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The sed FAQ
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3.1.3. Substitution switches

Standard versions of sed support 4 main flags or switches which may be added to the end of an "s///" command. They are:

       N      - Replace the Nth match of the pattern on the LHS, where
                N is an integer between 1 and 512. If N is omitted,
                the default is to replace the first match only.
       g      - Global replace of all matches to the pattern.
       p      - Print the results to stdout, even if -n switch is used.
       w file - Write the pattern space to 'file' if a replacement was
                done. If the file already exists when the script is
                executed, it is overwritten. During script execution,
                w appends to the file for each match.

GNU sed 3.02 and ssed also offer the /I switch for doing a case-insensitive match. For example,

     echo ONE TWO | gsed "s/one/unos/I"      # prints "unos TWO"

GNU sed 4.x and ssed add the /M switch, to simplify working with multi-line patterns: when it is used, ^ or $ will match BOL or EOL. \` and \' remain available to match the start and end of pattern space, respectively.

ssed supports two more switches, /S and /X, when its Perl mode is used. They are described in detail in section 6.7.3.H, below.

The sed FAQ
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   Reprinted courtesy of Eric Pement. Also available at Design by Interspire