RegExp matching string not starting with my

/^(?!my).*/

(?!expression) is a negative lookahead; it matches a position where expression doesn't match starting at that position.

^(?!my)\w+$

should work.

It first ensures that it's not possible to match my at the start of the string, and then matches alphanumeric characters until the end of the string. Whitespace anywhere in the string will cause the regex to fail. Depending on your input you might want to either strip whitespace in the front and back of the string before passing it to the regex, or use add optional whitespace matchers to the regex like ^\s*(?!my)(\w+)\s*$. In this case, backreference 1 will contain the name of the variable.

And if you need to ensure that your variable name starts with a certain group of characters, say [A-Za-z_], use

^(?!my)[A-Za-z_]\w*$

Note the change from + to *.