${parameter/pattern/string}
       ${parameter//pattern/string}
              The pattern is expanded to produce a pattern just as in pathname
              expansion.  Parameter is expanded and the longest match of  pat-
              tern  against  its  value is replaced with string.  In the first
              form, only the first match is replaced.  The second form  causes
              all  matches  of pattern to be replaced with string.  If pattern
              begins with #, it must match at the beginning  of  the  expanded
              value  of parameter.  If pattern begins with %, it must match at
              the end of the expanded value of parameter.  If string is  null,
              matches  of  pattern are deleted and the / following pattern may
              be omitted.  If parameter is @ or *, the substitution  operation
              is  applied to each positional parameter in turn, and the expan-
              sion is the resultant list.  If parameter is an  array  variable
              subscripted  with  @ or *, the substitution operation is applied
              to each member of the array in turn, and the  expansion  is  the
              resultant list.

${var1//a/x}