1. (x)
-> 'bar foo'.replace( /(..) (...)/, '$2 $1' )
-> "bfoo ar"
2. (x)
-> "foo bar foo bar".match(/(bar) (foo)/, '$2 $1');
-> ["bar foo", "bar", "foo"]
3. (x)
-> "foo bar foo bar".match(/(bar) (foo)/, '$&');
-> ["bar foo", "bar", "foo"]
4. x(?!y) Matches 'x' only if 'x' is not followed by 'y'
-> /d+(?!.)/.exec("3.141")
->["141"]
5.x(?=y) Matches 'x' only if 'x' is followed by 'y'.
-> /d+(?=.)/.exec("3.141")
-> ["3"]
6. {n}
-> /a{2}/.exec("cass")
-> null
-> /a{2}/.exec("caass")
-> ["aa"]
-> /a{2}/.exec("caaaaaass")
-> ["aa"]
6. {n,m}
-> /a{2,4}/.exec("caaaaaass")
-> ["aaaa"]
7. [xyz] Special characters like the dot(.) and asterisk (*) are not special inside a character set, so they don't need to be escaped
-> /[a-z.]+/.exec("test.i.ngaa11111")
-> ["test.i.ngaa"]
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions