Ruby feature: Regular Expression replace once

About a year ago, programmer Tony Arcieri posed a question on the Ruby language bug tracker:  Why was Thread.exclusive deprecated?

Among the answers to this question, Shyouhei Urabe’s one of the tersest feature descriptions yet: “we already have such thing, to some extent at least”:

/#{@mutex=Mutex.new}/o

1. The pair of slashes are Regular Expression delimiters.
2. A Regular Expression in Ruby allows String interpolation, just like the double-quoted String does. The #{} contains interpolated Ruby code.
3. The modifier o at the end of the Regular Expression stands for “once”. So, the regex engine would keep track of this replacement, and do it just once.

The Malmo.rb named this expression (//o) the face-palm operator.