Ruby regular expressions provide a /o option with the following meaning (from Pickaxe, forgive me): o - Substitute Once. Any #... substitutions in a particular regular expression literal will be performed just once, the first time it is evaluated. Otherwise, the substitutions will be performed every time the literal generates a Regexp object. JRuby currently ignores the modifier, which results in lower performance when a library is counting on the feature. For example, CodeRay (http://coderay.rubyforge.org/) uses it heavily, and runs about 10,000 times slower with jruby 1.1b1. Testing the feature is simple: assert Array('a'..'z').map { |c| c.to_s/#{c}/o }.compact.size == 1