How to embed regular expressions in other regular expressions in Ruby

Question

I have a string:

\'A Foo\'

and want to find \"Foo\" in it.

I have a regular expression:

/foo/

th

Answer 1

"what's wrong?"

Your assumption on how a Regexp is interpolated is wrong.

Interpolation via #{...} is done by calling to_s on the interpolated object:

d = Date.new(2017, 9, 8)
#=> #

d.to_s
#=> "2017-09-08"

"today is #{d}!"
#=> "today is 2017-09-08!"

and not just for string literals, but also for regular expression literals:

/today is #{d}!/
#=> /today is 2017-09-08!/

In your example, the object-to-be-interpolated is a Regexp:

foo_regex = /foo/

And Regexp#to_s returns:

[...] the regular expression and its options using the (?opts:source) notation.

foo_regex.to_s
#=> "(?-mix:foo)"

Therefore:

/A #{foo_regex}/i
#=> /A (?-mix:foo)/i

Just like:

"A #{foo_regex}"
#=> "A (?-mix:foo)"

In other words: because of the way Regexp#to_s is implemented, you can interpolate patterns without loosing their flags. It's a feature, not a bug.

If Regexp#to_s would return just the source (without options), it would work the way you expect:

def foo_regex.to_s
  source
end

/A #{foo_regex}/i
#=> /A foo/i

The above code is just for demonstration purposes, don't do that.