In ruby, the backticks are a system call, but they are interpolation. This is nice as I could do this
a = 20.sqrt
`cat #{a}`
But it is also annoying because I sometimes want \ in my code, but I need \\ within `` because it is interpolation and escaping. How can I avoid this?
Try this
Kernel.`('echo "#{a}"')
Which prints verbatim
#{a}
Fun fact, ` is actually a method on Kernel and you can call it just like any other method. And thus pass a single quote string as argument.
Form the string in a nonescaping context and interpolate it into the backticks:
s = %{echo "he\ny"}
puts `#{s}`
If you need to juggle with escape characters, you could instead use %q :
system(%q{echo '#&%$][/\'})
#=> #&%$][/\
If you want string interpolation, you can use %Q :
a = 20
system(%Q{echo '#{a}&%$][/\'})
#=> 20&%$][/
Here's a thread about it. Note that you can use any delimiter after %q and %Q : pick one that isn't in your string!
I used system here instead of %x{} or ticks. They're not equivalent, but I just wanted to show the string definition without making the line even more complex than it already is.
Related
I stumbled over this problem using the following simplified example:
line = searchstring.dup
line.gsub!(Regexp.escape(searchstring)) { '' }
My understanding was, that for every String stored in searchstring, the gsub! would cause that line is afterwards empty. Indeed, this is the case for many strings, but not for this case:
searchstring = "D "
line = searchstring.dup
line.gsub!(Regexp.escape(searchstring)) { '' }
p line
It turns out, that line is printed as "D " afterwards, i.e. no replacement had been performed.
This happens to any searchstring containing a space. Indeed, if I do a
p(Regexp.escape(searchstring))
for my example, I see "D\\ " being printed, while I would expect to get "D " instead. Is this a bug in the Ruby core library, or did I misuse the escape function?
Some background: In my concrete application, where this simplified example is derived from, I just want to do a literal string replacement inside a long string, in the following way:
REPLACEMENTS.each do
|from, to|
line.chomp!
line.gsub!(Regexp.escape(from)) { to }
end
. I'm using Regexp.escape just as a safety measure in the case that the string being replaced contains some regex metacharacter.
I'm using the Cygwin port of MRI Ruby 2.6.4.
line.gsub!(Regexp.escape(searchstring)) { '' }
My understanding was, that for every String stored in searchstring, the gsub! would cause that line is afterwards empty.
Your understanding is incorrect. The guarantee in the docs is
For any string, Regexp.new(Regexp.escape(str))=~str will be true.
This does hold for your example
Regexp.new(Regexp.escape("D "))=~"D " # => 0
therefore this is what your code should look like
line.gsub!(Regexp.new(Regexp.escape(searchstring))) { '' }
As for why this is the case, there used to be a bug where Regex.escape would incorrectly handle space characters:
# in Ruby 1.8.4
Regex.escape("D ") # => "D\\s"
My guess is they tried to keep the fix as simple as possible by replacing 's' with ' '. Technically this does add an unnecessary escape character but, again, that does not break the intended use of the method.
This happens to any searchstring containing a space. Indeed, if I do a
p(Regexp.escape(searchstring))
for my example, I see "D\\ " being printed, while I would expect to get "D " instead. Is this a bug in the Ruby core library, or did I misuse the escape function?
This looks to be a bug. In my opinion, whitespace is not a Regexp meta character, there is no need to escape it.
Some background: In my concrete application, where this simplified example is derived from, I just want to do a literal string replacement inside a long string […]
If you want to do literal string replacement, then don't use a Regexp. Just use a literal string:
line.gsub!(from, to)
how can i split a string "DESKTOP-AHDESI\Username" by slash in ruby 2.7.1p83
tmp = "DESKTOP-AHDESI\Username"
print tmp
tmp = tmp.split("\\")
print tmp
i got:
Ruby Error: NoMethodError undefined method `gsub!'
Problem
Your tmp variable is enclosed in double-quotes, and contains a backslash which is being interpreted as an escape rather than a character literal. You can see this easily by simply pasting your string into a REPL like irb:
"DESKTOP-AHDESI\Username" #=> "DESKTOP-AHDESIUsername"
You need to handle the backslash specially in both your String methods.
Solution
One way to handle this is to use Ruby's alternate quoting mechanism. For example:
%q(DESKTOP-AHDESI\Username).split '\\' #=> ["DESKTOP-AHDESI", "Username"]
This may not help you directly, though.
Wherever the value from tmp is coming from, you need to refactor the code to ensure that your String is properly escaped before you assign it to your variable, or otherwise pre-process it. String#dump won't really help much if the value you're assigning is unescaped before assignment, so you're going to have to fix this in whatever code you're using to generate or grab the string in the first place.
First of all, you are giving the wrong string. \ is the escape character when you use inside the "". So It will try to escape the next character U but this character doesn't have any Job so it will print U on the screen. Modify your string like below, it will work.
tmp = "DESKTOP-AHDESI\\Username"
p tmp
tmp = tmp.split("\\")
p tmp
Output
"DESKTOP-AHDESI\\Username"
["DESKTOP-AHDESI", "Username"]
How can I remove all newlines and spaces from a string in Ruby?
For example, if we have a string:
"123\n12312313\n\n123 1231 1231 1"
It should become this:
"12312312313123123112311"
That is, all whitespaces should be removed.
You can use something like:
var_name.gsub!(/\s+/, '')
Or, if you want to return the changed string, instead of modifying the variable,
var_name.gsub(/\s+/, '')
This will also let you chain it with other methods (i.e. something_else = var_name.gsub(...).to_i to strip the whitespace then convert it to an integer). gsub! will edit it in place, so you'd have to write var_name.gsub!(...); something_else = var_name.to_i. Strictly speaking, as long as there is at least one change made,gsub! will return the new version (i.e. the same thing gsub would return), but on the chance that you're getting a string with no whitespace, it'll return nil and things will break. Because of that, I'd prefer gsub if you're chaining methods.
gsub works by replacing any matches of the first argument with the contents second argument. In this case, it matches any sequence of consecutive whitespace characters (or just a single one) with the regex /\s+/, then replaces those with an empty string. There's also a block form if you want to do some processing on the matched part, rather than just replacing directly; see String#gsub for more information about that.
The Ruby docs for the class Regexp are a good starting point to learn more about regular expressions -- I've found that they're useful in a wide variety of situations where a couple of milliseconds here or there don't count and you don't need to match things that can be nested arbitrarily deeply.
As Gene suggested in his comment, you could also use tr:
var_name.tr(" \t\r\n", '')
It works in a similar way, but instead of replacing a regex, it replaces every instance of the nth character of the first argument in the string it's called on with the nth character of the second parameter, or if there isn't, with nothing. See String#tr for more information.
You could also use String#delete:
str = "123\n12312313\n\n123 1231 1231 1"
str.delete "\s\n"
#=> "12312312313123123112311"
You could use String#delete! to modify str in place, but note delete! returns nil if no change is made
Alternatively you could scan the string for digits /\d+/ and join the result:
string = "123\n\n12312313\n\n123 1231 1231 1\n"
string.scan(/\d+/).join
#=> "12312312313123123112311"
Please note that this would also remove alphabetical characters, dashes, symbols, basically everything that is not a digit.
In C#, I can write backslashes and other special characters without escaping by using # before a string, but I have to escape double-quotes.
C#
string foo = "The integer division operator in VisualBASIC is written \"a \\ b\"";
string bar = #"The integer division operator in VisualBASIC is written \"a \ b\""
In Ruby, I could use the single-quote string literal, but I'd like to use this in conjuction with string interpolation like "text #{value}". Is there an equivalent in Ruby to # in C#?
There is somewhat similar thing available in Ruby. E.g.
foo = %Q(The integer division operator in VisualBASIC is written "a \\ b" and #{'interpolation' + ' works'})
You can also interpolate strings in it. The only caveat is, you would still need to escape \ character.
HTH
You can use heredoc with single quotes.
foo = <<'_'
The integer division operator in VisualBASIC is written "a \ b";
_
If you want to get rid of the newline character at the end, then chomp it.
Note that this does not work with string interpolation. If you want to insert evaluated expressions within the string, you can use % operation after you create the string.
foo = <<'_'
text %{value}
_
foo.chomp % {value: "foo"}
I'm trying to do a simple string sub in Ruby.
The second argument to sub() is a long piece of minified JavaScript which has regular expressions contained in it. Back references in the regex in this string seem to be effecting the result of sub, because the replaced string (i.e., the first argument) is appearing in the output string.
Example:
input = "string <!--tooreplace--> is here"
output = input.sub("<!--tooreplace-->", "\&")
I want the output to be:
"string \& is here"
Not:
"string & is here"
or if escaping the regex
"string <!--tooreplace--> is here"
Basically, I want some way of doing a string sub that has no regex consequences at all - just a simple string replace.
To avoid having to figure out how to escape the replacement string, use Regex.escape. It's handy when replacements are complicated, or dealing with it is an unnecessary pain. A little helper on String is nice too.
input.sub("<!--toreplace-->", Regexp.escape('\&'))
You can also use block notation to make it simpler (as opposed to Regexp.escape):
=> puts input.sub("<!--tooreplace-->") {'\&'}
string \& is here
Use single quotes and escape the backslash:
output = input.sub("<!--tooreplace-->", '\\\&') #=> "string \\& is here"
Well, since '\\&' (that is, \ followed by &) is being interpreted as a special regex statement, it stands to reason that you need to escape the backslash. In fact, this works:
>> puts 'abc'.sub 'b', '\\\\&'
a\&c
Note that \\\\& represents the literal string \\&.