($): variable is a name for a container that holds one or more values; use $ to get the value.
(!): negation operator
(tr): transliterating one group of characters into another.
(s): substitution
(%): modulus operator:
\s : shortcut for whitespace
\s+ : match for one or more whitespace characters
/g : modifier-global
/d : modifier-delete found but unreplaced characters
== : equal
= : assignment operator
. : string values can be concatenated with the . operator
( * ): match the preceding item 0 or more times
( ^ ): the caret match the beginning of a line
$ : match the end of a line