Character Class Operators (`[:' ... `:]')
-----------------------------------------

  If the syntax bit `RE_CHARACTER_CLASSES' is set, then Regex
recognizes character class expressions inside lists.  A "character
class expression" matches one character from a given class.  You form a
character class expression by putting a character class name between an
"open-character-class operator" (represented by `[:') and a
"close-character-class operator" (represented by `:]').  The character
class names and their meanings are:

`alnum'
     letters and digits

`alpha'
     letters

`blank'
     system-dependent; for GNU, a space or tab

`cntrl'
     control characters (in the ASCII encoding, code 0177 and codes
     less than 040)

`digit'
     digits

`graph'
     same as `print' except omits space

`lower'
     lowercase letters

`print'
     printable characters (in the ASCII encoding, space tilde--codes
     040 through 0176)

`punct'
     neither control nor alphanumeric characters

`space'
     space, carriage return, newline, vertical tab, and form feed

`upper'
     uppercase letters

`xdigit'
     hexadecimal digits: `0'-`9', `a'-`f', `A'-`F'

These correspond to the definitions in the C library's `<ctype.h>'
facility.  For example, `[:alpha:]' corresponds to the standard
facility `isalpha'.  Regex recognizes character class expressions only
inside of lists; so `[[:alpha:]]' matches any letter, but `[:alpha:]'
outside of a bracket expression and not followed by a repetition
operator matches just itself.