Ruby

read the Docs:

If replacement is a String it will be substituted for the matched text. It may contain back-references to the pattern's capture groups of the form "\d", where d is a group number, or "\k<n>", where n is a group name. If it is a double-quoted string, both back-references must be preceded by an additional backslash. 

it is explained what to do with the blackslashes. and how much do you need.

Can you give a solution for my second example?

"abc".sub(/(b)/, '\\1') # doesn't works, should be "a\bc" 

I want to replace the 'b' with '\b'. How many backslashes do I need?

The #sub method has problems with consecutive backslashes in combination with backreferences.

need 5 or 6 (6 when using ")

"abc".sub(/(b)/, '\\\\\1') #=> "a\\bc" "abc".sub(/(b)/, "\\\\\1") #=> "a\\\x01c" "abc".sub(/(b)/, "\\\\\\1") #=> "a\\bc" 

but be careful that irb returns inspected value, that is important if you want to see \ or \ stuff

puts "abc".sub(/(b)/, '\\\\\1') #=> a\bc p "abc".sub(/(b)/, '\\\\\1') #=> a\\bc 

Thanks for help!

I don't unterstand, why five backslahes are required in single quotes. My understanding was that characters in single quotes won't get interpolated so two backslashes should be sufficient.