Regular expression for keeping

I have registers defined in vr_ad (with reg_def and reg_fld) and I have a macro that makes me possible to make writes to it, even to fields, e.g.:

wr_reg REG1 keeping {.fld1== val1}

I would like to have an expression that I can use inside the {} with which I could write the same value (e.g. 0) to all the fields of the register, something similar as the "for each" in case of a list. The problem is that I cannot use "for each" in this case as it is not a list but a register with several values. Could you please help me what the regular expression that I could use here is, to have all fields the same value?