three_hyphens_exactly.rb

# Ruby 1.9+
j = /(?<!\-)\-{3}(?![\-])/
 
positives = ["---", "x--- ", " ---x", "x --- x", "---!"]
negatives = ["--", "----", "x----", "----x", "---- x", "--!", "x--", "--x"]


positives.each {|t| puts "in #{t} I find a match a position #{t.match(j) ? t.match(j).offset(0) : 'NOWHERE!!1!'}, and I want to"}

negatives.each {|t| puts "in #{t} I find a match a position #{t.match(j) ? t.match(j).offset(0) : 'NOWHERE!!1!'}, and I don't want to"}