Well, regular languages are a specific, well defined subset of all formal languages.
If you want arbitrary parsing, use a parsing library?
If you want arbitrary parsing, use a parsing library?
However, that doesn't mean we shouldn't strive for richer feature (within the confine of regularity), or that a parsing library automatically does the job better.
For instance: intersection and complement are regular operators that works very well in regular expressions. lookahead is a regular operator (lookbehind and backreferences are not), etc. Those operators do not necessarily work at all in richer parsing libraries. When added to regular expressions, they should be well handled and optimized by regex engines (which is is currently not the case). There's still work to do, features to add, which require appropriate testing techniques.
Regular languages are closed under a host of more interesting operations than those that have 'grown with the capabilities of our pattern matching engines'.
As the author of Perl's regex engine Larry Wall puts it[0]:
> “Regular expressions” […] are only marginally related to real regular expressions. Nevertheless, the term has grown with the capabilities of our pattern matching engines, so I’m not going to try to fight linguistic necessity here. I will, however, generally call them “regexes” (or “regexen”, when I’m in an Anglo-Saxon mood).