BRE's need to escape () {} and can use \n as the nth backreference

 ^     first character on line
 $     last character on line
 .     matches any single character

 *     matches preceding element 0 or more times
 {m/n} matches preceding element at least m and no more than n times
 ?     matches preceding element 0 or 1 times (ERE only)
 +     matches preceding element 1 or more times (ERE only)
 |     or operator (ERE only)

 [ ]   matches any single character in brackets (a-z, A-Z, 0-9... allowed for character ranges)
 [^ ]  matches any single character not in brackets
 ( )   sub-expression

Character classes:
   [:alnum:] alphanumeric [A-Za-z0-9]
   [:alpha:] alphabetic [A-Za-z]
   [:blank:] space and tab
   [:cntrl:] control characters
   [:digit:] numbers [0-9]
   [:graph:] visible characters
   [:lower:] lowercase letters
   [:upper:] uppercase letters
   [:print:] visible and space characters
   [:punct:] punctuation characters []!"#$%^&'()*+,./:;=<>?\@`{}|~-_
   [:space:] whitespace characters
   [:xdigit:] hexadecimal digits [A-Fa-f0-9]