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3.5 The s Command

The syntax of the s (as in substitute) command is ‘s/regexp/replacement/flags’. The / characters may be uniformly replaced by any other single character within any given s command. The / character (or whatever other character is used in its stead) can appear in the regexp or replacement only if it is preceded by a \ character.

The s command is probably the most important in sed and has a lot of different options. Its basic concept is simple: the s command attempts to match the pattern space against the supplied regexp; if the match is successful, then that portion of the pattern space which was matched is replaced with replacement.

The replacement can contain \n (n being a number from 1 to 9, inclusive) references, which refer to the portion of the match which is contained between the nth \( and its matching \). Also, the replacement can contain unescaped & characters which reference the whole matched portion of the pattern space. Finally, as a GNU sed extension, you can include a special sequence made of a backslash and one of the letters L, l, U, u, or E. The meaning is as follows:

Turn the replacement to lowercase until a \U or \E is found,
Turn the next character to lowercase,
Turn the replacement to uppercase until a \L or \E is found,
Turn the next character to uppercase,
Stop case conversion started by \L or \U.

To include a literal \, &, or newline in the final replacement, be sure to precede the desired \, &, or newline in the replacement with a \.

The s command can be followed by zero or more of the following flags:

Apply the replacement to all matches to the regexp, not just the first.
Only replace the numberth match of the regexp.

Note: the posix standard does not specify what should happen when you mix the g and number modifiers, and currently there is no widely agreed upon meaning across sed implementations. For GNU sed, the interaction is defined to be: ignore matches before the numberth, and then match and replace all matches from the numberth on.

If the substitution was made, then print the new pattern space.

Note: when both the p and e options are specified, the relative ordering of the two produces very different results. In general, ep (evaluate then print) is what you want, but operating the other way round can be useful for debugging. For this reason, the current version of GNU sed interprets specially the presence of p options both before and after e, printing the pattern space before and after evaluation, while in general flags for the s command show their effect just once. This behavior, although documented, might change in future versions.

w file-name
If the substitution was made, then write out the result to the named file. As a GNU sed extension, two special values of file-name are supported: /dev/stderr, which writes the result to the standard error, and /dev/stdout, which writes to the standard output.1
This command allows one to pipe input from a shell command into pattern space. If a substitution was made, the command that is found in pattern space is executed and pattern space is replaced with its output. A trailing newline is suppressed; results are undefined if the command to be executed contains a nul character. This is a GNU sed extension.
The I modifier to regular-expression matching is a GNU extension which makes sed match regexp in a case-insensitive manner.
The M modifier to regular-expression matching is a GNU sed extension which causes ^ and $ to match respectively (in addition to the normal behavior) the empty string after a newline, and the empty string before a newline. There are special character sequences (\` and \') which always match the beginning or the end of the buffer. M stands for multi-line.


[1] This is equivalent to p unless the -i option is being used.