sed: The "s" Command
3.3 The 's' Command
===================
The 's' command (as in substitute) is probably the most important in
'sed' and has a lot of different options. The syntax of the 's' command
is 's/REGEXP/REPLACEMENT/FLAGS'.
Its basic concept is simple: the 's' command attempts to match the
pattern space against the supplied regular expression REGEXP; if the
match is successful, then that portion of the pattern space which was
matched is replaced with REPLACEMENT.
For details about REGEXP syntax ⇒Regular Expression Addresses
Regexp Addresses.
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.
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.
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:
'\L'
Turn the replacement to lowercase until a '\U' or '\E' is found,
'\l'
Turn the next character to lowercase,
'\U'
Turn the replacement to uppercase until a '\L' or '\E' is found,
'\u'
Turn the next character to uppercase,
'\E'
Stop case conversion started by '\L' or '\U'.
When the 'g' flag is being used, case conversion does not propagate
from one occurrence of the regular expression to another. For example,
when the following command is executed with 'a-b-' in pattern space:
s/\(b\?\)-/x\u\1/g
the output is 'axxB'. When replacing the first '-', the '\u' sequence
only affects the empty replacement of '\1'. It does not affect the 'x'
character that is added to pattern space when replacing 'b-' with 'xB'.
On the other hand, '\l' and '\u' do affect the remainder of the
replacement text if they are followed by an empty substitution. With
'a-b-' in pattern space, the following command:
s/\(b\?\)-/\u\1x/g
will replace '-' with 'X' (uppercase) and 'b-' with 'Bx'. If this
behavior is undesirable, you can prevent it by adding a '\E'
sequence--after '\1' in this case.
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:
'g'
Apply the replacement to _all_ matches to the REGEXP, not just the
first.
'NUMBER'
Only replace the NUMBERth match of the REGEXP.
interaction in 's' command 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.
'p'
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 FILENAME'
If the substitution was made, then write out the result to the
named file. As a GNU 'sed' extension, two special values of
FILENAME are supported: '/dev/stderr', which writes the result to
the standard error, and '/dev/stdout', which writes to the standard
output.(1)
'e'
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.
'I'
'i'
The 'I' modifier to regular-expression matching is a GNU extension
which makes 'sed' match REGEXP in a case-insensitive manner.
'M'
'm'
The 'M' modifier to regular-expression matching is a GNU 'sed'
extension which directs GNU 'sed' to match the regular expression
in 'multi-line' mode. The modifier 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. In addition, the period
character does not match a new-line character in multi-line mode.
---------- Footnotes ----------
(1) This is equivalent to 'p' unless the '-i' option is being used.
Info Catalog
sed: sed commands list
sed: sed scripts
sed: Common Commands