Matching/generating incorrect strings

[railreaper]

The regex ^(?=ca)([a-z]{4})$ should allow

cars
cast
cabs

..but not

car
cargo
castrate

...as they are not 4 letters long. exrex is generating strings starting with ca and followed by any 4 lowercase alphanumeric characters e.g. cabhzp. The example here shows how it should work.

(?=ca)(^[a-z]{4}$) is another example.