How do I make these string substitutions using hash keys in Ruby? - ruby

Question

I have a bunch of JSON files, processed in both Python and Ruby, that look something like this:
{
"KEY1": "foo",
"KEY2": "bar",
"URL": "https://{KEY2}.com/{KEY1}",
"IMPORTANT_THING": "repos/{KEY1}",
"NOTE": "This thing is {KEY1}{KEY2}ed",
"PYTHON_ONLY_THING": "{}/test/{}.py"
}
Note that the order that the keys will show up is not consistent, and I'd rather not change the JSON.
Here's my test code showing what I've tried so far:
my_config = {"KEY1"=>"foo",
"KEY2"=>"bar",
"URL"=>"https://{KEY2}.com/{KEY1}",
"IMPORTANT_THING"=>"repos/{KEY1}",
"NOTE"=>"This thing is {KEY1}{KEY2}ed",
"PYTHON_ONLY_THING"=>"{}/test/{}.py"}
my_config.each_key do |key|
# Braindead, hard-coded solution that works:
# my_config[key].gsub!("{KEY1}", my_config["KEY1"])
# my_config[key].gsub!("{KEY2}", my_config["KEY2"])
# More flexible (if it would work):
# my_config[key].gsub!(/{.*}/, my_config['\0'.slice(1,-2)])
my_config[key].gsub!(/{.*}/) {|s| my_config[s.slice(1,-2)]}
end
puts my_config
I'm using the braindead solution for now, which produces the expected output:
{"KEY1"=>"foo", "KEY2"=>"bar", "URL"=>"https://bar.com/foo", "IMPORTANT_THING"=>"repos/foo", "NOTE"=>"This thing is foobared", "PYTHON_ONLY_THING"=>"{}/test/{}.py"}
But I want to make it more flexible and maintainable. The first "better" solution throws an error apparently because slice operates on '\0' itself and not the match, plus I'm not sure it would match more than once. The currently uncommented solution doesn't work because the second part seems to operate on one letter at a time rather than each match like I expected, so it just removes the stuff in curly braces. Worse, it removes everything between the outer braces in the PYTHON_ONLY_THING, which is no good.
I figure I need to change both my regex and Ruby code if this is going to work, but I'm not sure where to look for more help. Or perhaps gsub isn't the right tool for this job. Any ideas?
I am using Ruby 2.3.7 on Linux x86_64.

Answer 1

Use String#gsub with an initial hash for replacements:
my_config.map do |k, v|
[
k,
v.gsub(/(?<={)[^}]+(?=})/, my_config).gsub(/{(?!})|(?<!{)}/, '')
]
end.to_h
#⇒ {"KEY1"=>"foo",
# "KEY2"=>"bar",
# "URL"=>"https://bar.com/foo",
# "IMPORTANT_THING"=>"repos/foo",
# "NOTE"=>"This thing is foobared",
# "PYTHON_ONLY_THING"=>"{}/test/{}.py"}
Starting with Ruby 2.4 (or using Rails) it might be done simpler using Hash#transform_values.
If you dislike the second gsubbing, transform the hash upfront:
my_substs = my_config.map { |k, v| ["{#{k}}", v] }.to_h
my_config.map do |k, v|
[k, v.gsub(/{[^}]+}/, my_substs)]
end.to_h

Answer 2

Here's a possible solution:
my_config = {"KEY1"=>"foo",
"KEY2"=>"bar",
"URL"=>"https://{KEY2}.com/{KEY1}",
"IMPORTANT_THING"=>"repos/{KEY1}",
"NOTE"=>"This thing is {KEY1}{KEY2}ed",
"PYTHON_ONLY_THING"=>"{}/test/{}.py"}
my_config.each_key do |key|
placeholders = my_config[key].scan(/{([^}]+)}/).flatten
placeholders.each do |placeholder|
my_config[key].gsub!("{#{placeholder}}", my_config[placeholder]) if my_config.keys.include?(placeholder)
end
end
puts my_config
By using scan, this will substitute all matches, not just the first match.
Using [[^}]+ in the regex, rather than .*, means you won't "swallow" too much in this part of the match. For example, if the input contains "{FOO} bar {BAZ}", then you want that pattern to only capture FOO and BAZ, not FOO} bar {BAZ.
Grouping the scan result, then calling flatten, is an easy way to reject what's outside the capture group, i.e. in this case the { and } characters. (This just makes the code a little less cryptic than using indexes like slice(1,-2)!
my_config.keys.include?(placeholder) checks whether this is actually . a known value, so you don't replace things with nil.