[...]                        Character Class

    [abc0-9] Character Class - match any character in brackets at  that
    position  in  regular  expression.     Ranges  of  characters   can  be
    abbreviated by using a hyphen.  The character immediately to the left of
    the  hyphen  defines  the   beginning  of  the  range;   the  character
    immediately to the right of the hyphen defines the end of the range.  A
    hyphen represents  itself when  "escaped", '\-',  or when  the first or
    last character in the class.  For example,

    To match A, E, I, O or U: [AEIOU]

    To match an uppercase letter:  [A-Z]

    To match a letter (lower or upper case):  [A-Za-z]

    To match a '+' or '-':  [+-] or [-+]