#!/bin/bash
s="05-620 Karpacz"
sed 's/^[0-9]\{2\}[ -][0-9]\{3\}[[:space:]][0-9A-Za-z]\{1,\}/50-370 Wroclaw/' <<< "$s"
sed 's/^[0-9]\{2\}[ -][0-9]\{3\}[[:space:]][[:alnum:]]\+/50-370 Wroclaw/' <<< "$s"
sed -E 's/^[0-9]{2}[ -][0-9]{3}[[:space:]]+[[:alnum:]]+/50-370 Wroclaw/' <<< "$s"