Character Class:
A character class is used to represent a set of characters. The following combinations are allowed in describing a character class:
- x: (where x is not one of the magic characters
^$()%.[]*+-?) represents the character x itself. .: (a dot) represents all characters.%a: represents all letters.%c: represents all control characters.%d: represents all digits.%l: represents all lowercase letters.%p: represents all punctuation characters.%s: represents all space characters.%u: represents all uppercase letters.%w: represents all alphanumeric characters.%x: represents all hexadecimal digits.%z: represents the character with representation 0.%_x_: (where x is any non-alphanumeric character) represents the character x. This is the standard way to escape the magic characters. Any punctuation character (even the non magic) can be preceded by a '=%=' when used to represent itself in a pattern.[set]: represents the class which is the union of all characters in set. A range of characters may be specified by separating the end characters of the range with a '=-='. All classes =%=_x_ described above may also be used as components in set. All other characters in set represent themselves. For example,[%w_](or[_%w]) represents all alphanumeric characters plus the underscore,[0-7]represents the octal digits, and[0-7%l%-]represents the octal digits plus the lowercase letters plus the '=-=' character.
The interaction between ranges and classes is not defined. Therefore, patterns like [%a-z] or [a-%%] have no meaning.
[^_set_]: represents the complement of set, where set is interpreted as above.
For all classes represented by single letters (%a, %c, etc.), the corresponding uppercase letter represents the complement of the class. For instance, %S represents all non-space characters.
The definitions of letter, space, and other character groups depend on the current locale. In particular, the class [a-z] may not be equivalent to %l.