Modify cheatsheet Regular_Expressions.rb - Add information about escape ...

Hi,

i extended the Dash Regex Cheatsheet, adding information about Escape-worthy Characters in PERL and POSIX implementations and added an entry about the standard escape character.

This is my first pull request, i hope i did everything in the right way - my first day on github as an active user too, so please tell me if i did anything wrong.

Best Regards,
Stephan