Answers

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.)

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.

Take care:

  $s1 = html_entity_decode(" Hello &#160; "); // the NBSP
  $s2 = "  exotic test ホ  ";

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


                        
                        
                            answered 2012-04-09T00:23:09.207


            
            
                
                    
                        
                            
                            
                        
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                        I don't know what you're trying to do with that endless recursive function you're defining, but if you just want a multibyte-safe trim, this will work.
function mb_trim($str) {
  return preg_replace("/^\s+|\s+$/u", "", $str); 
}


                        
                        
                            answered 2012-04-08T22:58:57.590
                            
                        
                        
                            
                                
                                
                            
                        
                        
                            
                            
                            
                                
                                
                            
                            
                        
                    
                
            
            
            
                
                    
                        
                            
                            
                        
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                        mb_ereg_replace seems to get around that:

function mb_trim($str,$regex = "(^\s+)|(\s+$)/us") {
    return mb_ereg_replace($regex, "", $str);
}


..but I don't know enough about regular expressions to know how you'd then add on the "charlist" parameter people would expect to be able to feed to trim() - i.e. a list of characters to trim - so have just made the regex a parameter.

It might be that you could have an array of special characters, then step through it for each character in the charlist and escape them accordingly when building the regex string.

                        
                        
                            answered 2012-05-24T14:11:49.507



        
        
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