diff --git a/Library/Homebrew/tap_constants.rb b/Library/Homebrew/tap_constants.rb
index b9edb7afeb..f5943ad124 100644
--- a/Library/Homebrew/tap_constants.rb
+++ b/Library/Homebrew/tap_constants.rb
@@ -4,7 +4,7 @@
 # Match taps' formulae, e.g. `someuser/sometap/someformula`
 HOMEBREW_TAP_FORMULA_REGEX = %r{^([\w-]+)/([\w-]+)/([\w+-.@]+)$}.freeze
 # Match taps' casks, e.g. `someuser/sometap/somecask`
-HOMEBREW_TAP_CASK_REGEX = %r{^([\w-]+)/([\w-]+)/([a-z0-9\-]+)$}.freeze
+HOMEBREW_TAP_CASK_REGEX = %r{^([\w-]+)/([\w-]+)/([a-z0-9\-_]+)$}.freeze
 # Match taps' directory paths, e.g. `HOMEBREW_LIBRARY/Taps/someuser/sometap`
 HOMEBREW_TAP_DIR_REGEX = %r{#{Regexp.escape(HOMEBREW_LIBRARY.to_s)}/Taps/(?<user>[\w-]+)/(?<repo>[\w-]+)}.freeze
 # Match taps' formula paths, e.g. `HOMEBREW_LIBRARY/Taps/someuser/sometap/someformula`