hi
can anyone let me know of a voltage regulator which produces a constant 2.5V regulated output when handling input ranging 2.5V-3V. it should be able to deliver around 50mA output current

You want what is called a "LDO Voltage Regulator"

Digikey or similar for possibles.

However you will not get 2.5 volts out of a regualtor on a 2.5v supply.

Close, but there will be some "drop out" voltage loss.
Maybe maintain 2.5 with the power supply as low as 2.55 or similar.