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The sed FAQ
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3.1.2. Escape characters on the right side of "s///"

The right-hand side (the replacement part) in "s/find/replace/" is almost always a string literal, with no interpolation of these metacharacters:

       .   ^   $   [   ]   {   }   (   )  ?   +   *

Three things are interpolated: ampersand (&), backreferences, and options for special seds. An ampersand on the RHS is replaced by the entire expression matched on the LHS. There is never any reason to use grouping like this:

       s/\(some-complex-regex\)/one two \1 three/

since you can do this instead:

       s/some-complex-regex/one two & three/

To enter a literal ampersand on the RHS, type '\&'.

Grouping and backreferences: All versions of sed support grouping and backreferences on the LHS and backreferences only on the RHS. Grouping allows a series of characters to be collected in a set, indicating the boundaries of the set with \( and \). Then the set can be designated to be repeated a certain number of times

       \(like this\)*   or   \(like this\)\{5,7\}.

Groups can also be nested "\(like \(this\) is here\)" and may contain any valid RE. Backreferences repeat the contents of a particular group, using a backslash and a digit (1-9) for each corresponding group. In other words, "/\(pom\)\1/" is another way of writing "/pompom/". If groups are nested, backreference numbers are counted by matching \( in strict left to right order. Thus, /..\(the \(word\)\) \("foo"\)../ is matched by the backreference \3. Backreferences can be used in the LHS, the RHS, and in normal RE addressing (see section 3.3). Thus,

       /\(.\)\1\(.\)\2\(.\)\3/;      # matches "bookkeeper"
       /^\(.\)\(.\)\(.\)\3\2\1$/;    # finds 6-letter palindromes

Seds differ in how they treat invalid backreferences where no corresponding group occurs. To insert a literal ampersand or backslash into the RHS, prefix it with a backslash: \& or \\.

ssed, sed16, and sedmod permit additional options on the RHS. They all support changing part of the replacement string to upper case (\u or \U), lower case (\l or \L), or to end case conversion (\E). Both sed16 and sedmod support awk-style word references ($1, $2, $3, ...) and $0 to insert the entire line before conversion.

     echo ab ghi | sed16 "s/.*/$0 - \U$2/"   # prints "ab ghi - GHI"

*Note:* This feature of sed16 and sedmod will break sed scripts which put a dollar sign and digit into the RHS. Though this is an unlikely combination, it's worth remembering if you use other people's scripts.

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