capitalize string using regular expressions

[colin smith]

Hi Everyone,

    Does anyone know how I can replace all occurences of  any strings that
end with ".txt" (lets say "ghjkl.txt" and "xyz.txt") with their uppercase
equivalents(ie. "GHJKL.txt" and "XYZ.txt").  There must be some way of doing
this using regular expressions. I've looked extensively through the emacs
help, but havent been able to figure it out.

Thanks a lot,
Colin.