I understand your concern, but I'm still trying understand why the OP question comes up in the context of your app.

Are you getting actual utf8 data from a file handle that does not use the ":utf8" PerlIO layer (so that perl begins by assuming it's just a raw byte stream)? And if that's the case, are you trying to work out a way to use "byte-semantics" regexen where possible, and "character-semantics" only when necessary?

If that's your situation, here's an easy, low-cpu-load method to check whether a raw byte string needs be tagged as utf8: 

   if ( length($string) > $string =~ tr/\x00-\x7f// ) {
      $string = decode( 'utf8', $string );
   }

[download]
(updated as per fenLisesi's reply -- thanks!)

Or, given that the original string is not tagged as a perl-internal utf8 scalar value (utf8 flag is off), this might be just as good or better: 

   if ( $string =~ /[\x80-\xff]/ ) {
      $string = decode( 'utf8', $string );
   }

[download]
I'm not actually sure whether one way is faster than the other, or whether the relative speed would depend on your data; "length()" and "tr///" are both pretty fast whereas a regex match is slower, but tr always processes the whole string, whereas that regex match will stop at the first non-ascii byte.

In reply to Re^3: Behaviour of Encode::decode_utf8 on ASCII by graff
in thread Behaviour of Encode::decode_utf8 on ASCII by jbert

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