METACHARACTER PURPOSE
\ accept as a regular character;
~ Matches the beginning of the string, unless /m is used to modify the expression.
. Matches any character except a new-line character, unless /s is used to modify the expression.
$ Matches the end of the string, unless /m is used to modify the expression.
| Expresses alternation.
( ) Groups expressions to assist in alternation and back referencing.
[ ] Looks for a set of characters.