The metacharacter "[" signals that the characters following, up to the
next "]", form a character class, that is, a regular expression that
matches any single character from the bracketed list. The character class
"[aA]" matches "a" or "A". A dash "-" is
used to signify a range of characters in the ASCII collating sequence.
For example, "[a-zA-Z]" matches any alphabetic character, while
"[0-9]" matches any numeric character. If the first
character in a character class is an "^", then any character not in
the class constitutes a match; for example, [^a] matches any character
except an "a".
