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Escape a string for a sed replace pattern

In my bash script I have an external (received from user) string, which I should use in sed pattern.

REPLACE="<funny characters here>" sed "s/KEYWORD/$REPLACE/g" 

How can I escape the $REPLACE string so it would be safely accepted by sed as a literal replacement?

NOTE: The KEYWORD is a dumb substring with no matches etc. It is not supplied by user.

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Alexander Gladysh Avatar asked Jan 02 '09 17:01

Alexander Gladysh


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How do you escape the sed string?

Sed needs many characters to be escaped to get their special meaning. For example, if you escape a digit in the replacement string, it will turn in to a backreference. Remember, if you use a character other than / as delimiter, you need replace the slash in the expressions above wih the character you are using.


1 Answers

Warning: This does not consider newlines. For a more in-depth answer, see this SO-question instead. (Thanks, Ed Morton & Niklas Peter)

Note that escaping everything is a bad idea. Sed needs many characters to be escaped to get their special meaning. For example, if you escape a digit in the replacement string, it will turn in to a backreference.

As Ben Blank said, there are only three characters that need to be escaped in the replacement string (escapes themselves, forward slash for end of statement and & for replace all):

ESCAPED_REPLACE=$(printf '%s\n' "$REPLACE" | sed -e 's/[\/&]/\\&/g') # Now you can use ESCAPED_REPLACE in the original sed statement sed "s/KEYWORD/$ESCAPED_REPLACE/g" 

If you ever need to escape the KEYWORD string, the following is the one you need:

sed -e 's/[]\/$*.^[]/\\&/g'

And can be used by:

KEYWORD="The Keyword You Need"; ESCAPED_KEYWORD=$(printf '%s\n' "$KEYWORD" | sed -e 's/[]\/$*.^[]/\\&/g');  # Now you can use it inside the original sed statement to replace text sed "s/$ESCAPED_KEYWORD/$ESCAPED_REPLACE/g" 

Remember, if you use a character other than / as delimiter, you need replace the slash in the expressions above wih the character you are using. See PeterJCLaw's comment for explanation.

Edited: Due to some corner cases previously not accounted for, the commands above have changed several times. Check the edit history for details.

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Pianosaurus Avatar answered Sep 17 '22 05:09

Pianosaurus