sed: Replace part of a line

Question

How can one replace a part of a line with sed?

The line

DBSERVERNAME     xxx 

should be replaced to:

DBSERVERNAME     yyy 

The value xxx can vary and there are two tabs between dbservername and the value. This name-value pair is one of many from a configuration file.

I tried with the following backreference:

echo "DBSERVERNAME    xxx" | sed -rne 's/\(dbservername\)[[:blank:]]+\([[:alpha:]]+\)/\1 yyy/gip' 

and that resulted in an error: invalid reference \1 on `s' command's RHS.

Whats wrong with the expression? Using GNU sed.

Answer 1

This works:

sed -rne 's/(dbservername)\s+\w+/\1 yyy/gip' 

(When you use the -r option, you don't have to escape the parens.)

Bit of explanation:

• -r is extended regular expressions - makes a difference to how the regex is written.
• -n does not print unless specified - sed prints by default otherwise,
• -e means what follows it is an expression. Let's break the expression down:
  • s/// is the command for search-replace, and what's between the first pair is the regex to match, and the second pair the replacement,
  • gip, which follows the search replace command; g means global, i.e., every match instead of just the first will be replaced in a line; i is case-insensitivity; p means print when done (remember the -n flag from earlier!),
  • The brackets represent a match part, which will come up later. So dbservername is the first match part,
  • \s is whitespace, + means one or more (vs *, zero or more) occurrences,
  • \w is a word, that is any letter, digit or underscore,
  • \1 is a special expression for GNU sed that prints the first bracketed match in the accompanying search.

Answer 2

Others have already mentioned the escaping of parentheses, but why do you need a back reference at all, if the first part of the line is constant?

You could simply do

sed -e 's/dbservername.*$/dbservername yyy/g'