I have this expression in the field calculator:

regexp_replace('2', '(\\d+)', rpad('\\1',3,'0') )

which yields '02' rather than '002' as expected

OTOH rpad('1', 3, '0') yields '001' as expected.

Weird! Is this a bug? or am I missing something? If so I'll log something in the issue tracker.

BTW I was delighted when I found I could use complex expressions including functions in the third parameter of regexp_replace! kudos to the QGIS developers!`