52

I'm curious, why does sed need 3 \ just to recognize one? I'd understand it needing 2, but 3 I don't.

EDIT: here's an example on my Windows computer, using Cygwin:

$ echo "sample_input\whatever" | sed "s/\\\/\//"
sample_input/whatever

If I don't add 3 backslashes, I get a 

sed: -e expression #1, char 7: unterminated s' command
markling
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Geo
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  • It doesn't. Show us why you think it does. – Ignacio Vazquez-Abrams Mar 03 '10 at 07:01
  • is your shell consuming an extra level of backslashing? – p00ya Mar 03 '10 at 07:07
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    one \ for the shell, one \ for the sed, one \ for the backslash = \\\ – Kimvais Mar 03 '10 at 07:24
  • @Kimvais: If it was as you stated there, it'd be four, as sed requires an escaped backslash (2) and the shell escaping of those would require one each. –  Mar 03 '10 at 07:26
  • Use single quotes for `sed` and you can get away with two backslashes. `echo "sample_input\whatever" | sed 's/\\/\//'` Hopefully someone will come up with the correct explanation for this behavior. – Amarghosh Mar 03 '10 at 08:06
  • @Amarghosh nope, that is not the case for win xp cmd.exe and sed from msys-1.0. I get the exact error as in the question and I used single quotes. – panny Feb 13 '13 at 15:27
  • You even need **four backslashes** and two quotes like in `sed "s/.*/\\\\""/"` to print out a single backslash followed by a quote `\"`. – Doomjunky Aug 01 '17 at 11:57
  • sorry for my question but you want to replace backslash with a slash? – Timo Jan 16 '18 at 13:56
  • It's because you use double quotes therefore the string is interpolated. See this [question](https://stackoverflow.com/questions/6697753/difference-between-single-and-double-quotes-in-bash). Using single quotes, this works for me in bash: `sed 's/\///'` – smac89 Mar 27 '21 at 08:56

7 Answers7

52

I was able to reproduce this behavior using Vista and Cygwin 1.7.0.

  • Two backslashes produce the error
  • either three or four backslashes work
  • Five gives the same error

Two backslashes become a single backslash in the shell which then in sed escapes the forward slash which is the middle delimiter.

\\/ -> \/ (which makes the forward slash a regular character instead of a delimiter)

Three of them: The first two become one in the shell which then escape the third one in sed

\\\/ -> \\/

Four: Each pair become single ones in the shell then the first resulting one escapes the second in sed

\\\\/ -> \\/

Edit:

Oh, I forgot to say that both single quotes and double quotes worked the same for me (cmd.exe doesn't make the distinction that Bash, et al, makes).

Dennis Williamson
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14

Your shell (probably bash) is doing its own escaping, and that's confusing you. You can use an echo command to see what is being passed, or it's easy to write a custom program (commonly named "showargs" or similar):

$ echo "s/\\\/\//"
s/\\/\//
$ echo "s/\\/\//"
s/\/\//

You can also use single quotes, which are treated differently in bash.

7

That's due to sh's double-quoted string parsing rule.

Posix specifies how sh parses double-quoted strings.

The backslash shall retain its special meaning as an escape character (see Escape Character (Backslash)) only when followed by one of the following characters when considered special: $ ` " \

In other words, sh lefts the backslash which is followed by characters other than $ ' " \.

So, if sh meets the double-quoted string sed "s/\\\/\//", sh parses it as follows.

  1. The first two \\ is changed into \. Because the first \ is followed by the second \.
  2. The third and fourth \ is still left in the string. Because both of them are followed by /, which is not special in double-quoted string.

After pasring, sh passes the string s/\\/\// to sed, which substitutes the first occurence of \ into /.

With same reasoning, when sh meets the string, "sed s/\\\\/\//", sh passes /\\/\// to sed, which also substitutes the first occurence of \ into /.

Community
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MS.Kim
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4

Please show an example of what you have in future. in sed, say you want to replace a "\" with pipe (|), for example

$ cat file
asklfja \ asf

$ sed 's/\\/|/g' file
asklfja | asf

$ sed 's%\\%|%g' file #using different delimiter
asklfja | asf

you just need to escape it once.

Edit: To @OP's example, since you are using cmd.exe and not bash/ksh, cmd.exe doesn't like single quotes. I cannot produce your scenario. This works for my GNU sed on windows using 2 slashes

eg

C:\test>echo "sample_input\whatever" | sed "s/\\/\//"
"sample_input/whatever"
ghostdog74
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2

In my version of CYGWIN, it works as the original poster says, but, works differently (normally) if I use single quotes.

$ echo "sample_input\whatever" | sed 's/\\/\//'
sample_input/whatever

$ echo "sample_input\whatever" | sed "s/\\/\//"
sed: -e expression #1, char 7: unterminated `s' command

Hmmm..

James K
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0

Replacing one backslash with two on my Cygwin requires this expression:

sed -e "s|\\|\\\\|g"

OutputLogic
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0

I guess, you assuming \\\n or \\\t as three backslashes there, but actually, its 2 backslashes and another patterns

   backslash          \\
   newline            \n
   tab                \t

also, / might need to escape because in s/.../, / is using for open and close parts.

so /\\\/\// would be \\ + \/ + \/ according to your updated example

YOU
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  • I like your method in explaining, could you please take a look at this and try to answer it? https://superuser.com/questions/1249497/sed-needs-n-to-append-new-line?noredirect=1#comment1834908_1249497 – Mohammed Noureldin Sep 12 '17 at 19:42