Why does String#gsub double content?

s = "#main= 'quotes'
s.gsub "'", "\\'" # => "#main= quotes'quotes"

This seems to be wrong, I expect to get "#main= \\'quotes\\'"

when I don't use escape char, then it works as expected.

s.gsub "'", "*" # => "#main= *quotes*"

So there must be something to do with escaping.

Using ruby 1.9.2p290

I need to replace single quotes with back-slash and a quote.

Even more inconsistencies:

"\\'".length # => 2
"\\*".length # => 2

# As expected
"'".gsub("'", "\\*").length # => 2
"'a'".gsub("'", "\\*") # => "\\*a\\*" (length==5)

# WTF next:
"'".gsub("'", "\\'").length # => 0

# Doubling the content?
"'a'".gsub("'", "\\'") # => "a'a" (length==3)

What is going on here?

You're getting tripped up by the specialness of \' inside a regular expression replacement string:

\0, \1, \2, ... \9, \&, \`, \', \+
Substitutes the value matched by the nth grouped subexpression, or by the entire match, pre- or postmatch, or the highest group.

So when you say "\\'", the double \\ becomes just a single backslash and the result is \' but that means "The string to the right of the last successful match." If you want to replace single quotes with escaped single quotes, you need to escape more to get past the specialness of \':

s.gsub("'", "\\\\'")

Or avoid the toothpicks and use the block form:

s.gsub("'") { |m| '\\' + m }

You would run into similar issues if you were trying to escape backticks, a plus sign, or even a single digit.

The overall lesson here is to prefer the block form of gsub for anything but the most trivial of substitutions.

s = "#main = 'quotes'

s.gsub "'", "\\\\'"

Since \it's \\equivalent if you want to get a double backslash you have to put four of ones.

You need to escape the \ as well:

s.gsub "'", "\\\\'"

Outputs

"#main= \\'quotes\\'"

A good explanation found on an outside forum:

The key point to understand IMHO is that a backslash is special in replacement strings. So, whenever one wants to have a literal backslash in a replacement string one needs to escape it and hence have [two] backslashes. Coincidentally a backslash is also special in a string (even in a single quoted string). So you need two levels of escaping, makes 2 * 2 = 4 backslashes on the screen for one literal replacement backslash.

source