Awws no.Parakeets belong in a flock.Please don't keep a single bird. Reguritating (spelling) is usually a sign of affection yes.They "feed" their mates by spitting back up food. (It can sometimes be a sign of illness too,so if it don't seem "normal" definately check with a vet...an
AViAN vet.Being the key word

)
Would the shrill & sharp chirping be that he's trying to call out to another bird? I have both parakeets& a tiel.My budgie calmed down lots once I got another bird (the tiel I got for christmas,no they don't live together,but in the same room,my bedroom) & he'll also have a friend,another keet,after I introduce them in a month.
Quarantining & slow introduction is important to birds.
But please..PLEASE don't keep just one keet.They are flock birds.