why ruby is prepends '\' character while I am trying to run below code. It is happening with only '#$'
It is happening with all ruby version.
puts '#$' => '\#$'
or
'#$' => '\#$'
or
'mypassord#$123' => 'mypassord\#$123'
Please share you experience here. Is it a ruby problem or anything?
No it is not a ruby problem. It is your problem. Since #$foo can be interpreted as interpolation of the global variable $foo, it is necessary to escape the # character. That is why there is a backslash.
To be more precise, there is no possibility of interpolation with the string "#$" ($ is an invalid global variable) or "#$123" ($123 is an invalid global variable), but it makes the inspection algorithm or the interpolation algorithm complicated to check the sequence after #$, so I guess that is why # is escaped even in such cases.
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I'm trying to write \1 in a string, but I can't do it. I would appreciate if somebody helped me with this strange behaviour. Here is an example with some explaining.
EDIT: Adding example output
puts "\1 <- null"
puts "\\1 <- slash one"
works!
but typing
"\1"
"\\1"
in the irb command line yields
"\1"
=> "\u0001
"\\1"
=> "\\1"
There are a few ways to get it:
"\\1"
'\1'
?\\ + ?1
Remember that the way it will show up is always "\\1", which means literal backslash, one, which is what you want. The way to know that this is correct is to use puts:
puts "\\1"
# => \1
Inside of double-quoted strings, backslashes have significant meaning. \n means the newline character. In single quoted strings, that's two characters: backslash and n.
You can even test this:
"\\1".chars
# => ["\\", "1"]
'\1'.chars
# => ["\\", "1"]
So you can see Ruby is interpreting that as two characters, not three. Don't be fooled by the second backslash inside a double-quoted string. That's how a literal backslash is represented.
Have you tried puts '\1'? (single quotes instead of double)
I'm not 100% sure what you're asking but if that helps, cheers.
Your command line shows "\1" because irb does .inspect on the object, which escapes the string. So essentially \1 is properly stored, but when it's displaying it, it adds another \ to indicate to you that it's escaped
When I'm in IRB and type \1, the value returned is \u0001 which is Ruby's way of
representing the character.
When I write puts('\1), the behavior is the same in IRB and when running
a script. I see a unicode character map as follows
0 0
0 1
This won't be the same output on all platforms (it depends on how unicode is
displayed). So that's probably why you see no output on the repl.it example.
I often see the gsub function being called with the pattern parameter enclosed in forward slashes. For example:
>> phrase = "*** and *** ran to the ###."
>> phrase.gsub(/\*\*\*/, "WOOF")
=> "WOOF and WOOF ran to the ###."
I thought maybe it had something to do with escaping asterisks, but using single quotes and double quotes works just as well:
>> phrase = "*** and *** ran to the ###."
>> phrase.gsub('***', "WOOF")
=> "WOOF and WOOF ran to the ###."
>> phrase.gsub("***", "WOOF")
=> "WOOF and WOOF ran to the ###."
Is it just convention to use forward slash? What am I missing?
Use forward slashes if you need to use regular expressions.
If you use a string argument with gsub, it will just do a plain character match.
In your example, you need backslashes to escape the asterisks when using a regular expression, because asterisks have a special meaning in regex (optionally match something any number of times). They are not necessary when using a string, because they are just matched exactly.
In your example, you probably don't need to use a regular expression, since it is a simple pattern. However, if you wanted to match *** only when it was at the beginning of a string (e.g. the first bunch in your example), then you would want to use a regex, for example:
phrase.gsub(/^\*{3}/, "WOOF")
For more information on regular expressions, see: http://www.regular-expressions.info/.
For more information on using regular expressions in Ruby, see: http://ruby-doc.org/core-2.2.0/Regexp.html.
To play with regular expressions as they work in Ruby, try: http://rubular.com/.
You are missing reading the documentation:
The pattern is typically a Regexp; if given as a String, any regular expression metacharacters it contains will be interpreted literally, e.g. '\d' will match a backlash followed by ‘d’, instead of a digit.
http://ruby-doc.org/core-2.1.4/String.html#method-i-gsub
In other words, you can give a string or a regular expression. Regular expressions can be delimited several ways:
Regexps are created using the /.../ and %r{...} literals, and by the Regexp::new constructor.
http://ruby-doc.org/core-2.2.2/Regexp.html
The benefit of %r and of the alternate %r delimiters is you can usually find a delimiter that doesn't collide with characters in the pattern, which would force escaping them, as in your example.
* has to be escaped because it has special meaning in a regex, but in a string it does not.
I got a regex in my code, which is to match pattern of url and threw error:
/^(http|https):\/\/([\w-]+\.)+[\w-]+([\w- .\/?%&=]*)?$/
The error was "empty range in char class error". I found the cause of that is in ([\w- .\/?%&=]*)? part. Ruby seems to recognize - in \w- . as an operator for range instead of a literal -. After adding escape to the dash, the problem was solved.
But the original regular expression ran well on my co-workers' machines. We use the same version of osx, rails and ruby: Ruby version is ruby 1.9.3p194, rails is 3.1.6 and osx is 10.7.5. And after we deployed code to our Heroku server, everything worked fine too. Why did only my environment have error regarding this regex? What is the mechanism of Ruby regex interpreting?
I can replicate this error on Ruby 1.9.3p194 (2012-04-20 revision 35410) [i686-linux], installed on Ubuntu 12.04.1 LTS using rvm 1.13.4. However, this should not be a version-specific error. In fact, I'm surprised it worked on the other machines at all.
A a simpler demonstration that fails just as well:
"abcd" =~ /[\w- ]/
This is because [\w- ] is interpreted as "a range beginning with any word character up to space (or blank)", rather than a character class containing a word, a hyphen, or a space, which is what you had intended.
Per Ruby's regular expression documentation:
Within a character class the hyphen (-) is a metacharacter denoting an inclusive range of characters. [abcd] is equivalent to [a-d]. A range can be followed by another range, so [abcdwxyz] is equivalent to [a-dw-z]. The order in which ranges or individual characters appear inside a character class is irrelevant.
As you saw, prepending a backslash escaped the hyphen, thus changing the nature of the regexp from a range to a character class, removing the error. However, escaping the hyphen in the middle of character class is not recommended, since it's easy to confuse the intended meaning of the hyphen in such cases. As m.buettner pointed out, always place hyphens either at the beginning or the end of a character class:
"abcd" =~ /[-\w ]/
Is there some way in Ruby that I can avoid having to put double-backslash in Ruby strings (like what can be done in C#):
For example, in C# was can prefix a string with # and then the backslash in the string does not need to be escaped:
#"C:\Windows, C:\ABC"
Without # we would need to escape the backslash:
"C:\\Windows, C:\\ABC"
Is there something similar in Ruby?
Use single quotes
my_string = 'C:\Windows'
See more in the Strings section here
You can also use %q and backslashes will be automatically escaped for you:
%q{C:\Windows} => "C:\\Windows"
I'm trying to learn RegEx in Ruby, based on what I'm reading in "The Rails Way". But, even this simple example has me stumped. I can't tell if it is a typo or not:
text.gsub(/\s/, "-").gsub([^\W-], '').downcase
It seems to me that this would replace all spaces with -, then anywhere a string starts with a non letter or number followed by a dash, replace that with ''. But, using irb, it fails first on ^:
syntax error, unexpected '^', expecting ']'
If I take out the ^, it fails again on the W.
>> text = "I love spaces"
=> "I love spaces"
>> text.gsub(/\s/, "-").gsub(/[^\W-]/, '').downcase
=> "--"
Missing //
Although this makes a little more sense :-)
>> text.gsub(/\s/, "-").gsub(/([^\W-])/, '\1').downcase
=> "i-love-spaces"
And this is probably what is meant
>> text.gsub(/\s/, "-").gsub(/[^\w-]/, '').downcase
=> "i-love-spaces"
\W means "not a word"
\w means "a word"
The // generate a regexp object
/[^\W-]/.class
=> Regexp
Step 1: Add this to your bookmarks. Whenever I need to look up regexes, it's my first stop
Step 2: Let's walk through your code
text.gsub(/\s/, "-")
You're calling the gsub function, and giving it 2 parameters.
The first parameter is /\s/, which is ruby for "create a new regexp containing \s (the // are like special "" for regexes).
The second parameter is the string "-".
This will therefore replace all whitespace characters with hyphens. So far, so good.
.gsub([^\W-], '').downcase
Next you call gsub again, passing it 2 parameters.
The first parameter is [^\W-]. Because we didn't quote it in forward-slashes, ruby will literally try run that code. [] creates an array, then it tries to put ^\W- into the array, which is not valid code, so it breaks.
Changing it to /[^\W-]/ gives us a valid regex.
Looking at the regex, the [] says 'match any character in this group. The group contains \W (which means non-word character) and -, so the regex should match any non-word character, or any hyphen.
As the second thing you pass to gsub is an empty string, it should end up replacing all the non-word characters and hyphens with empty string (thereby stripping them out )
.downcase
Which just converts the string to lower case.
Hope this helps :-)
You forgot the slashes. It should be /[^\W-]/
Well, .gsub(/[^\W-]/,'') says replace anything that's a not word nor a - for nothing.
You probably want
>> text.gsub(/\s/, "-").gsub(/[^\w-]/, '').downcase
=> "i-love-spaces"
Lower case \w (\W is just the opposite)
The slashes are to say that the thing between them is a regular expression, much like quotes say the thing between them is a string.