Programming Perl, 4th Edition

Bracketed Character Classes

An enumerated list of characters in square brackets is called a bracketed character class and matches any one of the characters in the list. For example, [aeiouy] matches a letter that can be a vowel in English. To match a right square bracket, either backslash it or place it first in the list.

Character ranges may be indicated using a hyphen^[95] and the a–z notation. Multiple ranges may be combined; for example, [0–9a–fA–F] matches one hex “digit”. You may use a backslash to protect a hyphen that would otherwise be interpreted as a range separator, or just put it at the beginning or end of the class (a practice which is arguably less readable but more traditional).

A caret (or circumflex, or hat, or up arrow) at the front of the bracketed character class inverts the class, causing it to match any single character not in the list. (To match a caret, either don’t put it first or, better, escape it with a backslash.) For example, [^aeiouy] matches any character that isn’t a vowel. Be careful with character class negation, though, because the universe of characters is expanding. For example, that bracketed character class matches consonants—and also matches spaces, newlines, and anything (including vowels) in Cyrillic, Greek, or nearly any other script, not to mention every ideograph in Chinese, Japanese, and Korean. And someday maybe even Cirth and Tengwar. (Linear B and Etruscan, for sure.) So it might be better to specify your consonants explicitly, such ...