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comp.lang.ruby

gsub("\\", "\\\\") seems unintuitive

John Woods

2/22/2008 5:28:00 PM

The following confusing behavior is noted in the pickaxe book (2nd ed)
on page 75:

# I would expect two backslashes in the result
irb> puts "\\".gsub("\\","\\\\")

# I would expect four backslashes in the result
irb> puts "\\".gsub("\\","\\\\\\\\")
\
I can certainly work around it, but it seems unintuitive. Is there a
reason why gsub behaves this way? Just curious...
10 Answers

William James

2/22/2008 5:36:00 PM

0

On Feb 22, 11:27 am, John Woods <jqwo...@gmail.com> wrote:
> The following confusing behavior is noted in the pickaxe book (2nd ed)
> on page 75:
>
> # I would expect two backslashes in the result
> irb> puts "\\".gsub("\\","\\\\")
> >
> # I would expect four backslashes in the result
> irb> puts "\\".gsub("\\","\\\\\\\\")
> \>
> I can certainly work around it, but it seems unintuitive. Is there a
> reason why gsub behaves this way? Just curious...

puts "\\".gsub("\\"){"\\\\"}

J. Cooper

2/22/2008 5:38:00 PM

0

John Woods wrote:
> The following confusing behavior is noted in the pickaxe book (2nd ed)
> on page 75:
>
> # I would expect two backslashes in the result
> irb> puts "\\".gsub("\\","\\\\")
> >
> # I would expect four backslashes in the result
> irb> puts "\\".gsub("\\","\\\\\\\\")
> \>
> I can certainly work around it, but it seems unintuitive. Is there a
> reason why gsub behaves this way? Just curious...

It's not a gsub thing, per se--it's a string thing. Backslashes are used
in strings to escape special characters. One such character is a " mark.
If you want to write a " mark in the middle of a string, you have to
escape it with a backslash:
"They call me \"Mellow Yellow\" etc."

If you didn't, then the " would signify the end of the string!
Similarly, in the example you listed, if you just did:
"\"
then you end up with a string that ISN'T ended! Because you escaped the
next ". So, if you want a literal backslash, you have to escape the
backslash too: "\\"

It just looks confusing because you are escaping the escape character :)


--
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Rob Biedenharn

2/22/2008 5:49:00 PM

0

On Feb 22, 2008, at 12:27 PM, John Woods wrote:
> The following confusing behavior is noted in the pickaxe book (2nd
> ed) on page 75:
>
> # I would expect two backslashes in the result
> irb> puts "\\".gsub("\\","\\\\")
> >
> # I would expect four backslashes in the result
> irb> puts "\\".gsub("\\","\\\\\\\\")
> \>
> I can certainly work around it, but it seems unintuitive. Is there a
> reason why gsub behaves this way? Just curious...


Notwithstanding the earlier responses...

Since the replacement string is evaluated 'twice', once as a ruby
string literal and then again by gsub to look for group refrences like
'\1', you need to provide two levels of escaping for a backslash.

\ is "\\"
so two of them is "\\\\"
and you want gsub to see that so it need to have them escaped: "\\\\\\
\\"

Whew! Yeah, it's unfortunate, but backslash is doing double-duty
here: introducing a group reference to the regular expression and
escaping characters in a string literal (just like "\n", but also
itself).

-Rob

Rob Biedenharn http://agileconsult...
Rob@AgileConsultingLLC.com

Kyle Schmitt

2/22/2008 5:54:00 PM

0

Just as a side note, this is typical in all real programming languages.
C, C++, Java, Perl, sh, etc.
I _believe_ it's also true in python, lithp/scheme, & (o)caml, but for
those I've either not used them, or not used them in so long I'm
unsure.

Some languages, like vb{6|script|.net}, use a doubled quote, but those
aren't really proper programing languages ;)

--Kyle

John Woods

2/22/2008 6:01:00 PM

0

Thanks Rob, that's exactly what I was missing -- the second round of
escaping is necessary to make escaped references to regex groups work.


-----Original Message-----
From: Rob Biedenharn
Sent: 02/22/2008 09:48 AM
> On Feb 22, 2008, at 12:27 PM, John Woods wrote:
>> The following confusing behavior is noted in the pickaxe book (2nd ed)
>> on page 75:
>>
>> # I would expect two backslashes in the result
>> irb> puts "\\".gsub("\\","\\\\")
>> >>
>> # I would expect four backslashes in the result
>> irb> puts "\\".gsub("\\","\\\\\\\\")
>> \>>
>> I can certainly work around it, but it seems unintuitive. Is there a
>> reason why gsub behaves this way? Just curious...
>
>
> Notwithstanding the earlier responses...
>
> Since the replacement string is evaluated 'twice', once as a ruby string
> literal and then again by gsub to look for group refrences like '\1',
> you need to provide two levels of escaping for a backslash.
>
> \ is "\\"
> so two of them is "\\\\"
> and you want gsub to see that so it need to have them escaped: "\\\\\\\\"
>
> Whew! Yeah, it's unfortunate, but backslash is doing double-duty here:
> introducing a group reference to the regular expression and escaping
> characters in a string literal (just like "\n", but also itself).
>
> -Rob
>
> Rob Biedenharn http://agileconsult...
> Rob@AgileConsultingLLC.com
>
>
>


Joe Peck

12/17/2008 9:08:00 PM

0

I stumbled onto this thread trying to figure out a similar problem...
--
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Joe Peck

12/17/2008 9:12:00 PM

0

Joe Peck wrote:
> I stumbled onto this thread trying to figure out a similar problem...

Dangit, I didn't mean to post the above message :)

Okay, I need to read a site name (like "joe.com") and put a \ in front
of the period (so it would be "joe\.com"). I'm having trouble with
this, I can't seem to get only ONE \ in front of the period...

>> "joe.com".gsub(/\./, "\\\.")
=> "joe\\.com"
>> "joe.com".gsub(/\./, "\\\\.")
=> "joe\\.com"

Anyone have a clue what to do? I'm probably missing something simple.
--
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Glen Holcomb

12/17/2008 9:21:00 PM

0

[Note: parts of this message were removed to make it a legal post.]

On Wed, Dec 17, 2008 at 2:12 PM, Joe Peck <joe@notsleepy.com> wrote:

> Joe Peck wrote:
> > I stumbled onto this thread trying to figure out a similar problem...
>
> Dangit, I didn't mean to post the above message :)
>
> Okay, I need to read a site name (like "joe.com") and put a \ in front
> of the period (so it would be "joe\.com"). I'm having trouble with
> this, I can't seem to get only ONE \ in front of the period...
>
> >> "joe.com".gsub(/\./, "\\\.")
> => "joe\\.com"
> >> "joe.com".gsub(/\./, "\\\\.")
> => "joe\\.com"
>
> Anyone have a clue what to do? I'm probably missing something simple.
> --
> Posted via http://www.ruby-....
>
>
That's because you are in irb. You are getting what you want try this:

puts "joe.com".gsub(/\./, "\\\\.")

Actually it might be more readable as gsub('.', '\.')

--
"Hey brother Christian with your high and mighty errand, Your actions speak
so loud, I can't hear a word you're saying."

-Greg Graffin (Bad Religion)

Todd Benson

12/17/2008 10:56:00 PM

0

On Wed, Dec 17, 2008 at 3:12 PM, Joe Peck <joe@notsleepy.com> wrote:
> Joe Peck wrote:
>> I stumbled onto this thread trying to figure out a similar problem...
>
> Dangit, I didn't mean to post the above message :)
>
> Okay, I need to read a site name (like "joe.com") and put a \ in front
> of the period (so it would be "joe\.com"). I'm having trouble with
> this, I can't seem to get only ONE \ in front of the period...
>
>>> "joe.com".gsub(/\./, "\\\.")
> => "joe\\.com"
>>> "joe.com".gsub(/\./, "\\\\.")
> => "joe\\.com"
>
> Anyone have a clue what to do? I'm probably missing something simple.

You are looking at a string with escaping characters (i.e. what it
would look like in double quotes). You can verify this by using #puts
or "joe.com".gsub('.', '\.').length.

hth,
Todd

Joe Peck

12/18/2008 3:13:00 PM

0

Okay, those last two explanations helped clear up my confusion. Thanks.
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