Ruby string interpolation document.

src/pages/ruby/ruby-string-interpolation/index.md

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+---
+title: Ruby String Interpolation
+---
+
+# String Interpolation
+
+String interpolation provides a more readable and concise syntax for building strings. You may be familiar with concatenation via the `+` or `<<` methods:
+
+```ruby
+"Hello " + "World!" #=> Hello World
+"Hello " << "World!" #=> Hello World
+```
+
+String interpolation provides a way to embed Ruby code directly into a string:
+
+```ruby
+place = "World"
+"Hello #{place}!" #=> Hello World!
+
+"4 + 4 is #{4 + 4}" #=> 4 + 4 is 8
+```
+
+Everything between `#{` and `}` is evaluated as Ruby code. In order to do so, the string must be surrounded by double quotes (`"`).
+
+Single quotes will return the exact string inside the quotes:
+
+```ruby
+place = "World"
+'Hello #{place}!' #=> Hello #{place}!
+```