Table 1 Regular Expession operator precedence: (based on the table in Dr. Aho's book)

 c = char     x = string    r,s = regular expression
-------------------------------------------------------
c          any non-operator character     Felis
\c         c literally and C escapes      catus\.
\:c        predefined macro:              \:c*(ie|ei)
              \:a - alphanumeric
              \:b - white space
              \:c - alphabetic
              \:d - decimal
              \:h - hex
              \:l - lower alpha
              \:u - upper alpha
.          any character but newline      c.t
<          beginning of word              <cat
>          end of word                    <cat>
^          beginning of line              ^cat>
$          end of line                    cat$
[x]        any character in x             [a-z0-9]
[^x]       any character not in x         [^AEIOU]
r*         zero or more r's               ca*t
r+         one or more r's                ca[b-t]+
r?         zero or one r                  c.?t
rs         r followed by s                ^$
r|s        either r or s                  kittycat
(r)        r                              (c)?(a+)t