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Single field substitutions extract a single subdomain part from the host/domain specification being rewritten. The available single field substitutions are shown in Table 2-4.
| Control Sequence | Usage | 
|---|---|
| $&n | Substitute the nth element, n=0,1,2,..,9, in the host specification (the part that did not match or matched a wildcard of some kind). Elements are separated by dots; the first element on the left is element zero. The rewrite fails if the requested element does not exist. | 
| $!n | Substitute the nth element, n=0,1,2,..,9, in the host specification (the part that did not match or matched a wildcard of some kind). Elements are separated by dots; the first element on the right is element zero. The rewrite fails if the requested element does not exist. | 
| $*n | Substitute the nth element, n=0,1,2,...,9, in the domain specification (the part that did match explicit text in the pattern). Elements are separated by dots; the first element on the left is element zero. The rewrite fails if the requested element does not exist. | 
| $#n | Substitute the nth element, n=0,1,2,...,9, in the domain specification (the part that did match explicit text in the pattern). Elements are separated by dots; the first element on the right is element zero. The rewrite fails if the requested element does not exist. | 
Suppose the address jdoe@vaxa.example.com matches the rewrite rule
      *.EXAMPLE.COM $U%$&0.example.com@mailhub.example.com  | 
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