re = /(?:\R\K|\G) /
str = ' - first
- second
this is a test
- first
- second
'
result = str.gsub(re, ' ')
puts result
cmUgPSAvKD86XFJcS3xcRykgLwpzdHIgPSAnICAtIGZpcnN0IAogIC0gc2Vjb25kCgp0aGlzIGlzIGEgdGVzdAoKICAtIGZpcnN0IAogIC0gc2Vjb25kCicKcmVzdWx0ID0gc3RyLmdzdWIocmUsICcmbmJzcDsnKQoKcHV0cyByZXN1bHQ=