String operations and `eval` - ruby

Question

I have an AWS lambda function that receives user's code from a browser as a string and runs eval in a sandboxed environment as in a standard REPL. I am not trying to interpolate a string. I am trying to have eval recognize and perform operations on strings.
I am somewhat limited in the operations I can perform. Basic regex replacement is cool, but I don't think I would be able to do anything more involved than that, but perhaps I'm mistaken.
This works for basic arithmetic operations, the creation of class instances, etc. However, it fails to perform string operations properly. When I pass:
eval `("3*4")`
it returns 12, which is great. However, if I pass:
eval(""str:" + " test"")
it fails to return "str: test". In fact it returns nothing.
It has been suggested that I escape the double quotes. In a REPL, replacing all double quotes with escaped ones, such as \", works.
eval("\"str \" + \"test\"") # => "str test"
However, when I try this with AWS, it returns "\"str \" + \"test\"".
I look forward to hearing your responses.

Answer 1

You shouldn't expect eval(""str:" + " test"") to work. The problem is not related to AWS lambda. If you try this on your own machine, using pry or irb, you will get a SyntaxError. That's because your interpreter can't understand that you are only passing one unified string to eval. So you need to escape all quotation marks inside your string:
eval("\"str \" + \"test\"")
If you have tested it in a REPL without escaping, and it worked, it seems that the REPL you are using, somehow changes your input before sending it to interpreter.

Answer 2

I have seemed to find a work around.
To begin, I will first clarify what my issue was. Say a user entered the following:
class Test
attr_accesssor :data
def initialize(data = nil)
#data = data
end
end
Followed by
Test.new(4).data
eval would properly return 4.
Now say, however, that the user instead wrote
Test.new("A nice, fine string").data
eval would return nothing.
I noticed, however, that if I tacked on a .inspect, as shown below:
Test.new("A nice, fine string").data.inspect
I would be returned A nice, fine string.
So my solution was to wrap the entirety of the user's code in parenthesis and then call .inspect.
Which was accomplished by the following line
code = "(" << code << ").inspect"
Thank you to everyone who took the time to help me out. I really appreciate all of the feedback and suggestions.

Answer 3

You need to be careful about string interpolation when using eval on strings:
str = %(This is my String. It's name is string. Now I can eval "Hooray for my string")
eval(str.inspect)
#=> "This is my String. It's name is string. Now I can eval \"Hooray for my string\""
however these will raise errors
eval(str)
#=> SyntaxError: (eval):1: unterminated string meets end of file
eval(puts str) # will output the string but raise an error after it.
#=> This is my String. It's name is string. Now I can eval "Hooray for my string"
# TypeError: no implicit conversion of nil into String
but this will work
eval("puts #{str.inspect}")
#=> This is my String. It's name is string. Now I can eval "Hooray for my string"