Learning Perl, 5th Edition

Character Classes

A character class, a list of possible characters inside square brackets ([]), matches any single character from within the class. It matches just one single character, but that one 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 usual to make a character class like [a-zA-Z] to match any one letter out of that set of 52.^[‡] 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.^[‖] Of course, a character class will be just part of a full pattern; it 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 characters left out, 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. (Note that 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 ...