♦️ String Interpolation: More Than Just #{variable}

2026-04-10

Ruby's string interpolation looks simple. "Hello, #{name}" and you're done, right? Wrong. You can shove entire expressions, method chains, and ternary operators inside those curly braces. Plus Ruby has a % operator for sprintf that'll make your formatted output sing.

Part 1: Basic Interpolation

Ruby uses #{} inside double-quoted strings (like Perl's $var in qq~~):

name = "Mike"
puts "Hello, #{name}"          # => Hello, Mike
puts %Q~Hello, #{name}~        # Same - %Q supports interpolation

Part 2: Expressions Inside Interpolation

Any Ruby expression works inside #{}:

"Count: #{count}"                          # variable
"Result: #{x > 0 ? 'yes' : 'no'}"          # ternary
"Today: #{Time.now.strftime('%Y-%m-%d')}"   # method call
"Debug: #{obj.inspect}"                     # inspect for debugging
"Sum: #{[1, 2, 3].sum}"                     # any expression
"Upper: #{"hello".upcase}"                  # string method

Part 3: Where Interpolation Works (and Doesn't)

Interpolation works in:

Double-quoted strings: "#{var}"
%Q~~ strings: %Q~#{var}~
%W~~ word arrays: %W~#{var} other~
Heredocs (double-quoted, the default): <<~END ... #{var} ... END
Backticks/%x~~: `echo #{var}`
Regex: %r~#{pattern}~

Interpolation does NOT work in:

Single-quoted strings: '#{var}' (stays literal)
%q~~ strings: %q~#{var}~ (stays literal)
%w~~ word arrays: %w~#{var}~ (stays literal)
Single-quoted heredocs: <<~'END'

Same rule as Perl: double-quote means interpolation, single-quote means literal.

Part 4: The sprintf Shortcut

The % operator on strings is a shortcut for sprintf:

# Single value
"%04d" % 42            # => "0042"
"%.2f" % Math::PI      # => "3.14"
"%s files" % count     # => "5 files"

# Multiple values (use array)
"%s: %d" % ["count", 5]                   # => "count: 5"
"%-10s %5d %8.2f" % ["item", 42, 3.14]    # => "item           42     3.14"

# Perl equivalent:
# sprintf("%04d", 42)
# sprintf("%s: %d", "count", 5)

Part 5: printf for Direct Output

# Like Perl's printf - prints directly instead of returning string
printf "%6d  %s\n", count, ip
printf "%-20s %s\n", filename, status

# Kernel#format is an alias for sprintf (returns string)
msg = format("Found %d errors in %s", count, filename)

Part 6: Format Specifiers Quick Reference

Specifier	Meaning	Example
`%s`	String	`"hello"`
`%d`	Integer	`42`
`%f`	Float	`3.14`
`%e`	Scientific	`3.14e+00`
`%x`	Hex	`2a`
`%o`	Octal	`52`
`%b`	Binary	`101010`
`%%`	Literal `%`	`%`

Width and padding:

%10s right-aligns in 10 chars
%-10s left-aligns in 10 chars
%04d zero-pads to 4 digits
%.2f gives 2 decimal places

The % operator is one of those things you'll use constantly once you know it exists. It's cleaner than sprintf for simple cases and way more readable than string concatenation.