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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
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| 34.11. Referencing Portions of a Search String |  | 34.13. Transformations on Text |
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