at least in the awks built into osx and debian have sub and gsub (and gawk adds gensub).

       sub(r, t, s)
              substitutes  t  for the first occurrence of the regular expression r in the string s.  If s is
              not given, $0 is used.

       gsub   same as sub except that all occurrences of the regular expression are replaced; sub  and  gsub
              return the number of replacements.