Given a string, determine if it's a valid identifier.
## Here is the syntax for valid identifiers:
* Each identifier must have at least one character.
* The first character must be picked from: alpha, underscore, or dollar sign. The first character cannot be a digit.
* The rest of the characters (besides the first) can be from: alpha, digit, underscore, or dollar sign. In other words, it can be any valid identifier character.
### Examples of valid identifiers:
* i
* wo_rd
* b2h
### Examples of invalid identifiers:
* 1i
* wo rd 
* !b2h
is_valid=lambda idn: bool(__import__("re").match("^[a-zA-Z_$][\w$]*$",idn))

is_valid=lambda idn: bool(__import__("re").match("^[a-zA-Z_$][\w$]*$",idn))
