It doesn't explain why you are getting what you get, but the use of \w, \W, [:alpha:], [:word:], etc is problematic in Perl, if the string you're matching against contains characters in the range 128-255, and no characters above. Then \w, [:alpha:] and such may match the accented characters, depending whether the string matched against has the UTF-8 flag (for that string) on or not. That is, it will match if the UTF-8 flag is on. If it's off, than in some cases if the pattern matched with has the UTF-8 flag on \w will match against accented characters - but not always. And, finally, if there's no UTF-8 it will be your locale that decides it - if you have an active locale.

Much safer is to use the appropriate Unicode attributes. \p{IsAlpha} will match all characters marked 'alpha' in the Unicode database, regardless of UTF-8 flags or locales. 

my $_ = "See on üks täppidega lause!";
for (split //) {print if /\p{IsAlpha}/}
say "";
$_ =~ s/\P{IsAlpha}//g;
say;
__END__
Seeonükstäppidegalause
Seeonükstäppidegalause

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In reply to Re: Why [[:alpha:]] doesn't involve diacritic characters in replace expression? by JavaFan
in thread Why [[:alpha:]] doesn't involve diacritic characters in replace expression? by wk

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