regex - Why does sed require 3 backslashes for a regular backslash?

Question

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_inputwhatever" | sed "s/\///"
sample_input/whatever

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

sed: -e expression #1, char 7: unterminated s' command

Answer 1

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).