One of the more unusual options of sed's substitution command is the numeric
flag that allows you to point to one particular match when there are many
possible matches on a particular line. It is used where a pattern repeats itself
on a line and the replacement must be made for only one of
those occurrences by position. For instance, a line, perhaps containing tbl input, might contain multiple tab characters.
Let's say that there are three tabs per line, and you'd like to replace the
second tab with >. The following
substitute command would do it:
s/TAB/>/2
TAB represents an actual tab character,
which is otherwise invisible on the screen. If the input is a one-line file such
as the following:
Column1TABColumn2TABColumn3TABColumn4
the output produced by running the script on this file will be:
Column1TABColumn2>Column3TABColumn4
Note that without the numeric flag, the substitute command would replace only
the first tab. (Therefore, 1 can be
considered the default numeric flag.) The range of the allowed numeric value is
from 1 to 512, though this may be implementation-dependent.
— DD