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 tabs. Let's say
that there are three tabs per line, and you'd like to replace the second
>. The following substitute command would do it:
[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:
the output produced by running the script on this file will be:
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 numeric value is from 1 to 512.
- from O'Reilly & Associates' sed & awk, Chapter 5