re = /((?<=NA - FY\d\d .....)|(?<=NA - FY\d\d ....))[(](?!([^)]*\b(ARR)\b[^(]*)).*?[)]/
str = 'NA - FY17 JCIG (Joint Coord Integration Grp) briefs
NA - FY17 JIPT (Joint Integr Product Tm) briefs
NA - FY17 PP (Phasing Plans)
NA - POM 17\\NA - FY17 SoW (Statement of Work)
NA - FY17 SRRB (Serv Requi Review Brd) Quad chart
NA - FY17 UFR (Unfunded Plan) - CYD (Current Year)
NA - POM 17\\NA - FY17 DR (Directors Reviews) briefs"
NA - FY17 Funding Docs"'
subst = ''
result = str.gsub(re, subst)
# Print the result of the substitution
puts result
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for Ruby, please visit: http://ruby-doc.org/core-2.2.0/Regexp.html