diff --git a/2020/ruby/day_04.rb b/2020/ruby/day_04.rb
new file mode 100644
index 0000000..b0e99ec
--- /dev/null
+++ b/2020/ruby/day_04.rb
@@ -0,0 +1,25 @@
+FIELDS = {
+  byr: ->(s) { s =~ /^\d{4}$/ && (1920..2002).cover?(s.to_i) },
+  iyr: ->(s) { s =~ /^\d{4}$/ && (2010..2020).cover?(s.to_i) },
+  eyr: ->(s) { s =~ /^\d{4}$/ && (2020..2030).cover?(s.to_i) },
+  hgt: ->(s) {
+    s =~ /^(\d+)(cm|in)$/
+    case $2
+    when "cm"
+      (150..193).cover?($1.to_i)
+    when "in"
+      (59..76).cover?($1.to_i)
+    else
+      false
+    end
+  },
+  hcl: ->(s) { s =~ /^#[0-9a-f]{6}$/ },
+  ecl: ->(s) { s =~ /^(amb|blu|brn|gry|grn|hzl|oth)$/ },
+  pid: ->(s) { s =~ /^\d{9}$/ },
+  cid: ->(s) { true },
+}
+
+puts ARGF.read.split("\n\n").map {|line| line.scan(/(\w+):(\S+)/).to_h }
+  .count {|passport|
+    FIELDS.map {|k,v| v[passport.fetch(k.to_s, "")] }.all?
+  }