Add negative look behind to exclude object methods

[roborourke]

This should prevent false negatives for object methods that match the name of any translation functions.

Related to #1535

[jrfnl]

@roborourke Thank you for your willingness to contribute,

However, the regexes you adjusted are for sprintf() placeholders, i.e. %2$s within strings, and would be broken by this "fix".
This is not the part of the code used to determine whether something is a function call or not. See #1267 for how it was actually fixed.

Closing as wrong solution for a problem which is already solved.