If the syntax bit
RE_CHAR_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:
These correspond to the definitions in the C library's <ctype.h>
facility. For example, ‘[:alpha:]’ corresponds to the standard
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.