Names starting  with an  underscore and  followed by  a single upper or
    lower case letter are reserved as predefined.  The following predefined
    names are currently available:

    {_a} == [A-Za-z]                Alphabetic
    {_b} == [{}()[]<>]              Brackets
    {_c} == [\x001-\x01f\x-7f]      Control character
    {_d} == [0-9]                   Digit
    {_e} == [DdEe][-+]?{_d}{1,3}    Exponent
    {_f} == [-+]?({_d}+\.{_d}*|{_d}*\.{_d}+)     Floating point number
    {_g} == {_f}({_e})?             float, optional exponent
    {_h} == [0-9A-Fa-f]             Hex-digit
    {_i} == [-+]?{_d}+              Integer
    {_n} == [A-Za-z0-9]             alpha-Numeric
    {_o} == [0-7]                   Octal digit
    {_p} == [\!-/:-@[-`{-\x07f]     Punctuation
    {_q} == {_s}[\"']               double or single Quote
    {_r} == {_f}{_e}                Real number
    {_s} == (^|[!\\](\\\\)*)        zero or even number of Slashes
    {_t} == [\s-~]                  printable character
    {_u} == [\!-~]                  graphical character
    {_w} == [\s\t]                  White space
    {_z} == [\t-\r\s]               space, \t, \n, \v, \f, \r, \s