Learning Perl, Fourth Edition

Character Classes

A character class, a list of possible characters inside square brackets ([ ]), matches any single character from within the class. It matches one character, but that character may be any of the ones listed.

For example, the character class [abcwxyz] may match any one of those seven characters. For convenience, you may specify a range of characters with a hyphen (-), so that class may also be written as [a-cw-z]. That didn’t save much typing, but it’s more common to make a character class like [a-zA-Z] to match any one letter out of that set of 52.^[183] You may use the same character shortcuts as in any double-quoted string to define a character, so the class [\000-\177] matches any seven-bit ASCII character.^[184] Of course, a character class will be just part of a full pattern and will never stand on its own in Perl. For example, you might see code that says something like this:

    $_ = "The HAL-9000 requires authorization to continue.";
    if (/HAL-[0-9]+/) {
      print "The string mentions some model of HAL computer.\n";
    }

Sometimes, it’s easier to specify the omitted characters rather than the ones within the character class. A caret (”^“) at the start of the character class negates it. That is, [^def] will match any single character except one of those three. And [^n\-z] matches any character except for n, hyphen, or z. (The hyphen is backslashed because it’s special inside a character class. But the first hyphen in /HAL-[0-9]+/ doesn’t need a backslash because hyphens ...