Function parse_regex 

pub fn parse_regex(input: impl AsRef<str>) -> Result<Regex, ParseError>

parse a string to a regular expression

the syntax of a regular expression is:

E -> 0
E -> e
E -> <ident>
E -> ~ E
E -> E *
E -> E +
E -> E ?
E -> E E
E -> E | E
E -> E & E

The <ident> needs to be a valid rust identifier