Programming Perl, 4th Edition

Building Character

To define your own property, you need to write a subroutine with the name of the property you want (see Chapter 7). For security reasons, this subroutine’s (unqualified) name must begin with either Is or In. The subroutine should be defined in the package that needs the property (see Chapter 10), which means that if you want to use it in multiple packages, you’ll either have to import it from a module (see Chapter 11), or inherit it as a class method from the package in which it is defined (see Chapter 12).

Once you’ve got that all settled, the subroutine should return data in the same format as the files in PATH_TO_PERLLIB/unicode/Is directory. That is, just return a list of characters or character ranges in hexadecimal, one per line. If there is a range, the two numbers are separated by a tab. Suppose you wanted a property that would be true if your character is in the range of either of the Japanese syllabaries, known as hiragana and katakana (together they’re known as kana). You can just put in the two ranges like this:

sub InKana {
    return <<'END';
3040    309F
30A0    30FF
END
}

Alternatively, you could define it in terms of existing property names:

sub InKana {
    return <<'END';
+utf8::InHiragana
+utf8::InKatakana
END
}

You can also do set subtraction using a “–” prefix. Suppose you only wanted the actual characters, not just the block ranges of characters. You could weed out all the undefined ones like this:

sub IsKana { return <<'END'; +utf8::InHiragana +utf8::InKatakana ...