Bug #21651: replacing a string with one backslash with two backslashes

• Status: Feedback
• Assignee: -
• Target version: -
• ruby -v: 3.4.4
• Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN

[tdrive (Stanislav Boldaev)]

ruby 2.7.8p225 (2023-03-30 revision 1f4d455848) [x86_64-linux]
ruby 3.4.4 (2025-05-14 revision a38531fd3f) +PRISM [x86_64-linux]

irb(main):002> "\\".gsub("\\", "\\\\")
=> "\\"
irb(main):003> "\\".gsub("\\", "\\ \\")
=> "\\ \\"

The replacement is happening, as you can see from the version with the space, but why is the result so strange without the space?
Similar code works fine in JS:

"\\".replace("\\", "\\\\")
> '\\\\'

How to replace one backslash with two backslashes?

[nobu (Nobuyoshi Nakada)]

• Status changed from Open to Feedback

From String@Substitution+Methods:

Note that \\ is interpreted as an escape, i.e., a single backslash.

Note also that a string literal consumes backslashes.
See String Literals for details about string literals.

This is what happens at the first line.

Since a space after a backslash is not a valid escape, the result at the second line is "\ \".

If you want to write a non-back-reference string & in
replacement, you need to first escape the backslash to prevent
this method from interpreting it as a back-reference, and then you
need to escape the backslashes again to prevent a string literal from
consuming them: "..\\\\&..".

For your example:

"\\".gsub("\\", "\\\\\\\\") # literally two backslashes
"\\".gsub("\\", "\\&\\&")   # double the matched substrings
"\\".gsub("\\") {"\\\\"}    # the block result used literally