The following confusing behavior is noted in the pickaxe book (2nd ed) on page 75:
# I would expect two backslashes in the result
> puts "\\".gsub("\\","\\\\")
\
# I would expect four backslashes in the result
> puts "\\".gsub("\\","\\\\\\\\")
\\
I can certainly work around it, but it seems unintuitive. Is there a reason why gsub behaves this way? Just curious...
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
> puts "\\".gsub("\\","\\\\")
\# I would expect four backslashes in the result
> 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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puts "\\".gsub("\\"){"\\\\"}
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
> puts "\\".gsub("\\","\\\\")
\# I would expect four backslashes in the result
> 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://agileconsultingllc.com
Rob@AgileConsultingLLC.com
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
> puts "\\".gsub("\\","\\\\")
\# I would expect four backslashes in the result
> puts "\\".gsub("\\","\\\\\\\\")
\\I can certainly work around it, but it seems unintuitive. Is there a reason why gsub behaves this way? Just curious...
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
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
> puts "\\".gsub("\\","\\\\")
\# I would expect four backslashes in the result
> 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://agileconsultingllc.com
Rob@AgileConsultingLLC.com
I stumbled onto this thread trying to figure out a similar problem...
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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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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('.', '\.')
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.
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--
"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)
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
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.
Okay, those last two explanations helped clear up my confusion. Thanks.
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