Programming Perl, 4th Edition

POSIX-Style Character Classes

Unlike Perl’s other character class shortcuts, the legacy POSIX-style character-class syntax notation, [:CLASS:], is available for use only when constructing other character classes—that is, inside an additional pair of square brackets. For example, /[.,[:alpha:][:digit:]]/ will search for one character that is either a literal dot (because it’s in a bracketed character class), a comma, an alphabetic character, or a digit. All may be used as character properties of the same name; for example, [.,\p{alpha}\p{digit}].

Except for “punct”, explained immediately below, the POSIX character class names can be used as properties with \p{} or \P{} with the same meanings. This has two advantages: it is easier to type because you don’t need to surround them with extra brackets; and, perhaps more importantly, because as properties their definitions are no longer affected by charset modifiers—they always match as Unicode. In contrast, using the [[:...:]] notation, the POSIX classes are affected by modifier flags.

The \p{punct} property differs from the [[:punct:]] POSIX class in that \p{punct} never matches nonpunctuation, but [[:punct:]] (and \p{POSIX_Punct} and \p{X_POSIX_Punct}) will. This is because Unicode splits what POSIX considers punctuation into two categories: Punctuation and Symbols. Unlike \p{punct}, the others just mentioned also will match the characters shown in Table 5-14.

Table 5-14. ASCII symbols that count as punctuation

Glyph	Code	Category	Script