sed pattern &

UNIX for Beginners Q & A
1

Hi,

I can't seem to understand what the sed and & do.. Why is the set of numbers appear twice ? how the command really work ?

echo "123 abc" | sed 's/[0-9]*/& www &/'


Output: 
123 www 123 abc
2

man sed yields:

       s/regexp/replacement/
              Attempt to match regexp against the pattern space.  If  success
              ful,   replace  that  portion  matched  with  replacement.   The
              replacement may contain the special character & to refer to that
              portion  of  the  pattern  space  which matched, and the special
              escapes \1 through \9 to refer  to  the  corresponding  matching
              sub-expressions in the regexp.
3

I understood the supposed function of sed with &, but this explanation (I read something similar before posting) doesn't explain to me why the set of numbers appears twice, and how the output came to be this way..

4

the numbers appear twice because there're 2 &-s separated by www.

echo "123 abc" | sed 's/[0-9]*/& www &/'
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