gsub oddity?

[Ralph Mason]

What am I missing here. I am trying to replace a single quote with the
string \'

irb(main):032:0> "replace 'quotes'.".gsub(/'/,"\\'")
=> "replace quotes'.quotes.."

should be

"replace \'quotes\'."

Thanks
Ralph

 

[Robert Klemme]

You're missing an extra level of processing applied to the replacement
string for gsub in order to let you include parts of what you're
replacing.

No, in this case he's missing extra backslashes:

irb(main):002:0> str='a\'o'
=> "a'o"
irb(main):003:0> str.gsub(/'/, '\\\\\'' )
=> "a\\'o"
irb(main):004:0> puts str.gsub(/'/, '\\\\\'' )
a\'o
=> nil

It does look a bit odd, but the explanation is logical: There are two
levels of escaping involved. The first level is that needed for the ruby
parser to get the expected characters into the replacement string. The
second level is used in order to be able to use a backslash literally in
the substitution string.

irb(main):005:0> puts '\\\\\''
\\'
=> nil

You need two backslashes in the substitution string to prevent gsub from
interpreting the backslash as escape char. That makes four backslashes
when entering the string. The fifth backslash escapes the single quote in
order to be able to include it into the string.

Regards

robert

 

[Mike Stok]

No, in this case he's missing extra backslashes:

irb(main):002:0> str='a\'o'
=> "a'o"
irb(main):003:0> str.gsub(/'/, '\\\\\'' )
=> "a\\'o"
irb(main):004:0> puts str.gsub(/'/, '\\\\\'' )
a\'o
=> nil

It does look a bit odd, but the explanation is logical: There are two
levels of escaping involved. The first level is that needed for the ruby
parser to get the expected characters into the replacement string. The
second level is used in order to be able to use a backslash literally in
the substitution string.

irb(main):005:0> puts '\\\\\''
\\'
=> nil

You need two backslashes in the substitution string to prevent gsub from
interpreting the backslash as escape char. That makes four backslashes
when entering the string. The fifth backslash escapes the single quote in
order to be able to include it into the string.

You can make life a little easier by picking a quote other than ' to
get rid of one backslash e.g.

[mike@ratdog mike]$ irb --simple-prompta\'o
=> nil

It can be tiresome to remember how many times a string will be scanned
in a context where \ characters matter, so I prefer using the block form
e.g.
a\'o
=> nil

....but I have never measured whether this is significantly more
expensive.

Hal, is this a FAQ? "How do I replace ' with \' in a string?" I'm sure
that this has come up a few times...

Hope this helps,

Mike

 

[Robert Klemme]

You can make life a little easier by picking a quote other than ' to
get rid of one backslash e.g.

Yes, although it's just a small improvement.

[mike@ratdog mike]$ irb --simple-prompta\'o
=> nil

It can be tiresome to remember how many times a string will be scanned
in a context where \ characters matter, so I prefer using the block form
e.g.
a\'o
=> nil

...but I have never measured whether this is significantly more
expensive.

It's definitely more expensive, which is not really a surprise given the
additional overhead of

- a block yield
- string replacement with {}

But even with a constant string there's still the block call overhead:

14:21:43 [ADMIN]: /c/temp/ruby/rx-replace-bm.rb
user system total real
direct 0.766000 0.000000 0.766000 ( 0.763000)
direct & 0.766000 0.000000 0.766000 ( 0.769000)
block 1.765000 0.000000 1.765000 ( 1.770000)
block fix 1.438000 0.000000 1.438000 ( 1.432000)
14:21:50 [ADMIN]: cat /c/temp/ruby/rx-replace-bm.rb
#!/usr/bin/ruby

require 'benchmark'
include Benchmark

N=100000

str = ' /* comment */ String s = "***/"; '

bm(10) do |x|
x.report("direct") do
for n in 1..N
str.gsub( /"/, '\\\\"' )
end
end

x.report("direct &") do
for n in 1..N
str.gsub( /"/, '\\\\\\&' )
end
end

x.report("block") do
for n in 1..N
str.gsub( /"/ ) {|m| "\\#{m}"}
end
end

x.report("block fix") do
for n in 1..N
str.gsub( /"/ ) {|m| '\\"'}
end
end
end
14:21:58 [ADMIN]:

Hal, is this a FAQ? "How do I replace ' with \' in a string?" I'm sure
that this has come up a few times...

I put it on the Wiki:
http://www.rubygarden.org/ruby?RegexpCookbook

Cheers

robert

 

[Hal Fulton]

Hal, is this a FAQ? "How do I replace ' with \' in a string?" I'm sure
that this has come up a few times...

I'm the maintainer of the comp.lang.ruby FAQ, which (mainly) deals with
the newsgroup itself, not language details.

Which reminds me, I've been terribly lax about that. My automated
process broke and I've not fixed it in three or four months. Mea culpa.

But I do think this is a FAQ from the Ruby language point of view.

/me scribbles note for potential second edition of TRW.


Cheers,
Hal

 

[Robert Klemme]

Did you not read the rest of my post, when I pointed that out and
supplied the solution? I answered the original question in terms
of what he was missing in the colloquial sense of being unaware of.

Apparently I misunderstood you here. I'm sorry.

Kind regards

robert